Aviation Maintenance Terms

24-bit Address

Unique address assigned to an aircraft during the registration process.

Aborted takeoff

A takeoff that is terminated prematurely when it is determined that some condition exists that makes takeoff or further flight dangerous.

Abradable strip

A strip of material in the compressor housing of some axial-flow gas turbine engines. The tip of the compressor blade touches the abradable strip, and wears, or abrades a groove in it. This groove ensures the minimum tip clearance.

Abradable tip (compressor blade tip)

The tip of some axial-flow compressor blades constructed so that it will abrade, or wear away, upon contact with the compressor housing, which ensures the minimum tip clearance between the blade and the housing.

Abrasion resistant PTFE

A solid insulation wall of PTFE with hard, nonconductive grit positioned midway in the wall thickness, and significantly improves the resistance of the PTFE material to damage from wear.

Absolute humidity

The actual amount of the water vapor in a mixture of air and water.

Absolute pressure

Equal to gauge pressure plus atmospheric pressure.Also known as psia.

Absolute pressure

Pressure referenced from zero pressure or a vacuum.

Absolute pressure

Pressure measured from zero pressure or a vacuum.

Absolute pressure regulator

A valve used in a pneumatic system at the pump inlet to regulate the compressor inlet air pressure to prevent excessive speed variation and/or overspeeding of the compressor.

Absolute temperature

Temperature measured relative to absolute zero. Absolute temperature scales include Kelvin and Rankine.

Absolute zero

The point at which all molecular motion ceases. Absolute zero is –460 °F and –273 °C.

Acceleration

The amount the velocity of an object is increased by a force during each second it is acted upon by that force. Acceleration is usually measured and expressed in terms of feet per second, per second (fps²).

Acceleration due to gravity

The acceleration of an object caused by gravity. On earth, it is measured as 32.2 feet per second per second (32.2 fps/s).

Accessory end

The end of a reciprocating engine on which many of the accessories are mounted. Also, called the anti- propeller end.

Accumulator

A hydraulic component that stores a non-compressible fluid, such as oil, under pressure. An accumulator has two compartments separated by a flexible or movable partition with one compartment containing compressed air. When oil is pumped into the other compartment, the partition moves over, further compressing the air which holds pressure on the oil.

Accumulator

A hydraulic component that consists of two compartments separated by a movable component, such as a piston, diaphragm, or bladder. One compartment is filled with compressed air or nitrogen, and the other is filled with hydraulic fluid and is connected into the system pressure manifold.An accumulator allows an incompressible fluid to be stored under pressure by the force produced by a compressible fluid. Its primary purposes are to act as a shock absorber in the system, and to provide a source of additional hydraulic power when heavy demands are placed on the system.

Acetylene

Gas composed of two parts of carbon and two parts of hydrogen. When burned in the atmosphere of oxygen, it produces one of the highest flame temperatures obtainable.

Acetylene regulator

Manually adjustable device used to reduce cylinder pressure to torch pressure and to keep the pressure constant. They are never to be used as oxygen regulators.

Actuator

A fluid power device that changes fluid pressure into mechanical motion.

Addition

The process in which the value of one number is added to the value of another.

Adherend

One of the members being bonded together by adhesive.

Adiabatic change

A physical change that takes place within a material in which heat energy is neither added to the material, nor taken away. If a container of gas is compressed, with no heat energy added to or taken from it, the gas will become hotter; its temperature will rise.

Administrator

The Federal Aviation Administrator or any person to whom he has delegated his authority in the matter concerned.

ADS-B Traffic Advisory System (ATAS)

An ADS-B In application that augments flight crew traffic situation awareness in support of the see-and-avoid responsibility by providing alerts of traffic that are detected in conflict with ownship. This application is named Traffic Situation Awareness with Alerts (TSAA) in RTCA DO-317B.

Advancing blade

The blade on a helicopter rotor whose tip is moving in the same direction the helicopter is moving.

Adverse yaw

A condition of flight at the beginning of a turn in which the nose of an airplane momentarily yaws in the opposite direction from the direction in which the turn is to be made.

Advisory

The level or category of alert for conditions that require flight crew awareness and may require subsequent flight crew response.

Advisory Circulars (AC)

Issued to inform the aviation public in a systematic way of nonregulatory material. An AC is issued to provide guidance and information in a designated subject area or to show a method acceptable to the Administrator for complying with a related 14 CFR part.

Aerodynamic coefficients

Non-dimensional coefficients for aerodynamic forces and moments.

Aerodynamic drag

The total resistance to the movement of an object through the air. Aerodynamic drag is composed of both induced drag and parasite drag. See induced drag and parasite drag.

Aerodynamic lift

The force produced by air moving over a specially shaped surface called an airfoil. Aerodynamic lift acts in a direction perpendicular to the direction the air is moving.

Aeromatic propeller

A patented variable-pitch propeller that has flyweights around the blade shanks and the blades angled back from the hub to increase the effects of aerodynamic and centrifugal twisting forces. This propeller automatically maintains a relatively constant rpm for any throttle setting.

Aeronautical Radio Incorporated (ARINC)

A corporation whose principal stockholders are the airlines. Its function is to operate certain communication links between airliners in flight and the airline ground facilities. ARINC also sets standards for communication equipment used by the airlines.

Aft-fan engine

A turbofan engine with the fan mounted behind the compressor section. The blades of an aft-fan are normally extensions of the free turbine blades.

Afterburner

A component in the exhaust system of a turbojet or turbofan engine used to increase the thrust for takeoff and for special flight conditions. Since much of the air passing through a gas turbine engine is used only for cooling, it still contains a great deal of oxygen. Fuel is sprayed into the hot, oxygen-rich exhaust in the afterburner, where it is ignited and burned to produce additional thrust.

Aging

A change in the characteristics of a material with time. Certain aluminum alloys do not have their full strength when they are first removed from the quench bath after they have been heat-treated, but they gain this strength after a few days by the natural process of aging.

Agonic line

A line drawn on an aeronautical chart along which there is no angular difference between the magnetic and geographic north poles.

Air bleed (carburetor component)

A small hole in the fuel passage between the float bowl and the discharge nozzle of a float carburetor. Air drawn into the liquid fuel through the air bleed breaks the fuel up into an emulsion, making it easy to atomize and vaporize.

Air carrier

An organization or person involved in the business of transporting people or cargo by air for compensation or hire.

Air cooling

The removal of unwanted heat from an aircraft engine by transferring the heat directly into the air flowing over the engine components.

Air impingement starter

A turbine engine starter that basically consists of a nozzle that blows a stream of compressed air against the turbine blades to rotate the compressor for starting the engine.

Air-cycle cooling system

A system for cooling the air in the cabin of a turbojet-powered aircraft. Compressor bleed air passes through two heat exchangers where it gives up some of its heat; then, it drives an expansion turbine where it loses still more of its heat energy as the turbine drives a compressor. When the air leaves the turbine, it expands and its pressure and temperature are both low.

Air-oil separator

A component in a turbine engine lubrication system that removes the air from the scavenged oil before it is returned to the oil tank.

Air/fuel mixture ratio

The ratio of the weight of the air to that of the fuel in the mixture fed into the cylinders of an engine.

Aircraft

A device that is used or intended to be used for flight in the air.

Aircraft communication addressing and reporting system (ACARS)

A two-way communication link between an airliner in flight and the airline’s main ground facilities. Data is collected in the aircraft by digital sensors and is transmitted to the ground facilities. Replies from the ground may be printed out so the appropriate flight crewmember can have a hard copy of the response.

Aircraft engine

An engine that is used or intended to be used for propelling aircraft. It includes turbosuperchargers, appurtenances, and accessories necessary for its functioning, but does not include propellers.

Aircraft Specifications

FAA recordkeeping documents issued for both type-certificated and non-typecertificated products which have been found eligible for U.S. airworthiness certification.

Aircraft Surveillance Application (ASA)

An application that uses aircraft surveillance data to provide benefits to the flight crew (refer to Table 1).

Aircraft Surveillance Applications System (ASAS)

An aircraft system based on airborne surveillance that provides assistance to the flight crew in operating their aircraft relative to other aircraft. Airborne Surveillance and Separation Assurance Processing (ASSAP). The processing subsystem that accepts surveillance inputs (e.g., ADS-B reports), performs surveillance processing to provide reports and tracks, and performs application-specific processing. Surveillance reports, tracks, and any application-specific alerts or guidance are output by ASSAP to the CDTI function. ASSAP surveillance processing consists of track processing and correlation of ADS-B, TIS-B, ADS-R and TCAS reports.

Airfoil

Any device that creates a force, or lift, based on Bernoulli’s principles or Newton’s laws, when air is caused to flow over the surface of the device.

Airfoil

Any surface designed to obtain a useful reaction, or lift, from air passing over it.

Airframe

The fuselage, booms, nacelles, cowlings, fairings, airfoil surfaces (including rotors but excluding propellers and rotating airfoils of engines), and landing gear of an aircraft and their accessories and controls.

Airplane

An engine-driven fixed-wing aircraft heavier than air, that is supported in flight by the dynamic reaction of the air against its wings.

Airship

An engine-driven lighter-than-air aircraft that can be steered.

Airspeed indicator

A flight instrument that measures the pressure differential between the pitot, or ram, air pressure, and the static pressure of the air surrounding the aircraft. This differential pressure is shown in units of miles per hour, knots, or kilometers per hour.

Airworthiness certificate

A document required to be onboard an aircraft that indicates the aircraft conforms to type design and is in condition for safe operation.

Airworthiness Directive

Airworthiness Directives (ADs) are legally enforceable regulations issued by the FAA in accordance with 14 CFR part 39 to correct an unsafe condition in a product. Part 39 defines a product as an aircraft, engine, propeller, or appliance.

Airworthiness directive

A regulation issued by the FAA that applies to aircraft, aircraft engines, propellers, or appliances, when an unsafe condition exists and that condition is likely to exist or develop in other products of the same type design.

Airworthiness Directive (AD note)

Airworthiness Directives (ADs) are legally enforceable rules issued by the FAA in accordance with 14 CFR part 39 to correct an unsafe condition in a product. 14 CFR part 39 defines a product as an aircraft, aircraft engine, propeller, or appliance.

Airworthiness directive (AD)

Issued by the FAA in response to deficiencies and/or unsafe conditions found in aircraft, engines, propellers, or other aircraft parts. Compliance with an AD is mandatory.

Airworthy

When an aircraft or one of its component parts meets its type design and is in a condition for safe operation.

Alclad

A registered trade name for clad aluminum alloy.

Alclad aluminum

Used to designate sheets that consist of an aluminum alloy core coated with a layer of pure aluminum to a depth of approximately 51⁄2 percent on each side.

Alert

A general term that applies to all advisories, cautions, and warning information; can include visual, aural, tactile, or other attention-getting methods.

Algebra

The branch of mathematics that uses letters or symbols to represent numbers in formulas and equations.

All-weather spark plug

A shielded spark plug designed for high altitude operation. The ceramic insulator is recessed into the shell to allow a resilient grommet on the ignition harness to provide a watertight seal. All weather spark plugs, also called high-altitude spark plugs, are identified by their 3/4-20 shielding threads.

Allowance

The difference of the upper and lower variation of a part.

Alodine

The registered trade name for a popular conversion coating chemical used to produce a hard, airtight, oxide film on aluminum alloy for corrosion protection.

Alodizing

A simple chemical treatment for all aluminum alloys to increase their corrosion resistance and to improve their paint bonding qualities.

Alpha control range (alpha mode)

The flight operating mode from takeoff through landing for a turbo-prop engine. Alpha mode includes operations from 95% to 100% of the engine’s rated rpm.

Alphanumeric symbols

Symbols made up of all of the letters in our alphabet, numerals, punctuation marks, and certain other special symbols.

Alteration

A change or modification to an aircraft from its previous state Alternating current. An electric current that reverses direction in a circuit at regular intervals.

Alternator

An electrical generator that produces alternating current. The popular DC alternator used on light aircraft produces three-phase AC in its stator windings. This AC is changed into DC by a six-diode, solid-state rectifier before it leaves the alternator.

Altimeter setting

The barometric pressure at a given location corrected to mean (average) sea level.

Altitude engine

A reciprocating aircraft engine having a rated takeoff power that is producible from sea level to an established higher altitude.

Altitude engine

An aircraft reciprocating engine equipped with a supercharger that allows it to maintain its rated sea- level horsepower to an established higher altitude.

Altitude engine

A reciprocating engine whose rated sea-level takeoff power can be produced to an established higher altitude.

Alumel

An alloy of nickel, aluminum, manganese, and silicon that is the negative element in a thermocouple used to measure exhaust gas temperature.

Amateur-built aircraft

Aircraft built by individuals as a hobby rather than by factories as commercial products. Amateur-built or home-built aircraft do not fall under the stringent requirements imposed by the FAA on commercially built aircraft.

Ambient air pressure

The pressure of the air that surrounds an object.

Ambient light

The visible light level measured at the surface of the part.

Ambient pressure

The pressure of the air surrounding a person or an object.

Ambient temperature

The temperature of the air surrounding a person or an object.

American wire gauge

The system of measurement of wire size used in aircraft electrical systems.

Ammeter

An instrument for measuring electric current in amperes.

Ampere

A unit of measure of the rate of electron flow or current in an electrical conductor. One ampere of current represents one coulomb of electrical charge (6.24 × 1018 charge carriers) moving past a specific point in one second.

Ampere (A)

The basic unit of current flow. One A is the amount of current that flows when a difference of potential of 1 V is applied to a circuit with a resistance of 1 Ω. One coulomb per second.

Amphibian

An airplane with landing gear that allows it to operate from both water and land surfaces.

Amplifier

An electronic circuit in which a small change in voltage or current controls a much larger change in voltage or current.

An airspeed indicator is a differential-pressure gauge

It measures the difference between static air pressure and pitot air pressure.

Analog electronics

Electronics in which values change in a linear fashion. Output values vary in direct relationship to changes of input values.

Analog indicator

An indicator that shows the value of the parameter being measured by a number marked on a graduated dial aligned with a movable pointer.

Analog-type indicator

An electrical meter that indicates values by the amount a pointer moves across a graduated numerical scale.

Aneroid

The sensitive component in an altimeter or barometer that measures the absolute pressure of the air. The aneroid is a sealed, flat capsule made of thin corrugated discs of metal soldered together and evacuated by pumping all of the air out of it. Evacuating the aneroid allows it to expand or collapse as the air pressure on the outside changes.

Angle of attack

The acute angle between the chordline of a propeller blade and the relative wind. The angle of attack is affected by both the engine rpm and the forward speed of the aircraft.

Angle of attack

The acute angle formed between the chord line of an airfoil and the direction of the air that strikes the airfoil.

Angle of attack indicator

An instrument that measures the angle between the local airflow around the direction detector and the fuselage reference plane.

Angle of incidence

The acute angle formed between the chord line of an airfoil and the longitudinal axis of the aircraft on which it is mounted.

Annealing

The process of heating a metal to a prescribed temperature, holding it there for a specified length of time, and then cooling the metal back to room temperature.

Annual inspection

An inspection required by the FAA once every 12 calendar months if other suitable inspections do not occur within that timeframe. An A&P technician with inspection authorization must perform this inspection.

Annual inspection

A complete inspection of the airframe and powerplant required for FAA-certificated aircraft operating under 14 CFR part 91 General Operating and Flight Rules, and not on one of the authorized special inspection programs. An annual inspection must be conducted every 12 calendar months, and it must be conducted by an aviation maintenance technician who holds an Airframe and Powerplant rating and an Inspection Authorization. The scope of an annual inspection is the same as that of a 100-hour inspection.

Annual rings

The rings that appear in the end of a log cut from a tree. The number of annual rings per inch gives an indication of the strength of the wood. The more rings there are and the closer they are together, the stronger the wood. The pattern of alternating light and dark rings is caused by the seasonal variations in the growth rate of the tree. A tree grows quickly in the spring and produces the light-colored, less dense rings. The slower growth during the summer, or latter part of the growing season, produces the dark-colored, denser rings.

Annular duct

A duct, or passage, that surrounds an object. The annular fan-discharge duct surrounds the core engine.

Annular orifice

A ring-shaped orifice, normally one that surrounds another orifice.

Annulus

A ring or groove around the outside of a circular body or shaft, or around the inside of a cylindrical hole.

Annunciator panel

A panel of warning lights visible to the flight crew. The lights are identified by the name of the system they represent and are often covered with colored lenses. Red lights indicate a dangerous condition and green indicate a safe condition.

Annunciator panel

A panel of warning lights in plain sight of the pilot. These lights are identified by the name of the system they represent and are usually covered with colored lenses to show the meaning of the condition they announce.

Anodizing

The most common surface treatment of nonclad aluminum alloy surfaces. The aluminum alloy sheet or casting is the positive pole in an electrolytic bath in which chromic acid or other oxidizing agent produces an aluminum oxide film on the metal surface. Aluminum oxide is naturally protective, and anodizing merely increases the thickness and density of the natural oxide film.

Anodizing

A hard, airtight, unbroken oxide film electrolytically deposited on an aluminum alloy surface to protect it from corrosion.

Anodizing

The electrolytic process in which a hard, airtight, oxide film is deposited on aluminum alloy for corrosion protection.

Antenna

A device designed to radiate or intercept electromagnetic waves.

Antenna

Aspecial device used with electronic communication and navigation systems to radiate and receive electromagnetic energy.

Antenna Offset Parameter

The distance from the nose of the aircraft to the GPS antenna. For large aircraft, this offset is important in accurately placing the aircraft symbol on the airport map.

Anti-ice system

A system that prevents the formation of ice on an aircraft structure.

Anti-icing

Prevention of the formation of ice on a surface.

Anti-icing additive

A chemical added to the turbine-engine fuel used in some aircraft. This additive mixes with water that condenses from the fuel and lowers its freezing temperature so it will not freeze and block the fuel filters. It also acts as a biocidal agent and prevents the formation of microbial contamination in the tanks.

Anti-propeller end

The end of a reciprocating engine that does not attach to the propeller. Also called the accessory end.

Anti-tear strips

Strips of fabric of the same material as the airplane is covered with, laid over the wing rib under the reinforcing tape.

Antidrag wire

A structural wire inside a Pratt truss airplane wing between the spars. Antidrag wires run from the rear spar inboard, to the front spar at the next bay outboard. Antidrag wires oppose the forces that try to pull the wing forward.

Antiservo tab

A tab installed on the trailing edge of a stabilator to make it less sensitive. The tab automatically moves in the same direction as the stabilator to produce an aerodynamic force that tries to bring the surface back to a streamline position. This tab is also called an antibalance tab.

Antiskid brake system

An electrohydraulic system in an airplane’s power brake system that senses the deceleration rate of every main landing gear wheel. If any wheel decelerates too rapidly, indicating an impending skid, pressure to that brake is released and the wheel stops decelerating. Pressure is then reapplied at a slightly lower value.

Antitear strip

Strips of aircraft fabric laid under the reinforcing tape before the fabric is stitched to an aircraft wing.

Apparent power

That power apparently available for use in an AC circuit containing a reactive component. It is the product of effective voltage times the effective current, expressed in volt-amperes.

Apparent power

The product of volts and amperes in AC circuits where the current and voltage are out of phase.

Appliance

Any instrument, mechanism, equipment, part, apparatus, appurtenance, or accessory, including communications equipment, that is used or intended to be used in operating or controlling an aircraft in flight, is installed in or attached to the aircraft, and is not part of an airframe, engine, or propeller.

Appliance

Application

The function(s) for which the ASA system is used.

Approved

Approved by the FAA or any person to whom the FAA has delegated its authority in the matter concerned, or approved under the provisions of a bilateral agreement between the United States and a foreign country or jurisdiction.

Aramid fiber

Fiber made from an organic compound of carbon, hydrogen, oxygen, and nitrogen. It has high strength and low density. It is flexible under load and is able to withstand impact, shock, and vibration. Kevlar is a well- known aramid fiber.

Arbor press

A press with either a mechanically or hydraulically operated ram used in a maintenance shop for a variety of pressing functions.

Archimedes’ principle

The buoyant force that a fluid exerts upon a submerged body is equal to the weight of the fluid the body displaces.

Arcing

Sparking between a commutator and brush or between switch contacts that is caused by induced current when a circuit is broken.

Area

A measurement of the amount of surface inside a two- dimensional object.

Area

The number of square units in a surface.

Area Navigation (RNAV)

A method of navigation that permits aircraft operation on any desired flight path within the coverage of station-referenced navigation aids or within the limits of the capability of self-contained aids, or a combination of these.

Arm

The horizontal distance that a part of the aircraft or a piece of equipment is located from the datum.

Arm

A measurement of distance, in inches, feet, etc., used in weight and balance calculations. Normally only the longitudinal arm is of practical importance. The three axial arms are longitudinal arm, lateral arm, and vertical arm.

Armature

The rotating part of an electric generator or motor.

Aromatic compound

A chemical compound such as toluene, xylene, and benzene that is blended with gasoline to improve its anti-detonation characteristics.

Articulating rod

See link rod.

Aspect ratio

The relationship of the length (wingtip to wingtip), or span, of an airfoil to its width, or chord.

Aspect ratio

The ratio of the length of an airfoil, such as a compressor blade, to its width.

Aspect ratio

The ratio of the length, or span, of an airplane wing to its width, or chord. For a nonrectangular wing, the aspect ratio is found by dividing the square of the span of the wing by its area. Aspect Ratio = span2 ÷ area.

Assembly drawing

A description of an object made up of two or more parts.

Asymmetrical airfoil

An airfoil section that is not the same on both sides of the chord line.

Asymmetrical lift

A condition of uneven lift produced by the rotor when a helicopter is in forward flight. Asymmetrical lift is caused by the difference between the airspeed of the advancing blade and that of the retreating blade.

Asymmetrical loading

The loading of a propeller disc that causes one side to produce more thrust than the other side.

Atom

The smallest particle composed of a nucleus that contains protons, neutrons, and electrons, which revolve around the nucleus

Atomize

The process of breaking a liquid down into tiny droplets or a fine spray. Atomized liquids vaporize easily.

Attenuate

To weaken, or lessen the intensity of, an activity.

Attitude indicator

A gyroscopic flight instrument that gives the pilot an indication of the attitude of the aircraft relative to its pitch and roll axes. The attitude indicator in an autopilot is in the sensing system that detects deviation from a level- flight attitude.

Augmenter tube

A long, stainless steel tube around the discharge of the exhaust pipes of a reciprocating engine. Exhaust gases flow through the augmenter tube and produce a low pressure that pulls additional cooling air through the engine compartment. Heat may be taken from the augmenter tubes and directed through the leading edges of the wings for thermal anti-icing.

Augmentor tube

A long, specially shaped stainless steel tube mounted around the exhaust tail pipe of a reciprocating engine. As exhaust gases flow through the augmentor tube, they produce a low pressure in the engine compartment that draws in cooling air through the cylinder fins.

Autoclave

A pressure vessel inside of which air can be heated to a high temperature and pressure raised to a high value. Autoclaves are used in the composite manufacturing industry to apply heat and pressure for curing resins.

Autogyro

A heavier-than-air rotor-wing aircraft sustained in the air by rotors turned by aerodynamic forces rather than by engine power. When the name Autogyro is spelled with a capital A, it refers to a specific series of machines built by Juan de la Cierva or his successors.

Autoignition system

A system on a turbine engine that automatically energizes the igniters to provide a relight if the engine should flame out.

Automatic adjuster

A subsystem in an aircraft disc brake that compensates for disc or lining wear. Each time the brakes are applied, the automatic adjuster is reset for zero clearance, and when the brakes are released, the clearance between the discs or the disc and lining is returned to a preset value. A malfunctioning automatic adjuster in a multiple-disc brake can cause sluggish and jerky operation.

Automatic Dependent Surveillance - Rebroadcast (ADS-R)

Retransmission of UAT ADS-B messages from aircraft on the 1090ES link and 1090ES messages on the UAT link. ADS-R ensures aircraft equipped with different links can receive messages from one another when equipped with ADS-B IN.

Automatic Dependent Surveillance Broadcast (ADS-B)

An advanced surveillance technology where ADS-B OUT equipped aircraft share position, altitude, velocity, and other information with ATC and other appropriately equipped aircraft.

Automatic Dependent Surveillance-Broadcast (ADS-B)

A function on an aircraft or surface vehicle operating within the surface movement area that periodically broadcasts its state vector (horizontal and vertical position, horizontal and vertical velocity) and other information. ADS-B is automatic because no external stimulus is required to elicit a transmission. It is dependent because it relies on on-board navigation sources and on-board broadcast transmission systems to provide surveillance information to other users.

Automatic Dependent Surveillance-Rebroadcast (ADS-R)

A service of the ground system that rebroadcasts ADS-B messages from one link technology onto another. For example, the SBS ground system provides ADS-R service to rebroadcast UAT messages on 1090 MHz and vice versa.

Automatic direction finder (ADF)

A radio receiver utilizing a directional loop antenna that enables the receiver to indicate the direction from which a radio signal is being received; also called a radio compass.

Automatic Flight Control System (AFCS)

A flight control system incorporating an automatic pilot with additional systems such as a VOR coupler, an ILS approach coupler, and an internal navigation system that is fully automatic, so the aircraft can be flown in a completely automatic mode.

Automatic flight control system (AFCS)

The full system of automatic flight control that includes the autopilot, flight director, horizontal situation indicator, air data sensors, and other avionics inputs.

Automatic intake valve

An intake valve opened by low pressure created inside the cylinder as the piston moves down. There is no mechanical means of opening it.

Automatic mixture control (AMC)

The device in a fuel metering system, such as a carburetor or fuel injection system, that keeps the air/fuel mixture ratio constant as the density of air changes with altitude.

Automatic pilot (autopilot)

An automatic flight control device that controls an aircraft about one or more of its three axes. The primary purpose of an autopilot is to relieve the pilot of the control of the aircraft during long periods of flight.

Autorotation

A rotorcraft or powered-lift flight condition in which the lifting rotor is driven entirely by action of the air when the rotorcraft or powered-lift is in motion.

Autosyn system

The registered trade name of a remote indicating instrument system. An Autosyn system uses an electromagnet rotor, excited with 400-hertz AC, and a three- phase distributed-pole stator.

Autosyn system

A synchro system used in remote indicating instruments. The rotors in an Autosyn system are two-pole electromagnets, and the stators are delta-connected, three-phase, distributed-pole windings in the stator housings. The rotors in the transmitters and indicators are connected in parallel and are excited with 26-volt, 400-Hz AC. The rotor in the indicator follows the movement of the rotor in the transmitter.

Auxiliary power unit (APU)

A small turbine or reciprocating engine that drives a generator, hydraulic pump, and air pump. The APU is installed in the aircraft and is used to supply electrical power, compressed air, and hydraulic pressure when the main engines are not running.

Auxiliary rotor

A rotor that serves either to counteract the effect of the main rotor torque on a rotorcraft or to maneuver the rotorcraft about one or more of its three principal axes.

Availability

An indication of the ability of a system or subsystem to provide usable service. Availability is expressed in terms of the probability of the system or subsystem being available at the beginning of an intended operation.

Aviation snips

Compound-action hand shears used for cutting sheet metal. Aviation snips come in sets of three. One pair cuts to the left, one pair cuts to the right, and the third pair of snips cuts straight.

Aviator’s oxygen

Oxygen that has had almost all of the water and water vapor removed from it.

Avionics

The science and technology of electronics as applied to aviation.

Avionics

The branch of technology that deals with the design, production, installation, use, and servicing of electronic equipment mounted in aircraft.

Axial bearing load

The load on a bearing parallel to the shaft on which the bearing is mounted. Thrust produces an axial load on a bearing.

Axial turbine

A turbine that is turned by a fluid flowing through it in a direction that is approximately parallel to the shaft on which the turbine wheel is mounted.

Axial-flow compressor

A type of compressor used in gas turbine engines. Air passes through the compressor in essentially a straight line, parallel to the axis of the compressor. The compressor is made of a number of stages of rotating compressor blades between stages of stationary stator vanes.

Axis of rotation

The center line about which a propeller rotates.

Azimuth

Angular distance measured on a horizontal circle in a clockwise direction from either north or south.

Azimuth

A horizontal angular distance, measured clockwise from a fixed reference direction to an object.

Babbitt

A soft silvery metal used for main bearing inserts in aircraft reciprocating engines. Babbitt is made of tin with small amounts of copper and antimony.

Back (propeller nomenclature)

The curved surface of a propeller blade. The back of a propeller blade corresponds to the upper surface of an airplane wing.

Back course

The reciprocal of the localizer course for an ILS (Instrument Landing System). When flying a back-course approach, the aircraft approaches the instrument runway from the end on which the localizer antennas are installed.

Back-suction mixture control

A type of mixture control used in some float carburetors that regulates the air/fuel mixture by varying the air pressure above the fuel in the float bowl.

Backhand welding

Welding in which the torch is pointed away from the direction the weld is progressing.

Backplate (brake component)

A floating plate on which the wheel cylinder and the brake shoes attach on an energizing- type brake.

Backup ring

A flat leather or Teflon ring installed in the groove in which an O-ring or T-seal is placed. The backup ring is on the side of the seal away from the pressure, and it prevents the pressure extruding the seal between the piston and the cylinder wall.

Baffle

A thin sheet metal shroud or bulkhead used to direct the flow of cooling air between and around the cylinder fins of an air-cooled reciprocating engine.

Balance

The condition of stability which exists in an aircraft when all weight and forces are acting in such a way as to prevent rotation about an axis or pivot point.

Balance cable

A cable in the aileron system of an airplane that connects to one side of each aileron. When the control wheel is rotated, a cable from the flight deck pulls one aileron down and relaxes the cable going to the other aileron. The balance cable pulls the other aileron up.

Balance panel

A flat panel hinged to the leading edge of some ailerons that produces a force which assists the pilot in holding the ailerons deflected. The balance panel divides a chamber ahead of the aileron in such a way that when the aileron is deflected downward, for example, air flowing over its top surface produces a low pressure that acts on the balance panel and causes it to apply an upward force to the aileron leading edge.

Balance tab

An adjustable tab mounted on the trailing edge of a control surface to produce a force that aids the pilot in moving the surface. The tab is automatically actuated in such a way it moves in the direction opposite to the direction the control surface on which it is mounted moves.

Balanced actuator

A linear hydraulic or pneumatic actuator that has the same area on each side of the piston.

Ballast

A weight installed or carried in an aircraft to move the center of gravity to a location within its allowable limits.

Balloon

A lighter-than-air aircraft that is not engine driven, and that sustains flight through the use of either gas buoyancy or an airborne heater.

Banana oil

Nitrocellulose dissolved in amyl acetate, so named because it smells like bananas.

Bank (verb)

The act of rotating an aircraft about its longitudinal axis.

Barometric Altitude Integrity Code (NICBARO)

Indicates if pressure altitude is provided by a single Gillham encoder or another, more robust altitude source. Because of the potential for an undetected error in a Gillham encoding, many Gillham installations are cross-checked against a second altitude source. NICBARO annotates the status of this cross-check.

Barometric scale

A small window in the dial of a sensitive altimeter in which the pilot sets the barometric pressure level from which the altitude shown on the altimeter is measured. This window is sometimes called the “Kollsman” window.

base

The electrode of a bipolar transistor between the emitter and the collector. Varying a small flow of electrons moving into or out of the base controls a much larger flow of electron between the emitter and the collector.

Base

In mathematics, used to refer to a particular mathematical object that is used as a building block. A base-a system is one that uses a as a new unit from which point counting starts again. (See decimal system.) In the mathematical expression an, read as “a to the nth power,” a is the base.

Base metal

The metal to be welded, brazed, soldered, or cut.

Basic Airborne (AIRB)

The basic airborne situation awareness application enhances the flight crew’s traffic situation awareness through the provision of an on-board graphical display of surrounding traffic that transmits ADS-B data of a sufficient quality. Pilots will use the CDTI to supplement and enhance out-the-window visual acquisition and listening to radio communications.

Basic empty weight

Standard empty weight plus optional equipment.

Bayonet stack

An exhaust stack with an elongated and flattened end. The gases leave the stack through a slot perpendicular to its length. Bayonet stacks decrease both exhaust back pressure and noise.

Bead

Zones that are in contact with the rim areas of the wheel, or the coils of steel wire that provide anchors for the plies.

Bead (tire component)

The high-strength carbon-steel wire bundles that give an aircraft tire its strength and stiffness where it mounts on the wheel.

Bead seat area

The flat surface on the inside of the rim of an aircraft wheel on which the bead of the tire seats.

Bearing strength (sheet metal characteristic)

The amount of pull needed to cause a piece of sheet metal to tear at the points at which it is held together with rivets. The bearing strength of a material is affected by both its thickness and the diameter of the rivet.

Beehive spring

A hardened-steel, coil-spring retainer used to hold a rivet set in a pneumatic rivet gun. This spring gets its name from its shape. It screws onto the end of the rivet gun and allows the set to move back and forth, but prevents it being driven from the gun.

Bell mouth

The shape of the inlet of an augmentor tube that forms a smooth converging duct. The bell mouth shape allows the maximum amount of air to be drawn into the tube.

Bell mouth inlet duct

A form of convergent inlet-air duct used to direct air into the compressor of a gas turbine engine. It is extremely efficient, and is used where there is little ram pressure available to force air into the engine. Bell mouth ducts are used in engine test cells and on engines installed in helicopters.

Bend allowance

The amount of material actually used to make a bend in a piece of sheet metal. Bend allowance depends upon the thickness of the metal and the radius of the bend, and is normally found in a bend allowance chart.

Bend radius

The radius of the inside of a bend.

Bend tangent line

A line made in a sheet metal layout that indicates the point at which the bend starts.

Benzene

A colorless, volatile, flammable, aromatic hydrocarbon liquid which has the chemical formula C6H6. Benzene, which is sometimes called benzoil, is used as a solvent, a cleaning fluid, and a fuel for some special types of reciprocating engines.

Bernoulli’s principle

The basic principle that explains the relation between kinetic energy and potential energy in fluids that are in motion. When the total energy in a column of moving fluid remains constant, any increase in the kinetic energy of the fluid (its velocity) results in a corresponding decrease in its potential energy (its pressure).

Bernoulli’s principle

Equivalent to the principle of conservation of energy, this principle states that the static pressure of a fluid (liquid or gas) decreases at points where the velocity of the fluid increases, provided no energy is added to or taken away from the fluid.

Bernoulli’s principle

A physical principle that explains the relationship between kinetic and potential energy in a stream of moving fluid. When energy is neither added to nor taken from the fluid, any increase in its velocity (kinetic energy) will result in a corresponding decrease in its pressure (potential energy).

Beta control range (Beta mode)

The range of operation of a turboprop powerplant used for in-flight approach and ground handling of the engine and aircraft. Typically, the Beta mode includes operations from 65% to 95% of the engine’s rated rpm.

Beta tube

A tube in a Garrett TPE331 turboprop powerplant that extends into the propeller pitch control to act as a follow- up device. It provides movement of the propeller blades in proportion to movement of the power lever.

Bezel

The rim that holds the glass cover in the case of an aircraft instrument.

Bezel

The rim which holds the glass cover in the case of an aircraft instrument.

Bias tire

A pneumatic tire in which the ply cords extend to the beads and are laid at alternate angles substantially less than 90 degrees to the centerline of the tread.

Bias-cut surface tape

A fabric tape in which the threads run at an angle of 45° to the length of the tape. Bias-cut tape may be stretched around a compound curve such as a wing tip bow without wrinkling.

Bidirectional fibers

Fibers in a piece of composite material arranged to sustain loads in two directions.

Bilge area

A low portion in an aircraft structure in which water and contaminants collect. The area under the cabin floorboards is normally called the bilge.

Bimetallic hairspring

A flat, spiral-wound spring made of two strips of metal laid side-by-side and welded together. The two metals have different coefficients of expansion, and as the temperature changes, the spiral either tightens or loosens. A bimetallic hair spring is used in a thermocouple temperature changes at the reference junction.

Bimetallic strip

A metal strip made of two different types of metal fastened together side by side. When heated, the two metals expand different amounts and the strip warps or bends.

Binary number system

The binary number system is a number system that has only two digits, 0 (zero) and 1. Binary numbers are made from a series of zeros and ones. An example of an 8-bit binary number is 11010010. The prefix “bi” in the word binary is a Latin root for the word “two.” Block diagrams. Used to show a simplified relationship of a more complex system of components.

Bipolar transistor

A solid-state component in which the flow of current between its emitter and collector is controlled by a much smaller flow of current into or out of its base. Bipolar transistors may be of either the NPN or PNP type.

Black box

A term used for any portion of an electrical or electronic system that can be removed as a unit. A black box does not have to be a physical box.

Black light

Electromagnetic radiation in the near ultraviolet range of wavelength.

Bladder-type fuel cell

A plastic-impregnated fabric bag supported in a portion of an aircraft structure so that it forms a cell in which fuel is carried.

Blade

One arm of a propeller from the hub to tip.

Blade

The component of a propeller that converts the rotation of the propeller shaft into thrust. The blade of a propeller corresponds to the wing of an airplane.

Blade angle

An angle between the chord line of a propeller blade section and a plane perpendicular to the axis of propeller rotation.

Blade station

See Stations.

Blade station

A reference position on a blade that is a specified distance from the center of the hub.

Blade track

The condition of a helicopter rotor in which each blade follows the exact same path as the blade ahead of it.

Blade track

The path established by the tip of each blade as it passes a fixed point in rotation.

Bleeder

A material such as glass cloth or mat that is placed over a composite lay-up to absorb the excess resin forced out of the ply fibers when pressure is applied.

Bleeding dope

Dope whose pigments are soluble in the solvents or thinners used in the finishing system. The color will bleed up through the finished coats.

Bleeding of brakes

The maintenance procedure of removing air entrapped in hydraulic fluid in the brakes. Fluid is bled from the brake system until fluid with no bubbles flows out.

Blending

A method of repairing damaged compressor and turbine blades. The damage is removed and the area is cleaned out with a fine file to form a shallow depression with generous radii. The file marks are then removed with a fine abrasive stone so the surface of the repaired area will match the surface of the rest of the blade.

Blimp

A cigar-shaped, nonrigid lighter-than-air flying machine.

Blisk

A turbine wheel machined from a single slab of steel. The disc and blades are an integral unit.

Blow-in doors

Spring-loaded doors in the inlet duct of some turbojet or turbofan engine installations that are opened by differential air pressure when inlet air pressure drops below that of the ambient air. Air flowing through the doors adds to the normal inlet air passing through the engine and helps prevent compressor stall.

Blush

A defect in a lacquer or dope finish caused by moisture condensing on the surface before the finish dries. If the humidity of the air is high, the evaporation of the solvents cools the air enough to cause the moisture to condense. The water condensed from the air mixes with the lacquer or dope and forms a dull, porous, chalky-looking finish called blush. A blushed finish is neither attractive nor protective.

Bond

The adhesion of one surface to another, with or without the use of an adhesive as a bonding agent.

Bonding

The process of electrically connecting all isolated components to the aircraft structure. Bonding provides a path for return current from electrical components, and a low-impedance path to ground to minimize static electrical charges. Shock-mounted components have bonding braids connected across the shock mounts.

Bonding

A general term applied to the process of electrically connecting two or more conductive objects. In aircraft, the purpose of bonding (except as applied to individual connections in the wiring and grounding systems) is to provide conductive paths for electric currents. This is accomplished by providing suitable low-impedance connections joining conductive aircraft components and the aircraft structure. Another purpose of bonding is to ensure the safe passage of current caused by lightning or static electricity through the aircraft structure.

Boost

A term for manifold pressure that has been increased above the ambient atmospheric pressure by a supercharger.

Boost pump

An electrically driven centrifugal pump mounted in the bottom of the fuel tanks in large aircraft. Boost pumps provide a positive flow of fuel under pressure to the engine for starting and serve as an emergency backup in the event an engine-driven pump should fail. They are also used to transfer fuel from one tank to another and to pump fuel overboard when it is being dumped. Boost pumps prevent vapor locks by holding pressure on the fuel in the line to the engine-driven pump. Centrifugal boost pumps have a small agitator propeller on top of the impeller to force vapors from the fuel before it leaves the tank.

Bootstrapping

An action that is self-initiating or self- sustaining. In a turbocharger system, bootstrapping describes a transient increase in engine power that causes the turbocharger to speed up, which in turn causes the engine to produce more power.

Bore

The diameter of a reciprocating engine cylinder.

Borescope

A device that enables the inspector to see inside areas that could not otherwise be inspected without disassembly.

Borescope

An inspection tool for viewing the inside of a turbine engine without disassembling it. The instrument consists of a light, mirror, and magnifying lens mounted inside a small-diameter tube that is inserted into a turbine engine through borescope inspection ports.

Borescope

A long, tubular optical instrument designed for remote visual inspection of surfaces.

Boss

An enlarged area in a casting or machined part. A boss provides additional strength to the part where holes for mounting or attaching parts are drilled.

Bottom

(verb) A condition in the installation of a propeller on a splined shaft when either the front or rear cone contacts an obstruction that prevents the cone from properly seating inside the propeller hub.

Boundary layer

The layer of air that flows next to an aerodynamic surface. Because of the design of the surface and local surface roughness, the boundary layer often has a random flow pattern, sometimes even flowing in a direction opposite to the direction of flight. A turbulent boundary layer causes a great deal of aerodynamic drag.

Bourdon tube

A pressure-indicating mechanism used in most oil pressure and hydraulic pressure gages. It consists of a sealed, curved tube with an elliptical cross section. Pressure inside the tube tries to straighten it, and as it straightens, it moves a pointer across a calibrated dial. Bourdon-tube pressure gauges are used to measure temperature by C measuring the vapor pressure in a sealed container of a volatile liquid, such as methyl chloride, whose vapor pressure varies directly with its temperature.

Bourdon tube

The major component in a gage-pressure measuring instrument. It is a thin-wall metal tube that has an elliptical cross section and is formed into a curve. One end of the tube is sealed and connected to an arm that moves the pointer across the instrument dial, and the open end is anchored to the instrument case. The pressure to be measured is directed into the open end, which causes the elliptical cross section to become more circular. As the cross section changes, the curve straightens and moves the pointer over the dial by an amount proportional to the amount of pressure.

Boyle’s law

States that the volume of an enclosed dry gas varies inversely with its absolute pressure, provided the temperature remains constant.

Brake horsepower

The power delivered at the propeller shaft (main drive or main output) of an aircraft engine.

Brashness

A condition of wood characterized by low resistance to shock and by an abrupt failure across the grain without splintering.

Brayton cycle

The constant-pressure cycle of energy transformation used by gas turbine engines. Fuel is sprayed into the air passing through the engine and burned. Heat from the burning air/fuel mixture expands the air and accelerates it as it moves through the engine. The Brayton cycle is an open cycle in that the intake, compression, combustion, expansion, and exhaust events all take place at the same time, but in different locations within the engine.

Braze welding

A welding process variation in which a filler metal, having a liquidus above 450 °C (840 °F) and below the solidus of the base metal is used. Unlike brazing, in braze welding the filler metal is not distributed in the joint by capillary action.

Brazing

A method of thermally joining metal parts by wetting the surface with a molten nonferrous alloy. When the molten material cools and solidifies, it holds the pieces together. Brazing materials melt at a temperature higher than 800 °F, but lower than the melting temperature of the metal on which they are used.

Brazing

The joining of two pieces of metal by wetting their surface with molten alloy of copper, zinc, or tin.

Break lines

Line on a drawing indicating that a portion of the object is not shown on the drawing.

British thermal unit (BTU)

The amount of heat energy needed to raise the temperature of one pound of pure water 1 °F.

British thermal unit (Btu)

The amount of heat required to change the temperature of 1 pound of water by 1 degree Fahrenheit.

British thermal unit (Btu)

The basic unit of heat energy in the English system. One Btu is the amount of heat energy needed to raise the temperature of one pound of pure water from 60 ˚F to 61˚F.

Buckets

The portions of aft-fan blades that are in the exhaust of the core engine. Buckets drive the fan from energy received from hot gases leaving the core engine.

Bucking bar

A heavy steel bar with smooth, hardened surfaces, or faces. The bucking bar is held against the end of the rivet shank when it is driven with a pneumatic rivet gun, and the shop head is formed against the bucking bar.

Buffeting

Turbulent movement of the air over an aerodynamic surface.

Bulb angle

An L-shaped metal extrusion having an enlarged, rounded edge that resembles a bulb on one of its legs.

Bulkhead

A structural partition that divides the fuselage of an aircraft into compartments, or bays.

Bungee cord

An elastic cord made of small strips of rubber encased in a loosely braided cloth tube that holds and protects the rubber, yet allows it to stretch. The energy in a stretched bungee cord may be used to crank a large aircraft engine.

Bungee shock cord

A cushioning material used with the nonshock absorbing landing gears installed on older aircraft. Bungee cord is made up of many small rubber bands encased in a loose-woven cotton braid.

Buoyancy

The upward force that any fluid exerts on a body submerged in it.

Burner

See combustor.

Burnish

To smooth the surface of a metal part that has been damaged by a deep scratch or gouge. Metal piled at the edge of the damage is pushed back into the damage with a smooth, hard steel burnishing tool.

Burnish (verb)

To smooth the surface of metal that has been damaged by a deep scratch or gouge. The metal piled up at the edge of the damage is pushed back into the damage with a smooth, hard steel burnishing tool.

Burr

A sharp rough edge of a piece of metal left when the metal was sheared, punched, or drilled.

Bus

A point within an electrical system from which the individual circuits get their power.

Bus or Bus bar

Solid copper strips to carry current between primary and secondary circuits; also used as jumpers.

Butt joint

A joint between two members aligned approximately in the same plane.

Butterfly valve

A flat, disc-shaped valve used to control the flow of fluid in a round pipe or tube. When the butterfly valve is across the tube, the flow is shut off, and when it is parallel with the tube, the obstruction caused by the valve is minimum, and the flow is at its greatest. Butterfly-type throttle valves are used to control the airflow through the fuel metering system.

Buttock line

A line used to locate a position to the right or left of the center line of an aircraft structure.

Buttock line (BL)

The longitudinal axis of the aircraft that serves as the reference location for positions to the left and right of center. The positions are usually dimensioned in inches

Butyl

Trade name for a synthetic rubber product made by the polymerization of isobutylene. Butyl withstands such potent chemicals as phosphate ester-base (Skydrol) hydraulic fluids.

Butyrate dope

A finish for aircraft fabric consisting of a film base of cellulose fibers dissolved in solvents with the necessary plasticizers, solvent, and thinners.

Bypass engine

Another name for a turbofan engine. See turbofan engine.

Bypass ratio

The ratio of the mass of air moved by the fan to the mass of air moved by the core engine.

Cable (electrical)

Assembly of one or more conductors within an enveloping protective sheath so constructed as to permit use of conductors separately or in a group.

Cage (verb)

To lock the gimbals of a gyroscopic instrument so it will not be damaged by abrupt flight maneuvers or rough handling.

Calendar month

A measurement of time used by the FAA for inspection and certification purposes. One calendar month from a given day extends from that day until midnight of the last day of that month.

Calendar month

The measurement of time used by the FAA for inspection and certification purposes. One calendar month from a given date extends from that date until midnight of the last day of that month.

Calibrated airspeed

The indicated airspeed of an aircraft, corrected for position and instrument error. Calibrated airspeed is equal to true airspeed in standard atmosphere at sea level.

Calibrated airspeed (CAS)

Indicated airspeed corrected for position error. See position error.

Calibration

A set of operations, performed in accordance with a definite document procedure, which compares the measurements performed by an instrument or standard, for the purpose of detecting and reporting, or eliminating by adjustment, errors in the instrument tested.

Calorie

The amount of heat required to change the temperature of 1 gram of water by 1 degree Centigrade.

Calorie

The amount of heat energy needed to raise the temperature of one gram of pure water 1 °C.

Cam

An eccentric, or lobe, on a rotating shaft that changes rotary motion into linear motion. A cam is mounted on the magnet shaft in a magneto to push upward on the insulated breaker point to separate, or open, the points when the magnet is in a particular location.

Cam engine

A reciprocating engine with axial cylinders arranged around a central shaft. Rollers on the pistons in the cylinders press against a sinusoidal cam mounted on the shaft to produce rotation of the shaft.

Cam-ground piston

A reciprocating engine piston that is not round, but is ground so that its diameter parallel to the wrist pin is slightly smaller than its diameter perpendicular to the pin. The mass of metal used in the wrist pin boss, the enlarged area around the wrist pin hole, expands when heated, and when the piston is at its operating temperature, it is perfectly round.

Camber

The curvature of a wing as viewed by cross section. A wing has upper camber on its top surface and lower camber on its bottom surface. The upper camber is more pronounced; the lower camber is comparatively flat. This causes the velocity of the airflow immediately above the wing to be much higher than that below the wing.

Camber (wheel alignment)

The amount the wheels of an aircraft are tilted, or inclined, from the vertical. If the top of the wheel tilts outward, the camber is positive. If the top of the wheel tilts inward, the camber is negative.

Can-annular combustor

A type of combustor used in some large turbojet and turbofan engines. It consists of individual cans into which fuel is sprayed and ignited. These cans mount on an annular duct which collects the hot gases and directs them uniformly into the turbine.

Canard

A horizontal control surface mounted ahead of the wing to provide longitudinal stability and control.

Canard

The forward wing of a canard configuration and may be a fixed, movable, or variable geometry surface, with or without control surfaces.

Canard configuration

A configuration in which the span of the forward wing is substantially less than that of the main wing.

Canted rate gyro

A rate gyro whose gimbal axis is tilted so it can sense rotation of the aircraft about its roll axis as well as its yaw axis.

Cantilever wing

A wing that is supported by its internal structure and requires no external supports. The wing spars are built in such a way that they carry all the bending and torsional loads.

Cap strip

The main top and bottom members of a wing rib. The cap strips give the rib its aerodynamic shape.

Capacitance (C)

The property of an electric conductor that characterizes its ability to store an electric charge.

Capacitance afterfiring

The continuation of the spark across the gap in a shielded spark plug after the air/fuel mixture in the cylinder is ignited. Afterfiring is caused by the return of electrical energy stored in the capacitance of the shielded ignition leads. Capacitance afterfiring is eliminated by the use of a resistor in the spark plug.

Capacitance-type fuel quantity measuring system

A popular type of electronic fuel quantity indicating system that has no moving parts in the fuel tank. The tank units are cylindrical capacitors, called probes, mounted across the tank, from top to bottom. The dielectric between the plates of the probes is either fuel or the air above the fuel, and the capacitance of the probe varies with the amount of fuel in the tank. The indicator is a servo-type instrument driven by the amplified output of a capacitance bridge.

Capacitive reactance (Xc)

The measure of a capacitor’s opposition to alternating current.

Capacitor

An electrical component that stores an electric charge.

Capacitor

An electrical component, formerly called a condenser, that consists of two large-area conductors, called plates, separated by an insulator. Electrons stored on one of the plates produces an electrostatic pressure difference between the plates.

Capillary tube

A soft copper tube with a small inside diameter. The capillary tube used with vapor-pressure thermometer connects the temperature sensing bulb to the Bourdon tube. The capillary tube is protected from physical damage by enclosing it in a braided metal wire jacket.

Capillary tube

A glass or metal tube with a tiny inside diameter. Capillary action causes the fluid to move within the tube.

Carbon monoxide detector

A packet of chemical crystals mounted in the aircraft flight deck or cabin where they are easily visible. The crystals change their color from yellow to green when they are exposed to carbon monoxide.

Carbon pile voltage regulator

A voltage regulator for a high output DC generator that uses a stack of pure carbon discs for the variable resistance element. A spring holds pressure on the stack to reduce its resistance when the generator output voltage is low. This allows maximum field current to flow. The field from an electro-magnet, whose strength varies directly with the generator voltage, opposes the spring to loosen the stack and increase its resistance when the generator voltage needs to be decreased. The increased resistance decreases the field current and reduces the output voltage.

Carbon track

A trail of carbon deposited by an arc across a high-voltage component such as a distributor block. Carbon tracks have a relatively low resistance to the high voltage and can cause misfiring and loss of engine power.

Carbon-pile voltage regulator

A type of voltage regulator used with high-output DC generators. Field current is controlled by varying the resistance of a stack of thin carbon discs. This resistance is varied by controlling the amount the stack is compressed by a spring whose force is opposed by the pull of an electromagnet. The electromagnet’s strength is proportional to the generator’s output voltage.

Carburizing flame

An oxyacetylene flame produced by an excess of acetylene. This flame is identified by a feather around the inner cone. A carburizing flame is also called a reducing flame.

Carcass (tire component)

The layers of rubberized fabric that make up the body of an aircraft tire.

Cartridge starter

A self-contained starter used on some military aircraft. A cartridge similar in size to a shotgun shell is ignited in the starter breech. The expanding gases drive a piston attached to a helical spline that converts the linear movement of the piston into rotary motion to rotate the crankshaft.

Cascade effect

The cumulative effect that occurs when the output of one series of components serves as the input to the next series.

Case hardening

A process in which the surface of a metal is changed chemically by introducing a high carbide or nitride content. Case hardening produces a hard, wear-resistant surface, or case, over a strong, tough core.

Case pressure

A low pressure that is maintained inside the case of a hydraulic pump. If a seal becomes damaged, hydraulic fluid will be forced out of the pump rather than allowing air to be drawn into the pump.

Casing

The structural part of a tire.

Catalyst

A substance used to change the speed, or rate, of a chemical action without being chemically changed itself.

Cathode-ray tube (CRT)

A display tube used for oscilloscopes and computer video displays. An electron gun emits a stream of electrons that is attracted to a positively charged inner surface of the face of the tube. Acceleration and focusing grids speed the movement of the electrons and shape the beam into a pinpoint size. Electrostatic or electromagnetic forces caused by deflection plates or coils move the beam over the face of the tube. The inside surface of the face of the tube is treated with a phosphor material that emits light when the beam of electrons strikes it.

Caution

The level or category of alert for conditions that require immediate flight crew awareness and subsequent flight crew response.

Cavitating

The creation of low pressure in an oil pump when the inlet system is not able to supply all of the oil the pump requires. Prolonged cavitation can damage pump components.

Cavitation

A condition that exist in a hydraulic pump when there is not enough pressure in the reservoir to force fluid to the inlet of the pump. The pump picks up air instead of fluid.

CDTI Assisted Visual Separation (CAVS)

The CDTI is used to assist the flight crew in acquiring and maintaining visual contact during visual separation on approach. The CDTI is also used in conjunction with visual, out-the-window contact to follow the preceding aircraft during the approach. The application is expected to improve both the safety and the performance of visual separation on approach. It may allow for the continuation of visual separation on approach even if visual contact out-the-window is momentarily lost.

Center of gravity

The location on an aircraft about which the force of gravity is concentrated.

Center of gravity

That point about which the aircraft would balance if suspended. For field weight and balance purposes/control, the center of gravity is normally calculated only along its longitudinal axis (nose to tail), disregarding both the lateral and vertical location.

Center of gravity (CG)

The point about which the nose-heavy and tail-heavy moments are exactly equal in magnitude.

Center of gravity range

The center of gravity range for an aircraft is the limits within which the aircraft must balance. It is identified as a forward-most limit (arm) and an aft-most limit (arm).

Center of lift

The location of the chord line of an airfoil at which all the lift forces produced by the airfoil are considered to be concentrated.

Center of pressure

The point on the chord line of an airfoil where all of the aerodynamic forces are considered to be concentrated.

Center of pressure

The point on the chordline of an airfoil where all aerodynamic forces are concentrated.

Center-line thrust airplane

A twin-engine airplane with both engines mounted in the fuselage. One is installed as a tractor in the front of the cabin. The empennage is mounted on booms.

Centering cam

A cam in the nose-gear shock strut that causes the piston to center when the strut fully extends. When the aircraft takes off and the strut extends, the wheel is straightened in its fore-and-aft position so it can be retracted into the wheel well. Charging stand (air conditioning service equipment). A handy and compact arrangement of air conditioning servicing equipment. A charging stand contains a vacuum pump, a manifold gauge set, and a method of measuring and dispensing the refrigerant.

Centrifugal compressor

A type of compressor that uses a vaned plate like impeller. Air is taken into the center, or eye, of the impeller and slung outward by centrifugal force into a diffuser where its velocity is decreased and its pressure increased.

Centrifugal force

The apparent force occurring in curvilinear motion acting to deflect objects outward from the axis of rotation. For instance, when pulling out of a dive, it is the force pushing the pilot down in their seat.

Centripetal force

The force in curvilinear motion acting toward the axis of rotation. For instance, when pulling out of a dive, it is the force that the seat exerts on the pilot to offset the centrifugal force.

Ceramic

Any of several hard, brittle, heat-resistant, noncorrosive materials made by shaping and then firing a mineral, such as clay, at a high temperature.

Certification

Implies that a certificate is in existence which certifies or states a qualification.

Channel-chromed cylinders

Reciprocating engine cylinders with hard chromium-plated walls. The surface of this chrome plating forms a spider web of tiny stress cracks. Deplating current enlarges the cracks and forms channels that hold lubricating oil on the cylinder wall.

Charles’ Law

States that all gases expand and contract in direct proportion to the change in the absolute temperature, provided the pressure is held constant.

Chatter

A type of rapid vibration of a hydraulic pump caused by the pump taking in some air along with the hydraulic fluid.

Check

A lengthwise separation of the wood, the greater part of which occurs across the rings of annual growth.

Check (wood defect)

Longitudinal cracks that extend across a log’s annual rings.

Check valve

A hydraulic or pneumatic system component that allows full flow of fluid in one direction but blocks all flow in the opposite direction.

Cheek (crankshaft)

The offset portion of a crankshaft that connects the crankpin to the main bearing journals.

Chemical conversion coating (Specification MIL-C-81706)

A chemical surface treatment used on aluminum alloys to inhibit corrosion and to provide a proper surface for paint finishing.

Chemical energy

Energy released from chemical reactions.

Chemical oxygen candle system

An oxygen system used for emergency or backup use. Solid blocks of material that release oxygen when they are burned are carried in special fireproof fixtures. When oxygen is needed, the candles are ignited with an integral igniter, and oxygen flows into the tubing leading to the masks.

Chevron seal

A form of one-way seal used in some fluid- power actuators. A chevron seal is made of a resilient material whose cross section is in the shape of the letter V. The pressure being sealed must be applied to the open side of the V.

Chip detector

A component in a lubrication system that attracts and holds ferrous metal chips circulating with the engine oil. Some chip detectors are part of an electrical circuit. When metal particles short across the two contacts in the detector, the circuit is completed, and an annunciator light is turned on to inform the flight crew that metal particles are loose in the lubrication system.

Choke nozzle

A nozzle in a gas turbine engine that limits the speed of gases flowing through it. The gases accelerate until they reach the speed of sound, and a normal shock wave forms that prevents further acceleration.

Choke of a cylinder

The difference in the bore diameter of a reciprocating engine cylinder in the area of the head and in the center of the barrel.

Choke-ground cylinder

A cylinder of a reciprocating engine that is ground so that its diameter at the top of the barrel is slightly smaller than the diameter in the center of the stroke. The large mass of metal in the cylinder head absorbs enough heat to cause the top end of the barrel to expand more than the rest of the barrel. At normal operating temperature, the diameter of a choke-ground cylinder is uniform throughout.

Chord

The distance from the blade leading edge to the trailing edge.

Chord

An imaginary straight line joining the leading and trailing edges of an airfoil.

Chord line

A straight line drawn between the leading and trailing edges of the blade.

Chordline

An imaginary line, passing through a propeller blade, joining the leading and trailing edges.

Chromel

An alloy of nickel and chromium used as the positive element in a thermocouple for measuring exhaust gas temperature.

Cigarette

A commonly used name for a spark plug terminal connector used with a shielded spark plug.

Circle

A closed plane figure with every point an equal distance from the center. A circle has the greatest area for its circumference of any enclosed shape.

Circuit

A closed path or mesh of closed paths usually including a source of EMF.

Circuit breaker

An electrical component that automatically opens a circuit any time excessive current flows through it. A circuit breaker may be reset to restore the circuit after the fault causing the excessive current has been corrected.

Circuit breaker

A protective device for opening a circuit automatically when excessive current is flowing through it.

Circular magnetism

A method of magnetizing a part for magnetic particle inspection. Current is passed through the part, and the lines of magnetic flux surround it. Circular magnetism makes it possible to detect faults that extend lengthwise through the part.

Circular magnetization

The induction of a magnetic field consisting of concentric circles of force about and within a part, which is achieved by passing electric current through the part.

Circumference (of a circle)

The linear measurement of the distance around a circle. The circumference is calculated by multiplying the diameter of the circle by 3.1416.

Circumferential coil spring (garter spring)

A coil spring formed into a ring. This type of spring is used to hold segmented ring-type carbon seals tightly against a rotating shaft.

Civil aircraft

Aircraft other than public aircraft.

Clad aluminum

A sheet of aluminum alloy that has a coating of pure aluminum rolled on one or both of its surfaces for corrosion protection.

Clamp-on ammeter

An electrical instrument used to measure current without opening the circuit through which it is flowing. The jaws of the ammeter are opened, slipped over the current- carrying wire, and then clamped shut. Current flowing through the wire produces a magnetic field which induces a voltage in the ammeter that is proportional to the amount of current.

Claret red

A dark purplish pink to a dark gray purplish red color.

Class

(2) As used with respect to the certification of aircraft, means a broad grouping of aircraft having similar characteristics of propulsion, flight, or landing. Examples include: airplane; rotorcraft; glider; balloon; landplane; and seaplane.

Class A fire

A fire with solid combustible materials such as wood, paper, and cloth as its fuel.

Class B fire

A fire that has combustible liquids as its fuel.

Class C fire

A fire which involves energized electrical equipment.

Class D fire

A fire in which a metal such as magnesium burns.

Cleco fastener

A patented spring-type fastener used to hold metal sheets in position until they can be permanently riveted together.

Close-grained wood

Wood with narrow and inconspicuous annual rings. The term is sometimes used to designate wood having small and closelyspaced pores, but in this sense the term “finetextured” is more often used.

Close-quarter iron

A small hand-held iron with an accurately calibrated thermostat. This iron is used for heat- shrinking polyester fabrics in areas that would be difficult to work with a large iron.

Closed angle

An angle formed in sheet metal that has been bent more than 90°.

Closed assembly time

The time elapsing between the assembly of glued joints and the application of pressure.

Closed-center hydraulic system

A hydraulic system in which the selector valves are installed in parallel with each other. When no unit is actuated, fluid circulates from the pump back to the reservoir without flowing through any of the selector valves.

Closed-center selector valve

A type of flow-control valve used to direct pressurized fluid into one side of an actuator, and at the same time, direct the return fluid from the other side of the actuator to the fluid reservoir. Closed-center selector valves are connected in parallel between the pressure manifold and the return manifold.

Closed-loop control

A type of control in which part of the output is fed back to the input. This allows the input to continually compare the command signals with the output to determine the extent to which the commands have been complied with.

Coast Interval

The elapsed time since a report from any source has been correlated with the track.

Coaxial

Rotating about the same axis. Coaxial rotors of a helicopter are mounted on concentric shafts in such a way that they turn in opposite directions to cancel torque.

Coaxial cable

A special type of electrical cable that consists of a central conductor held rigidly in the center of a braided outer conductor. Coaxial cable, commonly called coax, is used for attaching radio receivers and transmitters to their antenna.

Cockpit Display of Traffic Information (CDTI)

The pilot interface portion of the Aircraft Surveillance Applications System. This interface includes traffic display(s) and all the controls that interact with such a display. At a minimum, CDTI includes a graphical plan-view (top down) traffic display. Additional graphical and non-graphical display surfaces may also be included. The CDTI receives position information of traffic and Ownship from the airborne surveillance and separation assurance processing (ASSAP) function. The ASSAP receives such information from the surveillance sensors and Ownship position sensors.

Code of Federal Regulations (CFR)

Established by law to provide for the safe and orderly conduct of flight operations and to prescribe airmen privileges and limitations.

Coefficient of drag

A dimensionless number used in the formula for determining induced drag as it relates to the angle of attack.

Coefficient of lift

A dimensionless number relating to the angle of attack used in the formula for determining aerodynamic lift.

Coil shot

Production of longitudinal magnetization accomplished by passing current through a coil encircling the part being inspected.

Coin dimpling

A process of preparing a hole in sheet metal for flush riveting. A coining die is pressed into the rivet hole to form a sharp-edged depression into which the rivet head fits.

Coke

The solid carbon residue left when all volatile parts of a mineral oil have been evaporated by heat.

Cold section

The portion of a gas turbine engine ahead of the combustion section. The cold section includes the inlet, compressor, and diffuser.

Cold-cranking simulation

A method used for specifying the characteristics of a lubricating oil at low temperature. Oils rated by this test have the letter W (standing for Winter) in their designation. For example, SAE 15W50.

Cold-tank lubrication system

A turbine engine lubricating system in which the oil cooler is in the scavenge subsystem.

Collective pitch control

The helicopter control that changes the pitch of all of the rotor blades at the same time. Movement of the collective pitch control increases or decreases the lift produced by the entire rotor disc.

Collector ring

A ring made of thin corrosion-resistant steel tubing that encircles a radial engine and collects exhaust gases from each cylinder. The ring ends with a connection to the exhaust tail pipe.

Collodion

Cellulose nitrate used as a film base for certain aircraft dopes.

Combustion heater

A type of cabin heater used in some aircraft. Gasoline from the aircraft fuel tanks is burned in the heater.

Combustor (combustion chamber)

The section of a gas turbine engine in which fuel is injected. This fuel mixes with air from the compressor and burns. The intense heat from the combustion expands the air flowing through the combustor and directs it our through the turbine. Combustors are also called burners.

Commercial operator

A person who, for compensation or hire, engages in the carriage by aircraft in air commerce of persons or property, other than as an air carrier or foreign air carrier or under the authority ofPart 375 of this title. Where it is doubtful that an operation is for “compensation or hire”, the test applied is whether the carriage by air is merely incidental to the person’s other business or is, in itself, a major enterprise for profit.

Commutator

A mechanical rectifier mounted on the armature shaft of a DC generator or motor. It consists of a cylindrical arrangement of insulated copper bars connected to the armature coils. Carbon brushes ride on the copper bars to carry current into or out of the commutator, providing a unidirectional current from a generator or a reversal of current in the motor coils.

Compass

A device used to determine direction on the Earth’s surface. A magnetic compass utilizes the Earth’s magnetic field to establish direction.

Compass fluid

A highly refined, water-clear petroleum product similar to kerosene. Compass fluid is used to dampen the oscillations of magnetic compasses.

Compass rose

A location on an airport where an aircraft can be taken to have its compasses “swung.” Lines are painted on the rose to mark the magnetic directions in 30° increments.

Compass swinging

A maintenance procedure that minimizes deviation error in a magnetic compass. The aircraft is aligned on a compass rose, and the compensating magnets in the compass case are adjusted so the compass card indicates the direction marked on the rose. After the deviation error is minimized on all headings, a compass correction card is completed and mounted on the instrument panel next to the compass.

Compensated fuel pump

A vane-type, engine-driven fuel pump that has a diaphragm connected to the pressure regulating valve. The chamber above the diaphragm is vented to the carburetor upper deck where it senses the pressure of the air as it enters the engine. The diaphragm allows the fuel pump to compensate for altitude changes and keeps the carburetor inlet fuel pressure a constant amount higher than the carburetor inlet air pressure.

Compensating winding

A series winding in a compound- wound DC generator. The compensating windings are embedded in the faces of the field poles and their varying magnetic field works with the fields from the interpoles to effectively cancel the field distortion caused by armature current.

Compensator port (brake system component)

A small hole between a hydraulic brake master cylinder and the reservoir. When the brakes are released, this port is uncovered and the fluid in the master cylinder is vented to the reservoir. When the brake is applied, the master-cylinder piston covers the compensator port and allows pressure in the line to the brake to build up and apply the brakes. When the brake is released, the piston uncovers the compensator port. If any fluid has been lost from the brake, the reservoir will refill the master cylinder. A restricted compensator port will cause the brakes to drag or will cause them to be slow to release.

Composite

Something made up of different materials combined in such a way that the characteristics of the resulting material are different from those of any of the components.

Composite propeller blade

A propeller blade made from several materials such as metal, graphite, glass or aramid fibers, and foam.

Compound curve

A curve formed in more than one plane. The surface of a sphere is a compound curve.

Compound gauge (air conditioning servicing equipment)

A pressure gauge used to measure the pressure in the low side of an air conditioning system. A compound gauge is calibrated from zero to 30 inches of mercury vacuum, and from zero to about 150-psi positive gauge pressure.

Compressibility effect

The sudden increase in the total drag of an airfoil in transonic flight caused by formation of shock waves on the surface.

Compression failure

A type of structural failure in wood caused by the application of too great a compressive load. A compression failure shows up as a faint line running at right angles to the grain of the wood.

Compression ratio

The ratio of the volume of a cylinder with the piston at the bottom of its stroke to the volume of the cylinder with the piston at the top of its stroke.

Compression ratio (reciprocating engine)

The ratio of the volume of a cylinder of a reciprocating engine with the piston at the bottom of its stroke engine to the volume of the cylinder with the piston at the top of its stroke.

Compression ratio (turbine engine)

The ratio of the pressure of the air at the discharge of a turbine engine compressor to the pressure of the air at its inlet.

Compression strut

A heavy structural member, often in the form of a steel tube, used to hold the spars of a Pratt truss airplane wing apart. A compression strut opposes the compressive loads between the spars arising from the tensile loads produced by the drag and antidrug wires.

Compression wood

A defect in wood that causes it to have a high specific gravity and the appearance of an excessive growth of summerwood. In most species, there is little difference between the color of the springwood and the summerwood. Any material containing compression wood is unsuited for aircraft structural use and must be rejected.

Compression wood

Identified by its relatively wide annual rings, usually eccentric, and its relatively large amount of summer wood, usually more than 50 percent of the width of the annual rings in which it occurs. Compression wood shrinks excessively lengthwise as compared with normal wood.

Compressor (air conditioning system component)

The component in a vapor-cycle cooling system in which the low- pressure refrigerant vapors, after they leave the evaporator, are compressed to increase both their temperature and pressure before they pass into the condenser. Some compressors are driven by electric motors, others by hydraulic motors and, in the case of most light airplanes, are belt driven from the engine.

Compressor (air conditioning system component)

Compressor bleed air

Air that is tapped off from a turbine engine compressor. Compressor bleed air is used for anti- icing the inlet ducts and for cooling the turbine inlet guide vanes and first stage turbine blades. Bleed air is also used for certain airframe functions. See customer bleed air.

Compressor pressure ratio

See compression ratio (turbine engine).

Compressor stall

A condition in a turbine engine axial-flow compressor in which the angle of attack of one or more blades is excessive and the smooth airflow through the compressor is disrupted.

Compressor surge

A stall that affects the entire compressor and seriously restricts the airflow through the engine.

Computer aided design (CAD)

Using a computer in the design of a product.

Computer aided design drafting (CADD)

Using a computer in the design and drafting process.

Computer aided engineering (CAE)

Using a computer in the engineering of a product.

Computer aided manufacturing (CAM)

Using a computer in the manufacturing of a product.

Computer graphics

Drawing with the use of a computer.

Con-di ducts

The British name for a convergent-divergent duct. See convergent-divergent duct.

Concave surface

A surface that is curved inward. The outer edges are higher than the center.

Condenser

See capacitor.

Condenser (air conditioning system component)

The component in a vapor-cycle cooling system in which the heat taken from the aircraft cabin is given up to the ambient air outside the aircraft.

Conduction

The transfer of heat which requires physical contact between an object that has a large amount of heat energy and one that has a smaller amount of heat energy.

Conductor

A material that will carry electric current.

Conductor

A wire or other material suitable for conducting electricity.

Conductor (electrical)

A material that allows electrons to move freely from one atom to another within the material.

Conduit

A rigid metallic or nonmetallic casing, or a flexible metallic casing covered with a woven braid or synthetic rubber used to encase electrical cables.

Configuration, Maintenance, and Procedures (CMP) document

A document approved by the FAA that contains minimum configuration, operating, and maintenance requirements, hardware life-limits, and Master Minimum Equipment List (MMEL) constraints necessary for an airplane-engine combination to meet ETOPS type design approval requirements.

Conformal

A desirable property of map projections. A map projection (a function that associate points on the surface of an ellipsoid or sphere representing the earth to points on a flat surface such as the CDTI display) is said to be conformal if the angle between any two curves on the first surface is preserved in magnitude and sensed by the angle between the corresponding curves on the other surface.

Coning angle

The angle formed between the plane of rotation of a helicopter rotor blade when it is producing lift and a line perpendicular to the rotor shaft. The degree of the coning angle is determined by the relationship between the centrifugal force acting on the blades and the aerodynamic lift produced by the blades.

Constant (mathematical)

A value used in a mathematical computation that is the same every time it is used. For example, the relationship between the length of the circumference of a circle and the length of its diameter is a constant, 3.1416. This constant is called by the Greek name of Pi (π).

Constant differential mode (cabin pressurization)

The mode of pressurization in which the cabin pressure is maintained a constant amount higher than the outside air pressure. The maximum differential pressure is determined by the structural strength of the aircraft cabin.

Constant-displacement pump

A fluid pump that moves a specific volume of fluid each time it rotates; the faster the pump turns, the more fluid it moves. Some form of pressure regulator or relief valve must be used with a constant- displacement pump when it is driven by an aircraft engine.

Constant-displacement pump

A fluid pump that moves a specific volume of fluid each time it rotates.

Constant-pressure cycle of energy release

The cycle of energy transformation of a gas turbine engine. See Brayton cycle.

Constant-speed drive (CSD)

A special drive system used to connect an alternating current generator to an aircraft engine. The drive holds the generator speed (and thus its frequency) constant as the engine speed varies.

Constant-volume cycle of energy release

The cycle of energy transformation of a reciprocating engine. See Otto cycle.

Constantan

A copper-nickel alloy used as the negative lead of a thermocouple for measuring the cylinder head temperature of a reciprocating engine.

Contact

The term used between a person hand-propping an aircraft engine and the person in the flightdeck. When the person is ready to spin the propeller, he calls “contact”. The person in the flightdeck turns on the fuel, slightly opens the throttle, applies the brakes, and replies “contact”, and then turns the ignition switch to BOTH. The propeller is then pulled through to start the engine.

Contact

Electrical connectors in a switch, solenoid or relay that controls the flow of current.

Contactor (electrical component)

A remotely actuated, heavy-duty electrical switch. Contactors are used in an aircraft electrical system to connect the battery to the main bus.

Continuity tester

A troubleshooting tool that consists of a battery, a light bulb, and test leads. The test leads are connected to each end of the conductor under test, and if the bulb lights up, there is continuity. If it does not light up, the conductor is open.

Continuous Airworthiness Inspection Program

An inspection program that is part of a continuous airworthiness maintenance program approved for certain large airplanes (to which 14 CFR Part 125 is not applicable) turbojet multi-engine airplanes, turbopropeller-powered multi-engine airplanes, and turbine-powered rotorcraft.

Continuous magnetic particle inspection

A method of magnetic particle inspection in which the part is inspected by flowing a fluid containing particles of iron oxide over the part while the magnetizing current is flowing.

Continuous-duty solenoid

A solenoid-type switch designed to be kept energized by current flowing through its coil for an indefinite period of time. The battery contactor in an aircraft electrical system is a continuous-duty solenoid. Current flows through its coil all the time the battery is connected to the electrical system.

Continuous-flow oxygen system

A type of oxygen system that allows a metered amount of oxygen to continuously flow into the mask. A rebreather-type mask is used with a continuous-flow system. The simplest form of continuous- flow oxygen system regulates the flow by a calibrated orifice in the outlet to the mask, but most systems use either a manual or automatic regulator to vary the pressure across the orifice proportional to the altitude being flown.

Continuous-loop fire-detection system

A fire-detection system that uses a continuous loop of two conductors separated with a thermistor-type insulation. Under normal temperature conditions, the thermistor material is an insulator; but if it is exposed to a fire, the thermistor changes into a conductor and completes the circuit between the two conductors, initiating a fire warning.

Contrarotating

Rotating in opposite directions. Turbine rotors are contrarotating when the different stages have a common center, but turn in opposite directions.

Control horn

The arm on a control surface to which the control cable or push-pull rod attaches to move the surface.

Control panel

An upright panel, open or closed, where switches, rheostats, meters, etc., are installed for the control and protection of electrical machinery.

Control stick

The type of control device used in some airplanes. A vertical stick in the flight deck controls the ailerons by side-to-side movement and the elevators by fore- and-aft movement.

Control yoke

The movable column on which an airplane control wheel is mounted. The yoke may be moved in or out to actuate the elevators, and the control wheel may be rotated to actuate the ailerons.

Controllability

The characteristic of an aircraft that allows it to change its flight attitude in response to the pilot’s movement of the flight deck controls.

Controller Pilot Data Link Communication (CPDLC)

Provides direct data communication between the pilot and the air traffic controller through a data link.

Convection

The process by which heat is transferred by movement of a heated fluid (gas or liquid).

Conventional current

An imaginary flow of electricity that is said to flow from the positive terminal of a power source, through the external circuit to its negative terminal. The arrowheads in semiconductor symbols point in the direction of conventional current flow.

Convergent duct

A duct that has a decreasing cross section in the direction of flow.

Convergent-divergent duct

A duct that has a decreasing cross section in the direction of flow (convergent) until a minimum area is reached. After this point, the cross section increases (divergent). Convergent-divergent ducts are called CD ducts or con-di ducts.

Converging duct

A duct, or passage, whose cross-sectional area decreases in the direction of fluid flow.

Conversion coating

A chemical solution used to form an airtight oxide or phosphate film on the surface of aluminum or magnesium parts. The conversion coating prevents air from reaching the metal and keeps it from corroding.

Convex surface

A surface that is curved outward. The outer edges are lower than the center.

Core engine

The gas generator portion of a turboshaft, turboprop, or turbofan engine. The core engine consists of the portion of the compressor used to supply air for the engine operation, diffuser, combustors, and turbine(s) used to drive the compressor. The core engine provides the high-velocity gas to drive the fan and/or any free turbines that provide power for propellers, rotors, pumps, or generators.

Coriolis effect

The change in rotor blade velocity to compensate for a change in the distance between the center of mass of the rotor blade and the axis rotation of the blade as the blades flap in flight.

Cornice brake

A large shop tool used to make straight bends across a sheet of metal. Cornice brakes are often called leaf brakes.

Correlation

The process of determining that a new measurement belongs to an existing track.

Corrosion

The deterioration of metal by chemical or electrochemical attack.

Corrosion

Deterioration of a metal because of reaction with its environment. For further references see AC 43-4, Corrosion Control for Aircraft.

Corrosion

The electrochemical deterioration of a metal resulting from chemical reaction to the surrounding environment.

Corrosion, intergranular

Intergranular corrosion occurs in the grain boundaries of some alloys of aluminum. The presence of intergranular corrosion may be the result of fatigue, stress, or the continued presence of moisture such as under a decal, or in a fastener hole. Sometimes called exfoliation corrosion, when it occurs in forgings, extrusions or rolled sheets.

Corrosion, pitting

Visible cavities extending inward from the metal surface due to chemical or electro-chemical action.

Corrosion, surface

Surface loss of metal due to chemical or electro-chemical action. On polished surfaces begins as a dulling of surface, soon becoming frosted, followed by widespread pitting.

Corrugated metal

Sheets of metal that have been made more rigid by forming a series of parallel ridges or waves in its surface.

Cosine (cos)

A trigonometric function comparing two sides of a right triangle as follows: Cos = adjacent side hypotenuse Coulomb. A measure of electrical output. One coulomb is 6.24 × 1018 electrons.

Cotter pin

A split metal pin used to safety a castellated or slotted nut on a bolt. The pin is passed through the hole in the shank of the bolt and the slots in the nut, and the ends of the pin are spread to prevent it backing out of the hole.

Countersink

A tool that cuts a cone-shaped depression around a hole in order to allow a rivet or screw to set flush with the surface of the material.

Countersinking

Preparation of a rivet hole for a flush rivet by beveling the edges of the holes with a cutter of the correct angle.

Coverite surface thermometer

A small surface-type bimetallic thermometer that calibrates the temperature of an iron used to heat-shrink polyester fabrics.

Cowling

The removable cover that encloses an aircraft engine.

Crabbing

Pointing the nose of an aircraft into the wind to compensate for wind drift.

Crack

A physical opening or fissure within the body of a material. May be either internal within the material or at the surface (surface breaking). On a propeller, cracks can be started by cuts, nicks, or corrosion.

Crack

A partial separation of material caused by vibration, overloading, internal stresses, nicks, defective assemblies, fatigue, or rapid changes in temperature.

Crankcase

The housing that encloses the crankshaft, camshaft, and many of the accessory drive gears of a reciprocating engine. The cylinders are mounted on the crankcase, and the engine attaches to the airframe by the crankcase.

Crankshaft

The central component of a reciprocating engine. This high-strength alloy steel shaft has hardened and polished bearing surfaces that ride in bearings in the crankcase. Offset throws, formed on the crankshaft, have ground and polished surfaces on which the connecting rods ride. The connecting rods change the in-and-out motion of the pistons into rotation of the crankshaft.

Crazing

A form of stress-caused damage that occurs in a transparent thermoplastic material. Crazing appears as a series of tiny, hair-like cracks just below the surface of the plastic.

Creep

The deformation of a metal part that is continually exposed to high centrifugal loads and temperatures.

Creepage

The conducting of electrical current along a surface between two points at different potentials. The current’s ability to pass between two points increases with higher voltage and when deposits of moisture or other conductive materials exist on the surfaces.

Crewmember

A person assigned to perform duty in an aircraft during flight time.

Critical altitude

The maximum altitude at which, in standard atmosphere, it is possible to maintain, at a specified rotational speed, a specified power or a specified manifold pressure. Unless otherwise stated, the critical altitude is the maximum altitude at which it is possible to maintain, at the maximum continuous rotational speed, one of the following: (1) The maximum continuous power, in the case of engines for which this power rating is the same at sea level and at the rated altitude. (2) The maximum continuous rated manifold pressure, in the case of engines, the maximum continuous power of which is governed by a constant manifold pressure.

Critical altitude

The altitude above which a reciprocating engine will no longer produce its rated horsepower with its throttle wide open.

Critical engine

The engine whose failure would most adversely affect the performance or handling qualities of an aircraft.

Critical engine

The engine of a twin-engine airplane whose loss would cause the greatest yawing effect.

Critical Mach number

The flight Mach number at which there is the first indication of supersonic airflow over any part of the aircraft structure.

Critical Match number

The flight match number at which there is the first indication of air flowing over any part of the structure at a speed of Mach one, the local speed of sound.

Cross coat

A double coat of aircraft finishing material in which the second coat is sprayed at right angles to the first coat, before the solvents have evaporated from the first coat.

Cross coat

A double coat of dope or paint. It is sprayed on in one direction, and then immediately after the solvent flash-off, it is sprayed at right angles to the first coat.

Cross grain

Grain not parallel with the axis of a piece. It may be either diagonal or spiral grain or a combination of the two.

Cross-feed valve (fuel system component)

A valve in a fuel system that allows any of the engines of a multi-engine aircraft to draw fuel from any fuel tank. Cross-feed systems are used to allow a multi-engine aircraft to maintain a balanced fuel condition.

Cross-flow valve

An automatic flow-control valve installed between the gear-up and gear-down lines of the landing gear of some large airplanes. When the landing gear is released from its uplocks, its weight causes it to fall faster than the hydraulic system can supply fluid to the gear-down side of the actuation cylinder. The cross-flow valve opens and directs fluid from the gear-up side into the gear-down side. This allows the gear to move down with a smooth motion.

Cryogenic fluid

A liquid which boils at a temperature lower than about 110°K (-163°C) under normal atmospheric pressure.

Cryogenic liquid

Aliquid which boils at temperatures of less than about 110 °F (–163 °C) at normal atmospheric pressures.

Cuno filter

The registered trade name for a particular style of edge-type fluid filter. Cuno filters are made up of a stack of thin metal discs that are separated by thin scraper blades. Contaminants collect on the edge of the discs, and they are periodically scraped out and allowed to collect in the bottom of the filter case for future removal.

Cure

To change the properties of a thermosetting resin irreversibly by vulcanization or chemical reaction. May be accomplished by the addition of curing (cross-linking) agents, with or without a catalyst, and with or without heat or pressure.

Curing temperature

Temperature to which a resin or an assembly is subjected in order to cure the resin.

Current

The flow of electrical charge

Current

A general term used for electrical flow. See conventional current.

Current limiter

An electrical component used to limit the amount of current a generator can produce. Some current limiters are a type of slow-blow fuse in the generator output. Other current limiters reduce the generator output voltage if the generator tries to put out more than its rated current.

Curtiss Jenny (Curtiss JN4-D)

A World War I training airplane powered by a Curtiss OX-5 engine. It was widely available after the war and helped introduce aviation to the general public.

Cusp

A pointed end.

Customer bleed air

Air that is tapped off a turbine engine compressor and used for such airframe functions as the operation of air conditioning and pressurization systems.

Cut

A deep, long, narrow fissure usually caused by a sharp object.

Cutting torch

A device used in gas cutting of metals.

Cyclic pitch control

The helicopter control that allows the pilot to change the pitch of the rotor blades individually, at a specific point in their rotation. The cyclic pitch control allows the pilot to tilt the plane of rotation of the rotor disc to change the direction of lift produced by the rotor.

Cylinder

The component of a reciprocating engine which houses the piston, valves, and spark plugs and forms the combustion chamber.

Dacron

The registered trade name for a cloth woven from polyester fibers.

Dalton’s Law

States that a mixture of several gases which do not react chemically exerts a pressure equal to the sum of the pressures which the several gases would exert separately if each were allowed to occupy the entire space alone at the given temperature.

Damped oscillation

Oscillation whose amplitude decreases with time.

Damping

Limiting the duration of vibration by either electrical or mechanical means.

Data

Information that supports and/or describes the original aircraft design, alteration or repair including the following: (1) drawings, sketches, and or photographs; (2) engineering analysis; (3) engineering orders; and (4) operating limitations.

Data

The input for computer processing in the form of numerical information that represents characters or analog quantities.

Data Block

A block of information about selected traffic that is displayed somewhere around the edge of the CDTI display, rather than mixed in with the symbols representing traffic in the main part of the display.

Data bus

A wire or group of wires that are used to move data within a computer system.

Data Tag

A block of information that is displayed next to the traffic symbol in the main part of the CDTI display.

Database

A body of information that is available on any particular subject.

Dataplate performance

The performance specifications of a turbine engine observed and recorded by the engine manufacturer or overhauler and recorded on the engine dataplate. This data includes the engine speed at which a specified EPR is attained. When trimming the engine, the technician uses this data as the goal.

Dataplate specifications

Specification of each gas turbine engine determined in the manufacturer’s test cell when the engine was calibrated. This data includes the engine serial number with the EPR that produced a specific RPM. The technician refers to this information when trimming the engine.

Datum

Imaginary vertical plane from which all horizontal measurements are made or indicated when the aircraft is in level flight attitude.

Datum

An imaginary vertical plane or line from which all measurements of arm are taken. The datum is established by the manufacturer. Once the datum has been selected, all moment arms and the location of CG range are measured from this point.

Deaerator

A component in a turbine engine lubrication system that removes air from the scavenged oil before it is returned to the tank.

Debond/Disbond

An adhesive or cohesive failure or separation along a bonded interface between two or more surfaces.

Debonding

Separation of the bond between the skin laminates and the core of a composite structure.

Debooster valve

A valve in a power brake system between the power brake control valve and the wheel cylinder. This valve lowers the pressure of the fluid going to the brake and increases its volume. A debooster valve increases the smoothness of brake application and aids in rapid release of the brakes.

Decay

The breakdown of the structure of wood fibers. Wood that shows any indication of decay must be rejected for use in aircraft structure.

Deceleration

The amount the velocity of an object, measured in feet per second, is decreased by a force during each second it is acted upon by that force. Deceleration is usually expressed in terms of feet per second, per second (fps²).

Decibels

The unit for measuring sound intensity. One decibel is the smallest change in sound intensity the human ear can detect.

Deciduous

A type of tree that sheds its foliage at the end of the growing season. Hardwoods come from deciduous trees.

Decimal system

The number system, also called the base-ten system, based on the number 10. Consisting of ten symbols, or digits (0, 1, 2, 3, 4, 5, 6, 7, 8, 9), the main principle is that 10 is considered as a new unit from which point counting starts again.

Decomposition

The breakdown of the structure of wood fibers. Wood that shows any indication of decay must be rejected for use in aircraft structure.

Dedicated computer

A small digital computer, often built into an instrument or control device that contains a built-in program that causes it to perform a specific function.

Deep-vacuum pump

A vacuum pump capable of removing almost all of the air from a refrigeration system. A deep- vacuum pump can reduce the pressure inside the system to a few microns of pressure.

Deflator cap

A cap for a tire, strut, or accumulator air valve that, when screwed onto the valve, depresses the valve stem and allows the air to escape safely through a hole in the side of the cap.

Deflection

The ratio (in percent) of the deflected distance between the rim and tread to the undeflected distance.

Degradation

The alteration of material properties (e.g., strength, modulus, coefficient of expansion) which may result from deviations in manufacturing or from repeated loading and/or environmental exposure.

DeHaviland DH-4

An English designed observation airplane built in large quantities in the united States during World War I. After the war, surplus DH-4s were used for carrying the U.S. Mail.

Deicer system

A system that removes ice after it has formed on an aircraft.

Deicing

The removal of ice that has formed on a surface.

Delamination

Separation of the bond between the individual plies of a laminated composite structure.

Delamination

Separation between adjacent layers of a laminate.

Delamination

The separation of the layers of a laminated material.

Delivery air duct check valve

An isolation valve at the discharge side of the air turbine that prevents the loss of pressurization through a disengaged cabin air compressor.

Delta airplane

An airplane with a triangular-shaped wing. This wing has an extreme amount of sweepback on its leading edge, and a trailing edge that is almost perpendicular to the longitudinal axis of the airplane.

Delta connection (electrical connection)

A method of connecting three electrical coils into a ring or, as they are drawn on a schematic diagram as a triangle, a delta (D).

Delta P ∆P

Differential pressure.

Denier

A measure of the fineness of the yarns in a fabric.

Denominator

The lower part of a fraction (represented by the letter D in N⁄D), the quantity by which the numerator is divided.

Density

The weight of a substance per unit volume.

Density altitude

The altitude in standard air at which the density is the same as that of the existing air.

Density altitude

The altitude in standard air at which the density is the same as that of the existing air.

Density ratio (σ)

The ratio of the density of the air at a given altitude to the density of the air at sea level under standard conditions.

Dent

A depression in the blade as the result of impact from a blunt object.

Depression

Concave surface deformation.

Derated (electrical specification)

Reduction in the rated voltage or current of an electrical component. Derating is done to extend the life or reliability of the device.

Derating

A technique whereby a part is stressed in actual usage at values well below the manufacturer’s rating for the part. By decreasing mechanical, thermal, and electrical stresses, the probability of degradation or catastrophic failure is lessened.

Desiccant (air conditioning component)

A drying agent used in an air conditioning system to remove water from the refrigerant. A desiccant is made of silica-gel or some similar material.

Designated Application

An application that operates only on specifically-chosen (either by the flight crew or automation) traffic. They generally operate only for a specific flight operation.

Designated Traffic

Traffic upon which a designated application is to be conducted.

Desirable

The capability denoted as desirable is not required to perform the procedure, but would increase the utility of the operation.

Detail drawing

A description of a single part, given in such a manner as to describe bylines, notes, and symbols the specifications for size, shape, material, and methods of manufacture to be used in making the part.

Detailed inspection

A thorough examination of an item including disassembly. The overhaul of a component is considered to be a detailed inspection.

Detent

A spring-loaded pin or tab that enters a hole or groove when the device to which it is attached is in a certain position. Detents are used on a fuel valve to provide a positive means of identifying the fully on and fully off position of the valve.

Detergent oil

A type of mineral oil with metallic-ash- forming additives that protects the inside of an engine from sludge and varnish buildup. Used in automotive engines, it has proven unsuitable for use in aircraft engines.

Detonation

Uncontrolled burning of fuel in the cylinder of a reciprocating engine. Detonation causes explosive burning of the fuel which creates an increased cylinder pressure, excessive cylinder head temperature, and decreased engine performance.

Detonation

An explosion, or uncontrolled burning of the fuel-air mixture inside the cylinder of a reciprocating engine. Detonation occurs when the pressure and the temperature inside the cylinder become higher than the critical pressure and temperature of the fuel. Detonation is often confused with preignition.

Detonation

An uncontrolled explosion inside the cylinder of a reciprocating engine. Detonation occurs when the pressure and temperature of the fuel inside the cylinder exceeds the critical pressure and temperature of the fuel. Detonation may be caused by using fuel that has a lower octane rating or performance number than is specified for the engine.

Deviation error

An error in a magnetic compass caused by localized magnetic fields in the aircraft. Deviation error, which is different on each heading, is compensated by the technician “swinging” the compass. A compass must be compensated so the deviation error on any heading is no greater than 10 degrees.

Dew point

The temperature to which humid air must be cooled at constant pressure to become saturated.

Dewar bottle

A vessel designed to hold liquefied gases. It has double walls with the space between being evacuated to prevent the transfer of heat. The surfaces in the vacuum area are made heat-reflective.

Dewar bottle

A special container used to store liquid oxygen and liquid nitrogen. A Dewar bottle has an inner and an outer container, and the space between them forms a vacuum. The two surfaces within the vacuum are silvered to reflect heat away from the container walls.

Dial indicator

Measures variations in a surface by using an accurately machined probe mechanically linked to a circular hand whose movement indicates thousandths of an inch, or is displayed on a liquid crystal display (LCD) screen.

Diameter (circle)

The length of a line passing directly through the center of a circle. Twice the radius of the circle.

Diameter, propeller

The diameter of the circle circumscribed by the blade tips.

Die

Used for cutting external threads on round stock.

Difference

The answer to a subtraction problem.

Differential aileron travel

Aileron movement in which the upward-moving aileron deflects a greater distance than the one moving downward. The up aileron produces parasite drag to counteract the induced drag caused by the down aileron. Differential aileron travel is used to counteract adverse yaw.

Differential Ground Speed

Calculated by taking the difference between the magnitude of the ownship ground speed and the designated traffic ground speed. The assumption is that ownship is following the designated traffic approach path over the ground. Positive values indicate closure on the designated traffic.

Differential pressure

The difference between two pressures.

Differential pressure

A single pressure that is the difference between two opposing pressures.

Differential-voltage reverse-current cutout

A type of reverse-current cutout switch used with heavy-duty electrical systems. This switch connects the generator to the electrical bus when the generator voltage is a specific amount higher than the battery voltage.

Diffuser

A component in a gas turbine engine that decreases the velocity of air flowing through it and increases its pressure.

Digital multimeter

An electrical test instrument that can be used to measure voltage, current, and resistance. The indication is in the form of a liquid crystal display in discrete numbers.

Digitized image

A modified image picked up by the miniature TV camera in the end of a fiber-optic probe. This image is converted into a digital electronic signal that eliminates unwanted portions of the viewed area and allows the desired image to be enhanced for a clearer view of the inside of a turbine engine.

Dihedral

The positive angle formed between the lateral axis of an airplane and a line that passes through the center of the wing or horizontal stabilizer. Dihedral increases the lateral stability of an airplane.

Diluter-demand oxygen system

A popular type of oxygen system in which the oxygen is metered to the mask, where it is diluted with cabin air by an airflow-metering aneroid assembly which regulates the amount of air allowed to dilute the oxygen on the basis of cabin altitude. The mixture of oxygen and air flows only when the wearer of the mask inhales. The percentage of oxygen in the air delivered to the mask is regulated, on the basis of altitude, by the regulator. A diluter-demand regulator has an emergency position which allows 100 percent oxygen to flow to the mask, by-passing the regulating mechanism.

Dipole antenna

A half wavelength, center-fed radio antenna. The length of each of the two arms is approximately one fourth of the wavelength of the center frequency for which the antenna is designed.

Dipstick

A gage, in the form of a thin metal rod, used to measure the level of liquid in a reservoir. The dipstick is pushed into the reservoir until it contacts a built-in stop; then it is removed and visually inspected. The level of liquid in the reservoir is indicated by the amount of the dipstick wet by the liquid.

Direct Controller Pilot Communication (DCPC)

Direct communication established between the controller and the pilot without having to relay through another unit or going through a secondary means of delivery for that information. Currently, this is accomplished by conventional voice radio operations or CPDLC.

Direct current (DC)

Electricity that flows in one direction at all times.

Direct current electrode positive

The arrangement of direct current arc welding leads in which the work is the negative pole and the electrode is the positive pole of the welding arc.

Direct current wlectrode negative

The arrangement of direct current arc welding leads in which the work is the positive pole and the electrode is the negative pole of the welding arc.

Directional stability

Stability about the vertical axis of an aircraft, whereby an aircraft tends to return, on its own, to flight aligned with the relative wind when disturbed from the equilibrium state.

Dirigible

A large, cigar-shaped, rigid, lighter-than-air flying machine. Dirigibles are made of a rigid truss structure covered with fabric. Gas bags inside the structure contain the lifting gas, which is either helium or hydrogen.

Dirigible

A large, cigar shaped, lighter-than-air flying machine. Dirigibles differ from balloons in that they are powered and can be steered.

Disc area (helicopter specification)

The total area swept by the blades of a helicopter main rotor.

Discontinuity

An interruption in the normal physical structure or configuration of a part, such as a crack, lap, seam, inclusion, or porosity.

Discontinuity

An interruption in the normal physical structure or configuration of a part, such as a crack, forging lap, seam, inclusion, porosity, and the like. A discontinuity may or may not affect the usefulness of a part.

Display Range

The maximum distance from own-ship that is represented on the CDTI display. If the CDTI display is regarded as a map, then longer display ranges correspond to smaller map scales, and short display ranges correspond to larger map scales.

Dissimilar metal corrosion

Caused by contact between dissimilar metal parts in the presence of a conductor.

Distal tip

The tip, lens end, of a borescope.

Distributed pole stator winding

Alternator stator windings wound in a series of slots in the stator frame. A distributed pole stator is distinguished from a salient pole stator whose coils are wound around separate pole shoes that project inward from the field frame toward the rotor.

Distributor

A high-voltage selector switch that is gear- driven from the shaft of the rotating magnet in a magneto. The distributor rotor picks up the high voltage from the secondary winding of the coil and directs it to high-voltage terminals. From here, it is carried by high-tension ignition leads to the spark plugs.

Divergent duct

A duct that has an increased cross-sectional area in the direction of flow.

Divergent oscillation

Oscillation whose amplitude increases with time.

Diverging duct

A duct, or passage, whose cross-sectional area increases in the direction of fluid flow.

Dividend

In a division problem, the number to be divided by the divisor. In 6 ÷ 2 = 3, the dividend is 6.

Division

The process of finding how many times one number (the divisor) is contained in another number (the dividend).

Divisor

In a division problem, the number by which dividend is to be divided. In 6 ÷ 2 = 3, the divisor is 2.

Dope

Liquid applied to fabric to tauten it by shrinking, strengthen it, and render it airtight by acting as a filler.

Dope proofing

The treatment of a structure to be covered with fabric to keep the solvents in the dope from softening the protective coating on the structure.

Dope roping

A condition of aircraft dope brushed onto a surface in such a way that it forms a stringy, uneven surface rather than flowing out smoothly.

Dope-proofing

Protecting a surface from the chemicals and chafing qualities of dope and doped fabrics.

Doping

The process by which small amounts of additives called impurities are added to the semiconductor material to increase their current flow by adding a few electrons or a few holes.

Double-acting actuator (hydraulic system component)

A linear actuator moved in both directions by fluid power.

Double-acting hand pump (hydraulic system component)

A hand-operated fluid pump that moves fluid during both strokes of the pump handle.

Doubler

A piece of sheet metal used to strengthen and stiffen a repair in a sheet metal structure.

Downdraft carburetor

A carburetor that mounts on the top of a reciprocating engine. Air entering the engine flows downward through the carburetor.

Downtime

Any time during which an aircraft is out of commission and unable to be operated.

Downwash

Air forced down by aerodynamic action below and behind the wing of an airplane or the rotor of a helicopter. Aerodynamic lift is produced when the air is deflected downward. The upward force on the aircraft is the same as the downward force on the air.

Drag (helicopter rotor blade movement)

Fore-and-aft movement of the tip of a helicopter rotor blade in its plane of rotation.

Drag wire

A structural wire inside a Pratt truss airplane wing between the spars. Drag wires run from the front spar inboard, to the rear spar at the next bay outboard. Drag wires oppose the forces that try to drag the wing backward.

Dragging brakes

Brakes that do not fully release when the brake pedal is released. The brakes are partially applied all the time, which causes excessive lining wear and heat.

Drape

The ability of tape and broad goods to conform to a contoured shape.

Drill motor

An electric or pneumatic motor that drives a chuck that holds a twist drill. The best drill motors produce high torque, and their speed can be controlled.

Drip loop

A bundle installation method used to prevent water or other fluid contaminants from running down the wiring into a connector.

Drip stick

A fuel quantity indicator used to measure the fuel level in the tank when the aircraft is on the ground. The drip stick is pulled down from the bottom of the tank until fuel drips from its opened end. This indicates that the top of the gauge inside the tank is at the level of the fuel. Note the number of inches read on the outside of the gauge at the point it contacts the bottom of the tank, and use a drip stick table to convert this measurement into gallons of fuel in the tank.

Droop

A progressive decrease in RPOM with load in a gas turbine engine whose speed is governed with a fly-weight- type governor in the fuel control. As the load increases, the pilot valve drops down to meter more fuel. The lower position of the valve decreases the compression of the speeder spring and allows the flyweights to assume an on-speed position at a lower rpm.

Dry air pump

An engine-driven air pump which used carbon vanes. Dry pumps do not use any lubrication, and the vanes are extremely susceptible to damage from the solid airborne particles. These pumps must be operated with filters in their inlet so they will take in only filtered air.

Dry ice

Solidified carbon dioxide. Dry ice sublimates, or changes from a solid directly into a gas, at a temperature of –110 °F (–78.5 °C).

Dry rot

A term loosely applied to many types of wood decay but especially to that which, when in an advanced stage, permits the wood to be easily crushed to a dry powder. The term is actually a misnomer for any decay, since all fungi require considerable moisture for growth.

Dry rot

Decomposition of wood fibers caused by fungi. Dry rot destroys all strength in the wood.

Dry-sump engine

An engine that carries its lubricating oil supply in a tank external to the engine.

Dual ignition

An ignition system of an aircraft reciprocating engine that has two of every critical unit, including two spark plugs in each cylinder. Dual ignition provides safety in the event of one system malfunctioning, but more important, igniting the air/fuel mixture inside the cylinder at two locations provides more efficient combustion of the air/fuel mixture in the cylinder.

Dual-spool gas turbine engine

An axial-flow turbine engine that has two compressors, each driven by its own stage or stages of turbines.

Duct heater

A thrust augmentation system, similar to an afterburner, where fuel is added to the fan-discharge air and burned.

Duct losses

A decrease in pressure of the air flowing into a gas turbine engine caused by friction.

Ductility

The property of a material that allows it to be drawn into a thin section without breaking.

Dummy load (electrical load)

A noninductive, high-power, 50-ohm resistor that can be connected to a transmission line in place of the antenna. The transmitter can be operated into the dummy load without transmitting any signal.

Durability

A measure of engine life. Durability is usually measured in TBO hours.

Duralumin

The name for the original alloy of aluminum, magnesium, manganese, and copper. Duralumin is the same as the modern 2017 aluminum alloy.

Dutch roll

An undesirable, low-amplitude coupled oscillation about both the yaw and roll axes that affects many swept wing airplanes. Dutch roll is minimized by the use of a yaw damper.

Dutchman shears

A common name for compound-action sheet metal shears.

Duty cycle

A schedule that allows a device to operate for a given period of time, followed by a cooling down period before the device can be operated again.

Dwell chamber

A chamber in a turbine engine into which the scavenged oil is returned. Entrained air separates from the oil in the dwell chamber before it is picked up by the pressure pump.

Dwell time

The total time that a penetrant, emulsifier (or remover), or developer remains on the surface of the test part.

Dye penetrant inspection

An inspection method for surface cracks in which a penetrating dye is allowed to enter any cracks present and is pulled out of the crack by an absorbent developer. A crack appears as a line on the surface of the developer.

Dynamic pressure (q)

The pressure a moving fluid would have if it were stopped. Dynamic pressure is measured in pounds per square foot.

Dynamic stability

The property of an aircraft that causes it, when disturbed from straight-and level flight, to develop forces or moments that restore the original condition of straight and level

Dynamic stability

The stability that causes an aircraft to return to a condition of straight and level flight after it has been disturbed from this condition. When an aircraft is disturbed from the straight and level flight, its static stability starts it back in the correct direction; but it overshoots, and the corrective forces are applied in the opposite direction. The aircraft oscillates back and forth on both sides of the correct condition, with each oscillation smaller than the one before it. Dynamic stability is the decreasing of these restorative oscillations.

Dynamometer

A device used to measure the amount of torque being produced by an engine. The drive shaft of the engine is loaded with either an electric generator or a fluid pump, and the output of the generator or pump is measured and converted into units of torque. Torque at a specific rpm can be converted into brake horsepower.

Dynamometer Cycles

The test tire is required to withstand 50 takeoff cycles, and 8 taxi cycles as described below. The sequence of the cycles is optional.

Dyne

The unit of force that imparts an acceleration of one centimeter per second, per second to a mass of one gram. One dyne is equal to 2.248 ∙ 10-6 pounds.

E-gap angle

The position of the rotating magnet in a magneto when the breaker points are timed to open. The E-gap (efficiency gap) angle is several degrees of magnet rotation beyond the magnet’s neutral position. At this point, the magnetic field stress is the greatest, and the change in flux is the greatest, inducing the maximum voltage in the secondary winding.

Eccentric brushing

A special bushing used between the rear spar of certain cantilever airplane wings and the wing attachment fitting on the fuselage. The portion of the bushing that fits through the hole in the spar is slightly offset from that which passes through the holes in the fitting. By rotating the bushing, the rear spar may be moved up or down to adjust the root incidence of the wing.

Eddy current

Current induced into a conductor due to a mobbing or non-uniform magnetic field.

Eddy current damping (electrical instrument damping)

Decreasing the amplitude of oscillations by the interaction of magnetic fields. In the case of a vertical-card magnetic compass, flux from the oscillating permanent magnet produces eddy currents in a damping disc or cup. The magnetic flux produced by the eddy currents opposes the flux from the permanent magnet and decreases the oscillations.

Eddy current inspection

An inspection method where eddy currents are induced into the material to be tested. In aircraft manufacturing plants, eddy current is used to inspect castings, stampings, machine parts, forgings, and extrusions.

Edge distance

The distance between the center of a rivet hole and the edge of the sheet of metal.

Edge grain

Edge-grain lumber has been sawed parallel with the pith of the log and approximately at right angles to the growth rings; that is, the rings form an angle of 45 degrees or more with the surface of the piece.

Effective pitch

The actual distance a propeller advances in one revolution through the air.

Ejector

A form of jet pump used to pick up a liquid and move it to another location. Ejectors are used to ensure that the compartment in which the boost pumps are mounted is kept full of fuel. Part of the fuel from the boost pump flowing through the ejector produces a low pressure that pulls fuel from the main tank and forces it into the boostpump sump area.

Elastic limit

The maximum amount of tensile load, in pounds per square inch, a material is able to withstand without being permanently deformed.

Elastic limit

The maximum amount of tensile load, in pounds per square inch, that a material is able to withstand without permanent deformation.

Electrical energy

Electrical energy is converted to heat energy when an electric current flows through any form of resistance such as an electric iron, electric light, or an electric blanket.

Electrical potential

The electrical force caused by a deficiency of electrons in one location and an excess of electrons in another. Electrical potential is measured in volts.

Electrical steel

A low-carbon iron alloy that contains some silicon It is used as the core for transformers, field frames for generators and alternators, and the magnetic circuit of magnetos.

Electricity

One of the fundamental quantities in nature consisting of elementary particles, electrons and protons, which are manifested as a force of attraction or repulsion, and also in work that can be performed when electrons are caused to move; a material agency which, when in motion, exhibits magnetic, chemical, and thermal effects, and when at rest is accompanied by an interplay of forces between associated localities in which it is present.

Electromagnet

Temporary magnet which is magnetized by sending current through a coil of wire wound around an iron core.

Electromagnet

A magnet produced by an electrical current flowing through a coil of wire. The coil is normally wound around a core of soft iron which has an extremely low retentivity, allowing it to lose its magnetism as soon as the current stops flowing.

Electromagnetic radiation

A method of transmitting energy from one location to another. Current caused by high voltage in the secondary winding of a magneto produces electric and magnetic fields which oscillate back and forth at a high frequency and extend out into space in the form of waves. These waves of electromagnetic radiation are received as interference by the radio receivers in the aircraft.

Electromagnetic/Radio Frequency Interference (EMI/RFI)

Frequency spectrum of electromagnetic radiation extending from subsonic frequency to X-rays. This term should not be used in place of the term Radio Frequency Interference (RFI). (See radio frequency interference. ) Shielding materials for the entire EMI spectrum are not readily available.

Electromotive force

A force that causes electrons to move from one atom to another within an electrical circuit. An electromotive force, or EMF, is the difference in the electrical pressure, or potential, that exists between two points. An EMF may be produced by converting mechanical movement, pressure, chemical, light, or heat energy into electrical energy. The basic unit of EMF is the volt.

Electromotive force (EMF)

Difference of electrical potential measured in volts.

Electromotive force (EMF)

The pressure or force that causes electrons to flow in an electrical circuit.

Electromotive force (EMF)

The force that causes electrons to move from one atom to another within an electrical circuit. Electromotive force is an electrical pressure, and it is measured in volts.

Electron

A negative charge that revolves around the nucleus of an atom; a unit of a negative electrical charge.

Electron current

The actual flow of electrons in a circuit. Electrons flow from the negative terminal of a power source through the external circuit to its positive terminal. The arrowheads in semiconductor symbols point in the direction opposite to the flow of electron current.

Electron Volt (eV)

A unit of energy equal to the energy aquired by an electron falling though potential differences of one volt, approximately 1.602 x 10^19 joule.

Electronics

General term that describes the branch of electrical science and technology that treats the behavior and effects of electron emission and transmission.

Electrostatic field

A field of force that exists around a charged body.

Elevator downspring

A spring in the elevator control system that produces a mechanical force that tries to lower the elevator. In normal flight, this spring force is overcome by the aerodynamic force from the elevator trim tab. But in slow flight with an aft CG position, the trim tab loses its effectiveness and the downspring lowers the nose to prevent a stall.

Elevons

Movable control surfaces on the trailing edge of a delta wing or a flying wing airplane. These surfaces operate together to serve as elevators, and differentially to act as ailerons.

Empennage

The section of the airplane that consists of the vertical stabilizer, the horizontal stabilizer, and the associated control surfaces.

Empennage

The tail section of an airplane.

Empty weight

See standard empty weight.

Empty-weight center of gravity

The center of gravity of an aircraft when it contains only the items specified in the aircraft empty weight.

Empty-weight center of gravity range

The distance between the allowable forward and aft empty-weight CG limits.

Emulsion

A suspension of small globules of one material in another when the two materials will not mix. Oil and water will not mix, but they can be formed into an emulsion. An emulsion will separate into its components when it is allowed to sit.

Emulsion-type cleaner

A chemical cleaner which mixes with water or petroleum solvent to form an emulsion (a mixture which will separate if allowed to stand). It is used to loosen dirt, soot, or oxide films from the surface of an aircraft.

Enamel

A type of finishing material that flows out to form a smooth surface. Enamel is usually made of a pigment suspended in some form of resin. When the resin cures, it leaves a smooth, glossy protective surface.

Energizing brake

A brake that uses the momentum of the aircraft to increase its effectiveness by wedging the shoe against the brake drum. Energizing brakes are also called servo brakes. A single-servo brake is energizing only when moving in the forward direction, and a duo-servo brake is energizing when the aircraft is moving either forward or backward.

Energy

The capacity of a physical system to perform work. There are two types of energy, kinetic and potential.

Engine trimming

A maintenance procedure in which the fuel control on a gas turbine engine is adjusted to cause the engine to produce the required EGT or EPR at a specified rpm.

Enhanced Visual Acquisition (EVAcq)

This application is an enhancement for the outthe-window visual acquisition of aircraft traffic and potentially ground vehicles.

Entrained water

Water suspended in jet fuel. The amount of entrained water that can be held in the fuel is determined by the temperature of the fuel. When the fuel becomes cold, the water precipitates out and forms ice crystals on the fuel filter element.

Epicyclic reduction gears

A gear train in which a series of small planetary gears rotate around a central gear. More commonly called a planetary gear train.

Epoxy

One of various usually thermosetting resins capable of forming tight cross-linked polymer structures marked by toughness, strong adhesion, high corrosion, and chemical resistance, used especially in adhesives and surface coating.

Epoxy

A flexible, thermosetting resin that is made by polymerization of an epoxide. Epoxy has wide application as a matrix for composite materials and as an adhesive that bonds many different types of materials. It is noted for its durability and its chemical resistance.

Epoxy primer

A two-part catalyzed material used to provide a good bond between a surface and a surface coating.

Epoxy resin

A common thermosetting resin which exhibits exceptionally good adhesion, low cure shrinkage, and low water-absorption properties.

Equalizing resistor

A large resistor in the ground circuit of a heavy-duty aircraft generator through which all of the generator output current flows. The voltage drop across this resistor is used to produce the current in the paralleling circuit that forces the generators to share the electrical load equally.

Erosion

Loss of metal from metal surfaces by the action of small particles such as sand or water.

Erosion

Surface removal of material by mechanical action of grit, sand, water, etc., usually present on the leading edge close to the tip.

Estimation

The process of determining a track’s state based on new measurement information.

Ethanol

Alcohol made from cereal grains such as corn.

Ether

A volatile, highly flammable liquid that may be used to prime the cylinders of an aircraft engine when starting under extremely cold conditions.

Ethylene dibromide

A chemical compound added to aviation gasoline to convert some of the deposits left by the tetraethyl lead into lead bromides. These bromides are volatile and will pass out of the engine with the exhaust gases.

Ethylene dibromide

A colorless, poisonous liquid BrCH2CH2Br that is blended with leaded gasoline to help scavenge lead oxides.

Ethylene glycol

A form of alcohol used as a coolant for liquid-cooled engines and as an anti-icing agent.

Ethylene glycol

A form of alcohol used as a coolant for liquid-cooled aircraft engines. It is also used in automobile engines as a permanent antifreeze.

Eutectic

An alloy or solution that has the lowest possible constant melting point.

Eutectic material

An alloy or solution that has the lowest possible melting point.

Evacuation (air conditioning servicing procedure)

A procedure in servicing vapor-cycle cooling systems. A vacuum pump removes all the air from the system. Evacuation removes all traces of water vapor that could condense out, freeze, and block the system.

Evaporative cooling

See steam cooling.

Evaporator (air conditioning component)

The component in a vapor-cycle cooling system in which heat from the aircraft cabin is absorbed into the refrigerant. As the heat is absorbed, the refrigerant evaporates, or changes from a liquid into a vapor. The function of the evaporator is to lower the cabin air temperature.

Exceedance condition

A condition in which a parameter sensed by the EICAS exceeds the limits for which it is programmed.

Exciter

Small generator for supplying direct current to the alternator’s field windings.

Exfoliation corrosion

A form of intergranular corrosion that attacks extruded metals along their layer-like grain structure.

Exhaust cone

The fixed conical fairing centered in the turbine wheel. The exhaust cone straightens the flow and prevents the hot gases from circulating over the rear face of the turbine wheel.

Exhaust nozzle

The opening at the rear of the exhaust pipe.

Expandable sleeving

Open-weave braided sleeving used to protect wire and cables from abrasion and other hazards (commonly known by trade name EXPANDO).

Expander-tube brake

A brake that uses hydraulic fluid inside a synthetic rubber tube around the brake hub to force rectangular blocks of brake-lining material against the rotating brake drum. Friction between the brake drum and the lining material slows the aircraft.

Expansion wave

The change in pressure and velocity of a supersonic flow of air as it passes over a surface which drops away from the flow. As the surface drops away, the air tries to follow it. In changing its direction, the air speeds up to a higher supersonic velocity and its static pressure decreases. There is no change in the total energy as the air passes through an expansion wave, and so there is no sound as there is when air passes through a shock wave.

Expansion wave

The change in pressure and velocity of supersonic air as it passes over a surface that drops away from the flow. As the surface drops away, the air tries to follow it, and in changing its direction, the air speeds up to a higher supersonic speed, and its static pressure decreases. There is no change in the total amount of energy as air passes through an expansion wave.

Exploded view drawing

A pictorial drawing of two or more parts that fit together as an assembly. The view shows the individual parts and their relative position to the other parts before they are assembled.

Exponent (power)

A shorthand method of indicating how many times a number, called the base, is multiplied by itself. For example, in the number 43, the 3 is the power or exponent and 4 is the base. That is, 43 is equal to 4 × 4 × 4 = 64.

Extended Squitter

A long (112 bit) Mode S transmission that is spontaneously produced by the radio as opposed to a response to a Mode S Interrogation. Extended Squitter is the mechanism used to provide ADS-B messages from a Mode S transponder.

Extension lines

Used to extend the line showing the side or edge of a figure for the purpose of placing a dimension to that side or edge

External-combustion engine

A form of heat engine in which the fuel releases its energy outside of the engine. This released heat expands air which is used to perform useful work. Steam engines are a popular type of external combustion engine.

External-load attaching

The structural components used to attach an external load to an aircraft, including external-load containers, the backup structure at the attachment points, and any quick-release device used to jettison the external load.

Extrapolation

The process of predicting a track’s state forward in time based on the track’s last kinematic state.

Extreme pressure (EP) lubricant

A lubricant that reacts with iron to form iron chlorides, sulfides, or phosphides on the surface of a steel part. These compounds reduce wear and damage to surfaces in heavy rubbing contact. EP lubricants are specially suited for lubricating gear trains.

Extruded angle

A structural angle formed by passing metal heated to its plastic state through specially shaped dies.

FAA Form 337

The Major Repair and Alteration form that must be completed when an FAA-certificated aircraft or engine has been given a major repair or major alteration.

FAA Form 337

This form must be completed when a major repair or alteration is accomplished.

FAA Form 337

The FAA form that must be filled in and submitted to the FAA when a major repair or major alteration has been completed.

Face (propeller nomenclature)

The flat surface of a propeller that strikes the air as the propeller rotates. The face of a propeller corresponds to the bottom of an airplane wing.

Face surface

Flat, or very slightly curved, side of the propeller blade against which the relatively higher pressure acts to produce thrust when the propeller is rotated.

Fading of brakes

The decrease in the amount of braking action that occurs with some types of brakes that are applied for a long period of time. True fading occurs with overheated drum-type brakes. As the drum is heated, it expands in a bell-mouthed fashion. This decreases the amount of drum in contact with the brake shoes and decreases the braking action. A condition similar to brake fading occurs when there is an internal leak in the brake master cylinder. The brakes are applied, but as the pedal is held down, fluid leaks past the piston, and the brakes slowly release.

Fairing

A part of a structure whose primary purpose is to produce a smooth surface or a smooth junction where two surfaces join.

Fairlead

A plastic or wooden guide used to prevent a steel control cable rubbing against an aircraft structure.

Fan pressure ratio

The ratio of the fan-discharge pressure to the fan inlet pressure.

Feather (helicopter rotor blade movement)

Rotation of a helicopter rotor blade about its pitch-change axis.

Feathering propeller

A controllable-pitch propeller whose blades can be moved into a high pitch angle of approximately 90°. Feathering the propeller of an inoperative engine prevents it from wind-milling and greatly decreases drag.

Federal Aviation Administration Flight Standards District Office (FAA FSDO)

An FAA field office serving an assigned geographical area staffed with Flight Standards personnel who serve the aviation industry and the general public on matters relating to certification and operation of air carrier and general aviation aircraft.

Feeler gages

A type of measuring tool consisting of strips of precision-ground steel of accurately measured thickness. Feeler gages are used to measure the distance between close- fitting parts, such as the clearances of a mechanical system or the distance by which moving contacts are separated.

Ferrous metal

Iron, or any alloy containing iron.

Ferrous metal

Any metal that contains iron and has magnetic characteristics.

Ferrous metals

Metals having iron as their principal constituent.

Fiber optics

The technique of transmitting light or images through long, thin, flexible fibers of plastic or glass. Bundles of fibers are used to transmit complete images.

Fiber stop nut

A form of a self-locking nut that has a fiber insert crimped into a recess above the threads. The hole in the insert is slightly smaller than the minor diameter of the threads. When the nut is screwed down over the bolt threads, the opposition caused by the fiber insert produces a force that prevents vibration loosening the nut.

Fiberglass

The most common material used to reinforce structures in home-built and experimental aircraft. Available as mat, roving, fabric, etc. It is incorporated into both thermoset and thermoplastic resins. The glass fibers increase mechanical strength, impact resistance, stiffness, and dimensional stability of the matrix.

Field of View

The field of view of a CDTI is the geographical region within which the CDTI shows traffic. Some documents call this the field of regard.

File

A hand-held cutting tool used to remove a small amount of metal with each stroke.

Filiform corrosion

A thread, or filament-like corrosion which forms on aluminum skins beneath the finish.

Fill

Threads in a fabric that run crosswise of the woven material.

Fill threads

Threads in a piece of fabric that run across the width of the fabric, interweaving with the warp threads. Fill threads are often called woof, or weft, threads.

Fillet

A fairing used to give shape but not strength to an object. A fillet produces a smooth junction where two surfaces meet.

Finish

External coating or covering of an aircraft or part.

Finishing tape

Another name for surface tape. See surface tape.

Fire pull handle

The handle in an aircraft flight deck that is pulled at the first indication of an engine fire. Pulling this handle removes the generator from the electrical system, shuts off the fuel and hydraulic fluid to the engine, and closes the compressor bleed air valve. The fire extinguisher agent discharge switch is uncovered, but it is not automatically closed.

Fire resistant

(1) With respect to sheet or structural members the capacity to withstand the heat associated with fire at least as well as aluminum alloy in dimensions appropriate for the purpose for which they are used; and (2) With respect to fluid-carrying lines, fluid system parts, wiring, air ducts, fittings, and powerplant controls, means the capacity to perform the intended functions under the heat and other conditions likely to occur when there is a fire at the place concerned.

Fire sleeve

A covering of fire-resistant fabric used to protect flexible fluid lines that are routed through areas subject to high temperature.

Fire zone

A portion of an aircraft designated by the manufacturer to require fire-detection and/or fire-extinguishing equipment and a high degree of inherent fire resistance.

Fireproof

(1) With respect to materials and parts used to confine fire in a designated fire zone, the capacity to withstand at least as well as steel in dimensions appropriate for the purpose for which they are used, the heat produced when there is a severe fire of extended duration in that zone; and (2) With respect to other materials and parts, means the capacity to withstand the heat associated with fire at least as well as steel in dimensions appropriate for the purpose for which they are used.

Fishmouth splice

A type of splice used in a welded tubular structure in which the end of the tube whose inside diameter is the same as the outside diameter of the tube being spliced is cut in the shape of a V, or a fishmouth, and is slipped over the smaller tube welded. A fishmouth splice has more weld area than a butt splice and allows the stresses from one tube to transfer into the other tube gradually.

Fitting

An attachment device that is used to connect components to an aircraft structure.

Fixed fire-extinguishing system

A fire-extinguishing system installed in an aircraft.

Flame resistant

Not susceptible to combustion to the point of propagating a flame, beyond safe limits, after the ignition source is removed.

Flame tubes

Small-diameter metal tubes that connect can- type combustors in a turbine engine to carry the ignition flame to all of the combustion chambers. The British call combustion liners flame tubes.

Flameout

A condition of turbine engine operation when the fire unintentionally goes out. Improper air/fuel mixture or interruption of the air flow through the engine can cause a flameout.

Flameout

A condition in the operation of a gas turbine engine in which the fire in the engine unintentionally goes out.

Flammable

With respect to a fluid or gas, susceptible to igniting readily or to exploding.

Flap (aircraft control)

A secondary control on an airplane wing that changes its camber to increase both its lift and its drag.

Flap (helicopter rotor blade movement)

Up-and-down movement of the tip of a helicopter rotor blade.

Flap extended speed

The highest speed permissible with wing flaps in a prescribed extended position.

Flap overload valve

A valve in the flap system of an airplane that prevents the flaps being lowered at an airspeed which could cause structural damage. If the pilot tries to extend the flaps when the airspeed is too high, the opposition caused by the air flow will open the overload valve and return the fluid to the reservoir.

Flash point

The temperature to which a liquid must be raised for it to ignite, but not continue to burn when a flame is passed above it.

Flash point

The temperature to which a material must be raised for it to ignite, but not continue to burn, when a flame is passed above it.

Flash resistant

Not susceptible to burning violently when ignited.

Flashing the field

A maintenance procedure for a DC generator that restores residual magnetism to the field frame, A pulse of current from a battery is sent through the field coils in the direction in which current normally flows. The magnetic field produced by this current magnetizes the steel frame of the generator.

Flashover

An ignition system malfunction in which the high voltage in the magneto distributor jumps to the wrong terminal. Flashover causes the wrong spark plug to fire. This reduces the engine power and produces vibration and excessive heat.

Flat grain

Lumber has been sawed parallel with the pith of the log and approximately tangent to the growth rings; that is, the rings form an angle of less than 45 degrees with the surface of the piece.

Flat pattern layout

The pattern for a sheet metal part that has the material used for each flat surface, and for all of the bends, marked out with bend-tangent lines drawn between the flats and bend allowances.

Flat-rated engine

A turboprop engine whose allowable output power is less than the engine is physically capable of producing.

Flatspotting

A localized accelerated wear condition associated with landings; A temporary or permanent “out-of-round” condition resulting from a loaded hot tire having not rotated while cooling to the ambient temperature. This condition is most prevalent on bias nylon tires due to the “plastic memory” of the nylon cords.

Flight controller

The component in an autopilot system that allows the pilot to maneuver the aircraft manually when the autopilot is engaged.

Flight Crew

One or more cockpit crew members required for the operation of the aircraft.

Flight Information Service Broadcast (FIS-B)

FIS-B is a ground broadcast service provided over the UAT data link. The FAA FIS-B system provides pilots and flight crews of properly equipped aircraft with a cockpit display of certain aviation weather and aeronautical information.

Flight Information System - Broadcast (FIS-B)

A ground broadcast service provided over the UAT data link. The FAA FIS-B system provides pilots and flightcrews of properly equipped aircraft with a cockpit display of certain aviation weather and aeronautical information.

Flight Manual

A generic term used throughout this AC to represent the AFM, RFM, AFM supplement, or RFM supplement.

Flightcrew member

A pilot, flight engineer, or flight navigator assigned to duty in an aircraft during flight time.

Float carburetor

A fuel metering device that uses a float- actuated needle valve to maintain fuel level slightly below the edge of the discharge nozzle.

Flock

Pulverized wood or cotton fibers mixed with an adhesive. Flock, attached to a wire screen, acts as an effective induction air filter for small reciprocating engines.

Flow divider (reciprocating engine)

The valve in an RSA fuel injection system that divides the fuel from the fuel control unit and distributes it to all of the cylinders. It compares with the manifold valve in a Teledyne-Continental fuel injection system.

Flow divider (turbine engine)

A component in a turbine engine fuel system that routes all of the fuel to the primary nozzles or primary orifices when starting the engine or when the rpm is low. When the engine speed builds up, the flow divider shifts and opens a passage to send the majority of the fuel to the secondary nozzles or orifices.

Fluid

A form of material whose molecules are able to flow past one another without destroying the material. Gases and liquids are both fluids.

Fluid power

The transmission of force by the movement of a fluid. The most familiar examples of fluid power systems are hydraulic and pneumatic systems.

Fluorescent

A substance is said to be fluorescent when it will glow or fluoresce when excited by ultraviolet light. Some types of dye-penetrant material use fluorescent dyes which are pulled from the cracks by a developer and observed under “black” ultraviolet light.

Flutter

Rapid and uncontrolled oscillation of a flight control surface on an aircraft that is caused by a dynamically unbalanced condition.

Flux

Materials used to prevent, dissolve, or facilitate removal of oxides and other undesirable surface substances. Also, the name for magnetic fields.

Fly-by-wire

A method of control used by some modern aircraft in which control movement or pressures exerted by the pilot are directed into a digital computer where they are input into a program tailored to the flight characteristics of the aircraft. The computer output signal is sent to actuators at the control surfaces to move them the optimum amount for the desired maneuver.

Flying boat

An airplane whose fuselage is built in the form of a boat hull to allow it to land and takeoff from water. In the past, flying boats were a popular form of large airplane.

Flying wing

A type of heavier-than-air aircraft that has no fuselage or separate tail surfaces. The engines and useful load are carried inside the wing, and movable control surfaces on the trailing edge provide both pitch and roll control.

Foot-pound

A measure of work accomplished when a force of 1 pound moves an object a distance of 1 foot.

Force

Energy brought to bear on an object that tends to cause motion or to change motion.

Force

The intensity of an impetus, or the intensity of an input.

Forehand welding

Welding in which the torch is pointed in the direction the weld is progressing.

Foreign Object Damage (FOD)

Material that, if left on the runway, taxiway, or ramp, that could cause damage to tires, engines, or other aircraft structures; Damage resulting from contact with foreign objects.

Foreign object damage (FOD)

Any damage caused by any loose object to aircraft, personnel, or equipment. These loose objects can be anything from broken runway concrete to shop towels and safety wire.

Form drag

Parasite drag caused by the form of the object passing through the air.

Former

An aircraft structural member used to give a fuselage its shape.

Forward bias

A condition of operation of a semiconductor device such as a diode or transistor in which a positive voltage is connected to the P-type material and a negative voltage to the N-type material.

Forward wing

A forward lifting surface of a canard configuration or tandem-wing configuration airplane. The surface may be a fixed, movable, or variable geometry surface, with or without control surfaces.

Four-stroke cycle

A constant-volume cycle of energy transformation that has separate strokes for intake, compression, power, and exhaust.

Fraction

A number written in the form N⁄D in which N is the numerator and D is the denominator. For example, 5⁄16 is a fraction.

Fractional distillation

A method of separating the various components from a physical mixture of liquids. The material to be separated is put into a container and its temperature is increased. The components having the lowest boiling points boil off first and are condensed. Then, as the temperature is further raised, other components are removed. Kerosene, gasoline, and other petroleum products are obtained by fractional distillation of crude oil.

Fractional distillation

Procedure used for separating various components from a physical mixture of liquids. Crude oil is a mixture of many different types of hydrocarbon fuels which can be separated by carefully raising its temperature. The first products to be released, those having the lowest boiling points, are some of the gaseous fuels; next are gasoline, kerosene, diesel fuel, heavy fuel oils, lubricating oils, and finally, tar and asphalt.

Frangible

Breakable, or easily broken.

Frangible

Capable of being broken.

Free-turbine engine

A gas turbine engine with a turbine stage on a shaft independent of the shaft used to drive the compressor. Free turbines are used to drive the propeller reduction gear in a turboprop engine and the rotor transmission in a helicopter.

Freezing point

The temperature at which solids, such as wax crystals, separate from a hydrocarbon fuel as it is cooled.

Freon

The registered trade name for a refrigerant used in a vapor-cycle air conditioning system.

Frequency

The number of cycles (on/off) completed per unit of time. Usually expressed in Hertz.

Fretting corrosion

Corrosion damage between close-fitting parts which are allowed to rub together. The rubbing prevents the formation of protective oxide films and allows the metals to corrode.

Fretting corrosion

Occurs when two mating surfaces, normally at rest with respect to one another, are subject to slight relative motion.

Friction

The opposition to movement between objects.

Frise aileron

An aileron with its hinge line set back from the leading edge so that when it is deflected upward, part of the leading edge projects below the wing and produces parasite drag to help overcome adverse yaw.

Frost

Ice crystal deposits formed by sublimation when the temperature and dew point are below freezing.

Fuel grade

The rating system used for aviation gasoline. It rates fuel according to its antidetonation characteristics.

Fuel jettison system

A system installed in most large aircraft that allows the flight crew to jettison, or dump, fuel to lower the gross weight of the aircraft to its allowable landing weight. Boost pumps in the fuel tanks move the fuel from the tank into a fuel manifold. From the fuel manifold, it flows away from the aircraft through dump chutes is each wing tip. The fuel jettison system must be so designed and constructed that it is free from fire hazards.

Fuel totalizer

A fuel quantity indicator that gives the total amount of fuel remaining on board the aircraft on one instrument. The totalizer adds the quantities of fuel in all of the tanks.

Fuel-flow transmitter

A device in the fuel line between the engine-driven fuel pump and the carburetor that measures the rate of flow of the fuel. It converts this flow rate into an electrical signal and sends it to an indicator in the instrument panel.

Full-bodied

Not thinned.

Full-register position

The position of a magnet in a magneto when its poles are aligned with the pole shoes and the maximum amount of magnetic flux is flowing through the magnetic circuit.

Fully articulated rotor

A helicopter rotor whose blades are attached to the hub in such a way that they are free to flap, drag, and feather. See each of these terms.

Functional check

This test may require the use of appropriate test equipment.

Fungus (plural: fungi)

Any of several types of plant life that include yeasts, molds, and mildew.

Fuse

A protective device containing a special wire that melts when current exceeds the rated value for a definite period.

Fuse

A protective device containing a special wire that melts when current exceeds the rated value for a definite period.

Fuselage stations (FS)

Reference locations, usually given in inches, used to determine forward and aft positions on an aircraft. FS − 0 is the datum

Fusible plugs

Plugs in the wheels of high-performance airplanes that use tubeless tires. The centers of the plugs are filled with a metal that melts at a relatively low temperature. If a takeoff is aborted and the pilot uses the brakes excessively, the heat transferred into the wheel will melt the center of the fusible plugs and allow the air to escape from the tire before it builds up enough pressure to cause an explosion.

Galileo

A European satellite-based radio navigation system being developed that will provide a global positioning service.

Galling

Fretting or pulling out chunks of a surface by sliding contact with another surface or body.

Galvanic corrosion

Corrosion due to the presence of dissimilar metals in contact with each other.

Gas cylinder

A portable container used for transportation and storage of a compressed gas.

Gas generator

The basic gas turbine engine. It consists of the compressor, diffuser, combustor, and turbine. The gas generator is also called the core engine.

Gas Tungsten Arc Welding

(GTAW) an arc welding process which produces coalescence of metals by heating them with an arc between a tungsten (nonconsumable) electrode and the work. Shielding is obtained from a gas or gas mixture. Pressure may or may not be used and filler metal may or may not be used.

Gas turbine engine

An internal combustion engine that burns its fuel in a constant-pressure cycle and uses the expansion of the air to drive a turbine which, in turn, rotates a compressor. Energy beyond that needed to rotate the compressor is used to produce torque or thrust.

Gasket

A seal between two parts where there is no relative motion.

Gauge (rivet)

The distance between rows of rivets in a multirow seam. Gauge is also called transverse pitch.

Gauge pressure

Pressure referenced from existing atmospheric pressure.

Gauge pressure

Pressure referenced from the existing atmospheric pressure.

Gear-type pump

A constant-displacement fluid pump that contains two meshing large-tooth spur gears. Fluid is drawn into the pump as the teeth separate and is carried around the inside of the housing with teeth and is forced from the pump when the teeth come together.

General aviation

A term used to describe the total field of aviation operation except the military and airlines.

General Aviation Airworthiness Alerts

While these documents are no longer published, they are still available at www.faa.gov. These are used to alert technicians of problems that have been found in specific models of aircraft, and reported on Malfunction and Defect Reports. Airworthiness Alerts suggest corrective action, but compliance with the suggestion is not mandatory.

Generator

A mechanical device that transforms mechanical energy into electrical energy by rotating a coil inside a magnetic field. As the conductors in the coil cut across the lines of magnetic flux, a voltage is generated that causes current to flow.

Generator

A device for converting mechanical energy into electrical energy.

Generator series field

A set of heavy field windings in a generator connected in a series with the armature. The magnetic field produced by the series windings is used to change the characteristics of the generator.

Generator shunt field

A set of field windings in a generator connected in parallel with the armature. Varying the amount of current flowing in the shunt field windings controls the voltage output of the generator.

Geometric pitch

The distance a propeller would advance in one revolution if it were rotating in a solid.

Geopotential of the tropopause

The point in the standard atmosphere where the temperature stops dropping and becomes constant. This is the tropopause, or the dividing line between the troposphere and the stratosphere.

Gerotor pump

A form of constant-displacement pump that uses an external-tooth drive gear that meshes with and drives an internal-tooth gear that has one more space for a tooth than there are teeth on the drive gear. Both gears turn inside a close-tolerance housing. As the gears rotate, fluid flows between the teeth that are beginning to un-mesh, and is carried around the pump as the space continues to open up. On the discharge side of the pump, the teeth becomes smaller, fluid is forced out of the pump.

Gerotor pump

A form of constant-displacement gear pump. A gerotor pump uses an external-tooth spur gear that rides inside of and drives an internal-tooth rotor gear. There is one more tooth space inside the rotor than there are teeth on the drive gear. As the gears rotate, the volume of the space between two of the teeth on the inlet side of the pump increases, while the volume of the space between the two teeth on the opposite side of the pump decreases.

Gimbal

A support that allows a gyroscope to remain in an upright condition when its base is tilted.

Glass flight deck

An aircraft instrument system that uses a few color cathode-ray-tube displays to replace a large number of mechanically actuated instruments.

Glass flight deck

An aircraft instrument system that uses a few cathode-ray-tube displays to replace a large number of mechanically actuated instruments.

Glaze ice

Ice that forms when large drops of water strike a surface whose temperature is below freezing. Glaze ice is clear and heavy.

Glide slope

The portion of an ILS (Instrument Landing System) that provides the vertical path along which an aircraft descends on an instrument landing.

Glider

A heavier-than-air aircraft, that is supported in flight by the dynamic reaction of the air against its lifting surfaces and whose free flight does not depend principally on an engine.

Global Navigation Satellite System (GNSS)

The generic term for a satellite navigation system, such as GPS, that provides autonomous worldwide geo-spatial positioning and may include local or regional augmentations.

Global Positioning System (GPS)

A U.S. satellite-based radio navigation system that provides a global positioning service. The service provided by GPS for civil use is defined in the Global Positioning System Standard Positioning Service Performance Standard, 4th edition, dated September 2008, available at http://www.gps.gov/technical/ps/2008-SPS-performance- standard.pdf.

Global positioning system (GPS)

A navigation system that employs satellite transmitted signals to determine the aircraft’s location.

GNSS Time of Applicability

The time when the position output from the GNSS sensor is applicable.

GNSS Time of Measurement (TOM)

The time when the last GNSS signal used to determine the position arrives at the aircraft GNSS antenna.

Goniometer

Electronic circuitry in an ADF system that uses the output of a fixed loop antenna to sense the angle between a fixed reference, usually the nose of the aircraft, and the direction from which the radio signal is being received.

Gouge

A deep groove on a blade formed by a heavy pressure contact with a solid object.

Governor

A control used to automatically change the pitch of a constant speed propeller to maintain a constant engine rpm as air loads vary in flight.

Grain

The direction, size, arrangement, appearance, or quality of the fibers in wood or metal.

Grain- diagonal

Annual rings in wood at an angle with the axis of a piece as a result of sawing at an angle with the bark of the tree.

Gram

The basic unit of weight or mass in the metric system. One gram equals approximately 0.035 ounce.

Graphite

A form of carbon. Structural graphite is used in composite structure because of its strength and stiffness. Halon 1301 is bromotrifluoromethane.

Greige (pronounced “gray”)

The unshrunk condition of a polyester fabric as it is removed from the loom.

Grommet

An insulating washer that protects the sides of holes through which wires must pass/or a metal or plastic drain attached to fabric on aircraft.

Gross thrust

The thrust produced by a turbojet or turbofan engine when the engine is static or not moving. The air is considered to have no inlet velocity, and the velocity of the gas leaving the engine is considered to be the acceleration factor.

Gross weight

The total weight of the aircraft including its contents.

Ground

The voltage reference point in an aircraft electrical system. Ground has zero electrical potential. Voltage values, both positive and negative, are measured from ground. In the United Kingdom, ground is spoken of as “earth.” Ground-power unit (GPU). A service component used to supply electrical power to an aircraft when it is being operated on the ground.

Ground effect

The increased aerodynamic lift produced when an airplane or helicopter is flown nearer than half wing span or rotor span to the ground. This additional lift is caused by an effective increase in angle of attack without the accompanying increase in induced drag, which is caused by the deflection of the downwashed air.

Ground Speed

The magnitude of the horizontal velocity vector (see velocity). In these minimum operational performance standards (MOPS) it is always expressed relative to a frame of reference that is fixed with respect to the earth’s surface such as the WGS-84 ellipsoid.

Ground Track Angle

The direction of the horizontal velocity vector (see velocity) relative to the ground as noted in ground speed.

Ground-boosted engine

An aircraft reciprocating engine with a built-in supercharger that boosts the sea-level rated horsepower of the engine.

Grounding

The term is usually applied to a particular form of bonding that is the process of electrically connecting conductive objects to either conductive structure or some other conductive return path for the purpose of safely completing either a normal or fault circuit.

Gudgeon pin

The British name for a wrist pin, or piston pin. See wrist pin.

Guncotton

A highly explosive material made by treating cotton fibers with nitric and sulfuric acids. Guncotton is used in making the film base of nitrate dope.

Gusset

A small plate attached to two or more members of a truss structure. A gusset strengthens the truss.

Gyro (gyroscope)

The sensing device in an autopilot system. A gyroscope is a rapidly spinning wheel with its weight concentrated around its rim. Gyroscopes have two basic characteristics that make them useful in aircraft instruments: rigidity in space and precession. See rigidity in space and precession.

Gyrodyne

A rotorcraft whose rotors are normally engine-driven for takeoff, hovering, and landing, and for forward flight through part of its speed range, and whose means of propulsion, consisting usually of conventional propellers, is independent of the rotor system.

Gyroplane

A rotorcraft whose rotors are not engine-driven, except for initial starting, but are made to rotate by action of the air when the rotorcraft is moving; and whose means of propulsion, consisting usually of conventional propellers, is independent of the rotor system.

Gyroscopic precession

The characteristic of a gyroscope that causes it to react to an applied force as though the force were applied at a point 90° in the direction of rotation from the actual point of application. The rotor of a helicopter acts in much the same way as a gyroscope and is affected by gyroscopic precession.

Half-wave rectifier

An electrical rectifier circuit that converts AC into pulsating DC. Only one alternation of each cycle is present in the output.

Halogenated hydrocarbon

A hydrocarbon compound in which one or more hydrogen atoms have been replaced with atoms of one of the halogen elements such as fluorine, chlorine, or bromine.

Halon 1211

A halogenated hydrocarbon fire-extinguishing agent used in many HRD fire-extinguishing systems for powerplant protection. The technical name for Halon 1211 is bromochlorodifluoromethane.

Halon 1301

A halogenated hydrocarbon fire-extinguishing agent that is one of the best for extinguishing cabin and powerplant fires. It is highly effective and is the least toxic of the extinguishing agents available. The technical name for Hangar rash. Scrapes, bends, and dents in an aircraft structure caused by careless handling.

Hardwood

Wood from a broadleaf tree that sheds its leaves each year.

Harness

A cable harness is a group of cables or wires securely tied as a unit.

Hazard Classification

Refer to AC 25-1309-1(), System Design and Analysis, or AC 23.1309-1(), System Safety Analysis and Assessment for Part 23 Airplanes, as applicable.

Head of pressure

Pressure exerted by a column of fluid and created by the height of the column.

Heading indicator

A gyroscopic flight instrument that gives the pilot an indication of the heading of the aircraft.

Heat

The total kinetic energy of the molecules of any substance.

Heat engine

A mechanical device that converts the chemical energy in a fuel into heat energy. The heat energy is then converted into mechanical energy and useful work.

Heat exchanger

A device used to exchange heat from one medium to another. Radiators, condensers, and evaporators are all examples of heat exchangers. Heat always moves from the object or medium having the greatest level of heat energy to a medium or object having a lower level.

Heli-Coil insert

The registered trade name of a special helical insert used to restore threads stripped from a bolt hole, or to reinforce the threads in an aluminum casting. The damaged threads are drilled out and new threads are cut with a special oversize tap. A coil of stainless steel wire, with a cross section in the shape of a diamond, is screwed into the hole and serves as the new threads. Heli-Coil inserts are also used to provide durable threads in soft metal castings. Some spark plug holes in aluminum alloy cylinder heads are fitted with Heli-Coil inserts to minimize the wear caused by repeated removal and installation of the spark plugs.

Helical spline

A spline that twists, or winds, around the periphery of a shaft. Helical splines are used to change linear motion into rotary motion of the shaft on which the splines are cut.

Helical spring

A spring wound in the form of a helix, or coil.

Helicopter

A rotorcraft that, for its horizontal motion, depends principally on its engine-driven rotors.

Heliport

An area of land, water, or structure used or intended to be used for the landing and takeoff of helicopters.

Helix

A spiral.

Helix

A screw-like, or spiral, curve.

Henry

The basic unit of inductance, symbolized with the letter H. An electric circuit has an inductance of one henry when current changing at the rate of one ampere per second induces a voltage of one volt into the circuit.

Heptanes

An organic compound, CH3(CH2)5CH3, that is used as the low reference fuel for rating the antidetonation characteristics of aviation gasoline.

Hermaphrodite caliper

Generally used as a marking gauge in layout work. It should not be used for precision measurement.

Hermetically sealed

A complete seal, especially against the escape or entry of air.

Hertz

A unit of frequency equal to one cycle per second.

Hertz

One cycle per second.

Hidden lines

Indicates invisible edges or contours.

High Speed Tires

Tires operated in speed ranges of above 120 mph under TSO-C62c and above 160 mph under TSO-C62b.

High unmetered fuel pressure

Pressure in a Teledyne- Continental fuel injector pump that is adjusted by the variable orifice.

High-bypass ratio engine

A turbofan engine whose bypass ratio is 4:1 or greater.

High-pressure compressor

The second-stage compressor in a dual-spool gas turbine engine. The high pressure compressor is called the N2 compressor and is the one that is rotated by the starter for starting, and the one whose rpm is controlled by the fuel control.

Holding relay

An electrical relay that is closed by sending a pulse of current through the coil. It remains closed until the current flowing through its contacts is interrupted.

Home-built aircraft

See amateur-built aircraft.

Homebuilt aircraft

Aircraft that are built by individuals as a hobby rather than by factories as commercial products. Homebuilt, or amateur-built, aircraft are not required to meet the stringent requirements imposed on the manufacture of FAA-certified aircraft.

Honeycomb

Manufactured product consisting of a resin-impregnated sheet or metal material which has been corrugated or expanded into hexagon-shaped and other structural-shaped cells. Primarily used as core material for sandwich constructions.

Honing (cylinder wall treatment)

Scratching the surface of the cylinder wall with an abrasive to produce a series of grooves of microscopic depth and uniform pattern. The honed pattern holds oil to lubricate the cylinder walls.

Horizontal balance

The blade balance process that locates the center of blade mass along the radial direction to reduce unbalance of an assembled propeller.

Horizontal Figure of Merit (HFOM )

The radius of a circle in the horizontal plane, with its center being at the true position, that describes the region assured to contain the indicated horizontal position with at least 95 percent probability under fault-free conditions at the time of applicability.

Horizontal Protection Level Fault Detection (HPLFD)

The radius of a circle in the horizontal plane, with its center being at the true position, that describes the region assured to contain the indicated horizontal position. HPLFD is a horizontal region where the missed alert and false alert requirements are met for the chosen set of satellites when autonomous fault detection is used. It is a function of the satellite and user geometry and the expected error characteristics; it is not affected by actual measurements. Its value is predictable given reasonable assumptions regarding the expected error characteristics.

Horizontal Protection Level Fault Free (HPLFF)

Fault-free horizontal protection level. Refer to RTCA/DO-229D.

Horizontal Velocity

The component of velocity in a local horizontal plane. For Global Positioning System (GPS) sensors, that plane is tangent to the WGS-84 ellipsoid and is vertically displaced such that it contains the navigation sensors’ reference point. For inertial navigation system (INS) equipment, the local plane is perpendicular to the local gravity vector.

Horsepower

The most commonly used unit of mechanical power. One horsepower is equal to 33,000 foot-pounds of work done in one second.

Horsepower

A unit of mechanical power that is equal to 33,000 foot-pounds of work done in 1 minute, or 550 foot- pounds of work done in 1 second.

Horsepower

A measure of power equal to 550 foot-pounds per second or 33,000 foot-pounds per minute and 746 Watts.

Hot dimpling

A process used to dimple, or indent, the hole into which a flush rivet is to be installed. Hot dimpling is done by clamping the metal between heating elements and forcing the dies through the holes in the softened metal. Hot dimpling prevents hard metal from cracking when it is dimpled.

Hot section

The portion of a gas turbine engine that operates at a high temperature. The hot section includes the combustion, turbine, and exhaust sections.

Hot start

Occurs when the engine starts, but the exhaust gas temperature exceeds specified limits. This is usually caused by an excessively rich air-fuel mixture entering the combustion chamber.

Hot-tank lubricating system

A turbine engine lubricating system in which the oil cooler is located in the pressure subsystem. The oil is returned to the tank without being cooled.

Hot-wire cutter

A cutter used to shape blocks of Styrofoam. The wire is stretched tight between the arms of a frame and heated by electrical current. The hot wire melts its way through the foam.

Hub

Center section of the propeller that carries the blades and is attached to the engine shaft.

Hub (propeller component)

The high-strength component inside a propeller that attaches the blades to the engine propeller shaft.

Humidity

The amount of water vapor in the air.

Hung start

Occurs when the engine starts normally, but the rpm remains at some low value rather than increasing to the normal starting rpm. This is often the result of insufficient power to the starter, or the starter cutting off before the engine starts self-accelerating.

Hybrid compressor engine

A gas turbine engine that has both centrifugal and axial-flow compressors.

Hybrid spark plug

A fine-wire spark plug that has a platinum center electrode and iridium ground electrodes.

Hydraulic actuator

The component in a hydraulic system that converts hydraulic pressure into mechanical force. The two main types of hydraulic actuators are linear actuators (cylinders and pistons) and rotary actuators (hydraulic motors). I Hydraulic fuse. A type of flow control valve that allows a normal flow of fluid in the system but, if the flow rate is excessive, or if too much fluid flows for normal operation, the fuse will shut off all further flow.

Hydraulic lock

A condition in which oil drains into the lower cylinders of a reciprocating engine and leaks past the piston rings to fill the combustion chamber. If the oil is not removed before the engine is started, it can cause serious damage.

Hydraulic motor

A hydraulic actuator that converts fluid pressure into rotary motion. Hydraulic motors have an advantage in aircraft installations over electric motors, because they can operate in a stalled condition without the danger of a fire.

Hydraulic power pack

A small, self-contained hydraulic system that consists of a reservoir, pump, selector valves, and relief valves. The power pack is removable from the aircraft as a unit to facilitate maintenance and service.

Hydraulics

The system of fluid power which transmits force through an incompressible fluid.

Hydrocarbon

An organic compound that contains only carbon and hydrogen. The vast majority of fossil fuels, such as gasoline and turbine-engine fuel, are hydrocarbons.

Hydromechanical

Any device that combines fluid pressures with mechanical actions to achieve a desired result. In a hydromechanical fuel control used for a turbine engine, hydraulic servos are used in conjunction with the mechanical linkages.

Hydrometer

An instrument for determining the specific gravity of liquids.

Hydroplaning

A condition that exists when a high-speed airplane is landed on a water-covered runway. When the brakes are applied, the wheels lock up and the tires skid on the surface of the water in much the same way a water ski rides on the surface. Hydroplaning develops enough heat in a tire to ruin it.

Hydrostatic test

A pressure test used to determine the serviceability of high-pressure oxygen cylinders. The cylinders are filled with water and pressurized to 5⁄3 of their working pressure. Standard-weight cylinders (DOT 3AA) must by hydrostatically tested every five years, and lightweight cylinders (DOT 3HT) must be tested every three years.

Hyperbolic navigation

Electronic navigation systems that determine aircraft location by the time difference between reception of two signals. Signals from two stations at different locations will be received in the aircraft at different times. A line plotted between two stations along which the time difference is the same forms a hyperbola.

Hypersonic speed

Speed of greater than Mach 5 (5 times the speed of sound).

Hypotenuse

The side of a right triangle that is opposite the right angle. The hypotenuse is the longest side of a right triangle

Hypoxia

A physiological condition in which a person is deprived of the needed oxygen. The effects of hypoxia normally disappear as soon as the person is able to breathe air containing sufficient oxygen.

Ice bridging

A spark plug failure that occurs when starting a reciprocating engine in extremely cold weather. When a cylinder fires, the air/fuel mixture is converted into carbon dioxide and water vapor. The water vapor condenses on the spark plug electrodes and forms ice that bridges the electrode gap and prevents the plug firing until the ice is melted. This normally requires removing the spark plugs from the engine.

Icebox rivet

A solid rivet made of 2017 or 2024 aluminum alloy. These rivets are too hard to drive in the condition they are received from the factory, and must be heat-treated to soften them. They are heated in a furnace and then quenched in cold water. Immediately after quenching they are soft, but within a few hours at room temperature they become quite hard. The hardening can be delayed for several days by storing them in a subfreezing icebox and holding them at this low temperature until they are to be used.

Idle thrust

The jet thrust obtained with the engine power control level set at the stop for the least thrust position at which it can be placed.

Igniter

The component in a turbine-engine ignition system that provides a high-energy spark for igniting the air/fuel mixture in the combustion chamber for starting.

Impact damage

Damage (visible or not) resulting from a blade striking or being struck, while in flight or on the ground, by an object such as a snow bank, runway light, maintenance stand, birds, etc.

Improper fraction

A fraction with the numerator equal to or greater than the denominator.

Impulse coupling

A spring-loaded coupling between a magneto shaft and the drive gear inside the engine. When the engine is rotated for starting, the impulse coupling locks the magnet so it cannot turn. The spring in the coupling winds up as the crankshaft continues to turn, and when the piston is near top center, the coupling releases and spins the magnet, producing a hot and retarded spark.

In-flight shutdown (IFSD)

For ETOPS only, when an engine ceases to function (when the airplane is airborne) and is shutdown, whether self induced, flightcrew initiated or caused by an external influence. The FAA considers IFSD for all causes: for example, flameout, internal failure, flightcrew initiated shutdown, foreign object ingestion, icing, inability to obtain or control desired thrust or power, and cycling of the start control, however briefly, even if the engine operates normally for the remainder of the flight. This definition excludes the airborne cessation of the functioning of an engine when immediately followed by an automatic engine relight and when an engine does not achieve desired thrust or power but is not shutdown.

In-Trail Procedure (ITP)

A procedure that allows an aircraft to climb-through or descend-through another aircraft’s altitude in order to make a desired flight level change.

Incandescent

Glowing because of intense heat.

Inch-pound

A measure of work accomplished when a force of 1 pound moves an object a distance of 1 inch.

Inclusion

Material foreign to base metal but contained in it.

Inconel

The registered trade name for an alloy of chromium, iron, and nickel. Inconel is similar to stainless steel, but cannot be hardened by heat treatment.

Indicated airspeed

The speed of an aircraft as shown on its pitot static airspeed indicator calibrated to reflect standard atmosphere adiabatic compressible flow at sea level uncorrected for airspeed system errors.

Indicated airspeed (IAS)

The airspeed as shown on an airspeed indicator with no corrections applied.

Induced current

Electrical current produced in a conductor when it is moved through or crossed by a magnetic field.

Induced drag

Aerodynamic drag produced by an airfoil when it is producing lift. Induced drag is affected by the same factors that affect induced lift.

Inductance

The ability of a coil or conductor to oppose a change in current flow.

Inductance (L)

The ability of a coil or conductor to oppose a change in current flow.

Induction time

The time allowed an epoxy or polyurethane material between its initial mixing and its application. This time allows the materials to begin their cure.

Inductive reactance

An opposition to the flow of AC or changing DC caused by inductance in the circuit. Inductive reactance, whose symbol is XL, causes a voltage drop, but it does not use power nor produce heat.

Inductive reactance

The opposition to the flow of current which inductances put in a circuit.

Inductor

A coil of wire that produces inductance in an electrical circuit.

Inertia

The tendency of a body to resist acceleration. A body at rest will remain at rest or a body in motion will stay in motion in a straight line unless acted on by an outside force.

Inertia starter

A starter for a large reciprocating engine that uses energy stored in a rapidly spinning flywheel to turn the crankshaft.

Infrared radiation

Electromagnetic radiation whose wavelengths are longer than those of visible light.

Ingot

A large block of metal that was molded as it was poured from the furnace. Ingots are further processed into sheets, bars, tubes, or structural beams.

Inlet guide vanes

A set of stator vanes in front of the first stage of compression in a gas turbine engine. The inlet guide vanes deflect the air entering the compressor in the correct direction for optimum operation. Inlet guide vanes may be fixed, or their angle may be controlled hydraulically by fuel from the fuel control.

Inline engine

A reciprocating engine with all of the cylinders arranged in a straight line.

Innerliner

The integral rubber lining of a tubeless tire that is engineered to prevent the diffusion of the inflation gas into the casing.

Innertube

A gas-tight rubber device placed inside a tube-type tire casing for the purpose of containing the inflation gas. It is provided with an integral valve assembly.

Inside calipers

Calipers with outward curved legs for measuring inside diameters, such as diameters of holes.

Inspection Authorization (IA)

An authorization that may be issued to an experienced aviation maintenance technician who holds both an Airframe and Powerplant rating. It allows the holder to conduct annual inspections and to approve an aircraft or aircraft engine for return to service after a major repair or major alteration.

Installation drawing

A drawing that includes all necessary information for a part or an assembly in the final installed position in the aircraft.

Instrument

A device using an internal mechanism to show visually or aurally the attitude, altitude, or operation of an aircraft or aircraft part. It includes electronic devices for automatically controlling an aircraft in flight.

Insulator

A material that will not conduct current to an appreciable degree.

Insulator

A material that does not conduct electrical current very well or not at all. Examples are glass, ceramic, and plastic.

Integral fuel tank

An aircraft fuel tank made by sealing off part of the structure so fuel can be carried in the structure itself.

Integral fuel tank

A fuel tank which is formed by sealing off part of the aircraft structure and using it as a fuel tank. An integral wing tank is called a “wet wing.” Integral tanks are used because of their large weight saving. The only way of repairing an integral fuel tank is by replacing damaged sealant and making riveted repairs, as is done with any other part of the aircraft structure.

Integrated circuit

Small, complete circuit built up by vacuum deposition and other techniques, usually on a silicon chip, and mounted in a suitable package.

Intercooler

An air-to-air heat exchanger installed between a turbosupercharger and the carburetor. Intercoolers decrease the temperature of compressed air to prevent detonation.

Interference angle (poppet valve dimension)

The difference between the valve seat and the valve face angles. Normally, the valve seats are ground with between 0.5° and 1° greater angle than the valve face. This allows the face to touch the seat with a line contact that provides the best sealing.

Interference drag

Parasite drag caused by air flowing over one portion of the airframe interfering with the smooth flow of air over another portion.

Interference fit

A type of fit used when assembling certain mechanical devices. The hole is made smaller than the part that fits into it. The material containing the hole is heated to expand the hole, and the part that fits into the hole is chilled to shrink it. The parts are assembled, and when they reach the same temperature their fit is so tight they will not loosen in service.

Intergranular corrosion

The formation of corrosion along the grain boundaries within a metal alloy.

Intergranular corrosion

An attack along the grain boundaries of an alloy that commonly results from a lack of uniformity in the alloy structure.

Interlocked-grained wood

Wood in which the fibers are inclined in one direction in a number of rings of annual growth, then gradually reverse and are inclined in an opposite direction in succeeding growth rings, then reverse again.

Intermittent-duty solenoid

A solenoid-type switch whose coil is designed for current to flow through it for only a short period of time. The coil will overheat if current flows through it too long.

Internal timing

The adjustment of the breaker points of a magneto so they will begin to open at the time the magnet is in its E-gap position.

Internal-combustion engine

A form of heat engine in which the fuel and air mixture is burned inside the engine to heat and expand the air so it can perform useful work.

International Civil Aviation Organization (ICAO)

A United Nations organization that is responsible for developing international standards, and recommending practices, and procedures covering a variety of technical fields of aviation.

Interpole

A field pole in a compound-wound DC generator used to minimize armature reaction. Interpoles are located between each of the regular field poles, and their coils are in series with the armature winding so all of the armature current flows through them. The magnetic field produced by the interpole coils cancels the distortion caused by the armature field and allows the brushed to remain in the neutral plane where there is no potential difference between the commutator segments. Keeping the brushes in the neutral plane minimizes sparking.

Inverted engine

An inline or V-engine in which the cylinders are mounted below the crankshaft.

Inverter

A device for converting direct current to alternating current.

Ion

An atom or group of atoms in which the number of electrons is different from the number of protons. It is a positive ion if the number of electrons is less than the number of protons, and a negative ion if the number of electrons is greater than the number of protons.

Iridium

A very hard, brittle, highly corrosion-resistant, whitish-yellow, metallic chemical element. Iridium is used for the fine-wire electrodes in spark plugs that must operate in engines using fuel with an exceptionally high lead content.

Iso-octane

An organic compound used as the high reference fuel for rating the antidetonation characteristics of aviation gasoline (CH3)2CHCH2C(CH3)3.

Iso-octane

A hydrocarbon, C8H18, which has very high critical pressure and temperature. Iso-octane is used as the high reference for measuring the antidetonation characteristics of a fuel.

Isobaric mode

The mode of pressurization in which the cabin pressure is maintained at a constant value regardless of the outside air pressure.

Isogonic line

A line drawn on an aeronautical chart along which the angular difference between the magnetic and geographic north poles is the same.

Isometric drawings

A drawing that uses a combination of the views of an orthographic projection and tilts the object forward so that portions of all three views can be seen in one view

Isopropyl alcohol

A colorless liquid used in the manufacture of acetone and its derivatives and as a solvent and anti-icing agent.

Isothermal change

A physical change that takes place within a material in which heat energy is added to or taken from the material as needed to keep its temperature constant.

Jackscrew

A hardened steel rod with strong threads cut into it. A jackscrew is rotated by hand or with a motor to apply a force or to lift an object.

Jet fuel

Fuel designed and produced to be used in aircraft gas turbine engines.

Jet propulsion

A method of propulsion by accelerating a relatively small mass of air through a large change in velocity.

Jet pump

A special venturi in a line carrying air from certain areas in an aircraft that need an augmented flow of air through them. High-velocity compressor bleed air is blown into the throat of a venturi where it produces a low pressure that pulls air from the area to which it is connected. Jet pumps are often used in the lines that pull air through galleys and toilet areas.

Jeweler’s file

A small, fine-cut, metalworking file used by jewelry manufacturers.

Joggle

A small offset near the edge of a piece of sheet metal. It allows one sheet of metal to overlap another sheet while maintaining a flush surface.

Jointer

A woodworking power tool used to smooth edges of a piece of wood.

Joule

A measure of energy. In terms of electrical energy, one joule is equal to one watt-second.

Joule

The amount of work done by a force of one newton when it acts through a distance of one meter

Journal (bearing)

A hardened and polished surface on a rotating shaft that rides in a plain bearing.

K-factor

A factor used in sheet metal work to determine the setback for other than a 90° bend. Setback = K ∙ (bend radius + metal thickness). For bends of less than 90°, the value of K is less than 1; for bends greater than 90°, the value of K is greater than 1.

Kerosene

A light, almost colorless, hydrocarbon liquid obtained from crude oil through the fractional distillation process. Kerosene is the base for turbine engine fuel.

Kevlar

The registered trade name by DuPont for a patented aramid fiber.

Kevlar

A patented synthetic aramid fiber noted for its flexibility and light weight. It is to a great extent replacing fiberglass as a reinforcing fabric for composite construction.

Key (verb)

To initiate an action by depressing a key or a button.

kHz (kilohertz)

1,000 cycles per second.

Kick-in pressure

The pressure at which an unloading valve causes a hydraulic pump to direct its fluid into the system manifold.

Kick-out pressure

The pressure at which an unloading valve shuts off the flow of fluid into the system pressure manifold and directs it back to the reservoir under a much reduced pressure.

Kilogram

One thousand grams.

Kinematic viscosity

The ratio of the absolute viscosity of a fluid to its density. Kinematic viscosity is measured in centistokes.

Kinetic energy

Energy that exists because of motion.

Kinetic energy

Energy due to motion, defined as one half mass times velocity squared.

Kirchhoff’s Law (voltage)

A basic law of electrical currents stating that the algebraic sum of the applied voltage and the voltage drop around any closed circuit is zero

Kite

A framework, covered with paper, cloth, metal, or other material, intended to be flown at the end of a rope or cable, and having as its only support the force of the wind moving past its surfaces.

Knot (measure of speed)

A speed measurement that is equal to one nautical mile per hour. One knot is equal to 1.15 statute mile per hour.

Knot (wood defect)

A hard, usually round section of a tree branch embedded in a board. The grain of the knot is perpendicular to the grain of the board. Knots decrease the strength of the board and should be avoided where strength is needed.

Kollsman window

The barometric scale window of a sensitive altimeter. See barometric scale.

Koroseal lacing

A plastic lacing material available in round or rectangular cross sections and used for holding wire bundles and tubing together. It holds tension on knots indefinitely and is impervious to petroleum products.

Kraft paper

A tough brown wrapping paper, like that used for paper bags.

L/D ratio

A measure of efficiency of an airfoil. It is the ratio of the lift to the total drag at a specified angle of attack.

Labyrinth seal

A type of air and/or seal used around the main –shaft bearings in a gas turbine engine. The seal consists of a series of rotating blades that almost contact the seal land. A small amount of air flows between the seal and the land to prevent oil flowing past the seal.

Laminar flow

Airflow in which the air passes over the surface in smooth layers with a minimum of turbulence.

Laminate

A product obtained by bonding two or more laminae of the same material or of different materials.

Laminated wood

A piece of wood built up of plies or laminations that have been joined either with glue or with mechanical fastenings. The term is most frequently applied where the plies are too thick to be classified as veneer and when the grain of all plies is parallel.

Land (piston)

The portion of a piston between the ring grooves.

Land (splined shaft)

The portion of a splined shaft between the grooves.

Landing gear extended speed

The maximum speed at which an aircraft can be safely flown with the landing gear extended.

Landing gear operating speed

The maximum speed at which the landing gear can be safely extended or retracted.

Landing gear warning system

A system of lights used to indicate the condition of the landing gear. A red light illuminates when any of the gears are in an unsafe condition; a green light shows when all of the gears are down and locked, and no light is lit when the gears are all up and locked. An aural warning system is installed that sounds a horn if any of the landing gears are not down and locked when the throttles are retarded for landing.

Large aircraft

Aircraft of more than 12,500 pounds, maximum certificated takeoff weight.

Laser tachometer

A highly accurate tachometer that shines a laser beam on a rotating element that has reflective tape or a contrasting mark. The reflected laser beam is converted into electrical pulses which are counted and displayed on a monitoring instrument.

Last-chance oil filter

A small filter installed in the oil line to the bearing jet in a gas turbine engine. This filter traps any contaminants that have passed the main filter and holds them until the engine is disassembled for overhaul.

Latency

The time incurred between two particular interfaces. Total latency is the delay between the time of a measurement and the time that the measurement is reported at a particular interface (the latter minus the former). Components of the total latency are elements of the total latency allocated between different interfaces. Each latency component will be specified by naming the interfaces between which it applies.

Latent heat

Heat that is added to a material that causes a change in its state without changing its temperature.

Lateral axis

An imaginary line, passing through the center of gravity of an airplane, and extending across it from wing tip to wing tip.

Lateral stability

The stability about the longitudinal axis of an aircraft; the rolling stability, or the ability of an airplane to return to level flight due to a disturbance that causes one of the wings to drop.

Lay-up

The placement of the various layers of resin- impregnated fabric in the mold for a piece of laminated composite material.

Leading edge

The edge of the blade that leads the direction of rotation and first encounters the air.

Leading edge

The thick edge at the front of a propeller blade.

Leakage field

The magnetic field forced out into the air by the distortion of the field within a part, caused by the presence of a discontinuity or change in section configuration.

Lean die-out

A condition in which the fire in a gas turbine engine goes out because the air/fuel mixture ratio is too lean to sustain combustion.

Lean mixture

A air/fuel mixture that contains more than 15 parts of air to 1 part of fuel, by weight.

Lever

The simplest machine. There are three basic parts in all levers: the fulcrum “F,” a force or effort “E,” and a resistance “R.” Load cell. A component in an electronic weighing system that is placed between the jack and the jack pad on the aircraft. The load cell contains strain gauges whose resistance changes with the weight on the cell.

Light-sport aircraft

An aircraft, other than a helicopter or powered-lift that, since its original certification, has continued to meet the following: (1) A maximum takeoff weight of not more than— (i) 1,320 pounds (600 kilograms) for aircraft not intended for operation on water; or (ii) 1,430 pounds (650 kilograms) for an aircraft intended for operation on water. (2) A maximum airspeed in level flight with maximum continuous power (VH) of not more than 120 knots CAS under standard atmospheric conditions at sea level. (3) A maximum never-exceed speed (VNE) of not more than 120 knots CAS for a glider. (4) A maximum stalling speed or minimum steady flight speed without the use of lift-enhancing devices (VS1) of not more than 45 knots CAS at the aircraft’s maximum certificated takeoff weight and most critical center of gravity. (5) A maximum seating capacity of no more than two persons, including the pilot. (6) A single, reciprocating engine, if powered. (7) A fixed or ground-adjustable propeller if a powered aircraft other than a powered glider. (8) A fixed or feathering propeller system if a powered glider. (9) A fixed-pitch, semi-rigid, teetering, two-blade rotor system, if a gyroplane. (10) A nonpressurized cabin, if equipped with a cabin. (11) Fixed landing gear, except for an aircraft intended for operation on water or a glider. (12) Fixed or retractable landing gear, or a hull, for an aircraft intended for operation on water. (13) Fixed or retractable landing gear for a glider.

Lightening hole

A hole cut in a piece of structural material to get rid of weight without losing any strength. A hole several inches in diameter may be cut in a piece of metal at a point where the metal is not needed for strength, and the edges of the hole are flanged to give it rigidity. A piece of metal with properly flanged lightening holes is more rigid than the metal before the holes were cut.

Lighter-than-air aircraft

Aircraft that can rise and remain suspended by using contained gas weighing less than the air that is displaced by the gas.

Lightning strikes

Damage to blades caused by lightning usually manifested by localized burning, discoloration, melted metal, and/or pits.

Line boring

A method of assuring concentricity of bored holes. A boring bar extends through all of the holes and cuts the inside diameters so they all have the same center.

Linear actuator

A fluid power actuator that uses a piston moving inside a cylinder to change pressure into linear, or Lacquer. A finishing material made of a film base, solvents, straight-line, motion. plasticizers, and thinners. The film base forms a tough film over the surface when it dries. The solvents dissolve the film Linear change. A change in which the output is directly base so it can be applied as a liquid. The plasticizers give proportional to the input, the film base the needed resilience, and the thinners dilute the lacquer so it can be applied with a spray gun. Lacquer is sprayed on the surface as a liquid, and when the solvents and thinners evaporate, the film base remains as a tough decorative and protective coating.

Link rod

The rod in a radial engine that connects one of the piston wrist pins to a knuckle pin on the master rod. Also called articulating rods.

Linter

The short fiber left on the cotton seed after ginning.

Liquid cooling

The removal of unwanted heat from an aircraft engine by transferring the heat into a liquid and then passing the heated liquid through a liquid-to-air heat exchanger (radiator) to transfer the heat into the ambient air.

Load factor

The ratio of a specified load to the total weight of the aircraft. The specified load is expressed in terms of any of the following: aerodynamic forces, inertia forces, or ground or water reactions.

Loaded Radius

The distance from the axle centerline to a flat surface for a tire initially inflated to the rated inflation pressure and then loaded to its rated load against the flat surface.

Loadmeter

A current meter used in some aircraft electrical systems to show the amount of current the generator or alternator is producing. Loadmeters are calibrated in percent of the generator rated output.

Localizer

That section of an ILS that produces the directional reference beam.

Localizer

The portion of an ILS (Instrument Landing System) that directs the pilot along the center line of the instrument runway.

Lodestone

A magnetized piece of natural iron oxide.

Logic flow chart

A type of graphic chart that can be made up for a specific process or procedure to help follow the process through all of its logical steps.

Long-range communication system (LRCS)

A system that uses satellite relay, data link, high frequency, or another approved communication system which extends beyond line of sight.

Long-range navigation system (LRNS)

An electronic navigation unit that is approved for use under instrument flight rules as a primary means of navigation, and has at least one source of navigational input, such as inertial navigation system or global positioning system.

Longitudinal axis

An imaginary line, passing through the center of gravity of an airplane, and extending lengthwise Laminated wood. A type of wood made by gluing several through it from nose to tail.

Longitudinal magnetism

A method of magnetizing through a solenoid, or coil, that encircles the part so the lines of magnetic flux pass lengthwise through the part. Longitudinal magnetism makes it possible to detect faults that extend across the part.

Longitudinal magnetization

The magnetic field is produced in a direction parallel to the long axis of the part. This is accomplished by placing the part in a solenoid excited by electric current.

Longitudinal stability

Stability of an aircraft along its longitudinal axis and about its lateral axis. Longitudinal stability is also called pitch stability.

Longitudinal stability

The tendency for an aircraft nose to pitch up or pitch down, rotating around the lateral axis (wingtip to wingtip)

Low bypass ratio engine

A turbofan engine whose bypass ratio is less than 2:1.

Low unmetered fuel pressure

Pressure in a Teledyne- Continental fuel injector pump that is adjusted by the relief valve.

Low-pressure compressor

The first-stage compressor in a dual-spool gas turbine engine. The low-pressure compressor is called the N1 compressor and its speed is not governed. It seeks its own best speed as the atmospheric conditions change so it can furnish a relatively constant mass of air to the inlet of the second-stage compressor.

Lubber line

A reference on a magnetic compass and directional gyro that represents the nose of the aircraft. The heading of the aircraft is shown on the compass card opposite Left-right indicator. The course-deviation indicator used the lubber line with a VOR navigation system.

M&D (Malfunction and Defect) report

A small postcard- like form (FAA Form 8330) used by repair stations, maintenance shops, and technicians to report an unacceptable condition to the FAA. Information on these forms provides the basis for the General Airworthiness Alerts and subsequent Airworthiness Directives.

Mach number

The ratio of true airspeed to the speed of sound.

Mach number

The ratio of the speed of an object through the air to the speed of sound under the same atmospheric conditions. An object traveling at the speed of sound is traveling at Mach one (M1.0).

Mach number

A measurement of speed based on the ratio of the speed of the aircraft to the speed of sound under the same atmospheric conditions. An airplane flying at Mach 1 is flying at the speed of sound.

Magnesyn system

The registered trade name of a remote indicating instrument system. A Magnesyn system uses a permanent magnet as its rotor and a toroidal coil excited by 400-hertz AC as its stator. A small magnet in the center of the indicator coil follows the movement of a larger magnet in the transmitter coil.

Magnetic bearing

The direction to or from a radio transmitting station measured relative to magnetic north.

Magnetic field

The space around a source of magnetic flux in which the effects of magnetism can be determined.

Magnetic field

The invisible, but measurable, force surrounding a permanent magnet or current-carrying conductor. This field is produced when the orbital axes of the electrons of the atoms in the material are all in alignment.

Magnetic flux

Lines of magnetic force that are assumed to leave a magnet at its north end and return to its south end. Lines of flux tend to be as short as possible and cannot cross each other.

Magnetic particle inspection

A method of non-destructive inspection for ferrous metal components. The part being inspected is magnetized and then flooded with a solution of iron oxide suspended in a light oil, much like kerosene. Any flaw, either on the surface or just below the surface, forms a north and south pole, and the iron oxide attracted to these poles helps locate the flaw. The iron oxide is normally treated with a fluorescent dye, and the inspection is conducted in a darkened booth. When an ultraviolet light (black light) is shone on the part, the treated iron oxide shows up as a brilliant line.

Magnetic particle inspection

A method of detecting invisible cracks and other defects in ferromagnetic materials such as iron and steel. The inspection process consists of magnetizing the part and then applying ferromagnetic particles to the surface area to be inspected.

Main rotor

The rotor that supplies the principal lift to a rotorcraft.

Maintenance

Inspection, overhaul, repair, preservation, and the replacement of parts, but excludes preventive maintenance. (1) That might appreciably affect weight, balance, structural strength, performance, powerplant operation, flight characteristics, or other qualities affecting airworthiness; or (2) That is not done according to accepted practices or cannot be done by elementary operations.

Maintenance

This includes inspection, overhaul, repair, preservation, and the replacement of parts, but excludes preventive maintenance.

Major alteration

An alteration not listed in the aircraft, aircraft engine, or propeller specifications that might appreciably affect weight, balance, structural strength, powerplant operation, flight characteristics, or other qualities affecting airworthiness; an alteration not done according to accepted practices, or one that cannot be done by elementary operations.

Major alteration

An alteration not listed in the aircraft, aircraft engine, or propeller specifications. It is one that might appreciably affect weight, balance, structural strength performance, powerplant operation, flight characteristics, or other qualities affecting airworthiness, or that cannot be made with elementary operations.

Major alteration

An alteration not listed in the aircraft, aircraft engine, or propeller specifications: (1) that might appreciably affect weight, balance, structural strength, performance, powerplant operation, flight characteristics, or other qualities affecting airworthiness; or (2) that is not done according to accepted practices or cannot be done by elementary operations.

Major overhaul

The disassembly, cleaning, and inspection of an engine and the repair and replacement of all parts that do not meet the manufacturer’s specification.

Major repair

A repair: (1) That, if improperly done, might appreciably affect weight, balance, structural strength, performance, powerplant operation, flight characteristics, or other qualities affecting airworthiness; or (2) That is not done according to accepted practices or cannot be done by elementary operations.

Major repair

A repair to a component that if improperly done might appreciably affect weight, balance, structural strength, performance, powerplant operation, flight characteristics, or other qualities affecting airworthiness; a repair not done according to accepted practices, or one that cannot be done by elementary operations.

Major repair

A repair to an aircraft structure or component that if improperly made might appreciably affect weight, balance, structural strength, performance, powerplant operation, flight characteristics, or other qualities affecting airworthiness, or that is not done according to accepted practices, or cannot be made with elementary operation.

Major repairs

A repair that (1) if improperly done, might appreciably affect weight, balance, structural strength, performance, powerplant operation, flight characteristics, or other qualities affecting airworthiness, or (2) is not done according to accepted practices, or cannot be done by elementary operations.

Malfunction or Defect Report

A report (FAA Form 8010-4) providing the FAA and industry with a very essential service record of mechanical difficulties encountered in aircraft operations. Such reports contribute to the correction of conditions or situations which otherwise will continue to prove costly and/or adversely affect the airworthiness of aircraft.

Mandrel

A precision steel bar on which a propeller is mounted for balancing. The mandrel is placed across two perfectly level knife-edge plates, and the propeller is allowed to rotate until it stops with its heavy point at the bottom.

Manifold cross-feed fuel system

A type of fuel system commonly used in large transport category aircraft. All fuel tanks feed into a common manifold, and the dump chutes and the single-point fueling valves are connected to the manifold. Fuel lines to each engine are taken from the manifold.

Manifold pressure

Absolute pressure as measured at the appropriate point in the induction system and usually expressed in inches of mercury.

Manifold pressure

The absolute pressure of the air inside the induction system of a reciprocating engine.

Manifold pressure

The absolute pressure of the air in the induction system of a reciprocating engine.

Manifold pressure gauge

A pressure gauge that measures the absolute pressure inside the induction system of a reciprocating engine. When the engine is not operating, this instrument shows the existing atmospheric pressure.

Manifold valve

See flow divider (reciprocating engine).

Manufacturer’s maintenance manual

A manual provided by an aircraft manufacturer that outlines the methods, techniques, and practices prescribed for each person performing maintenance, alteration, or preventive maintenance on an aircraft, engine, propeller, or appliance.

Marker beacon

A radio navigation aid used in an instrument approach to identify distance to the runway. As the aircraft crosses over the marker-beacon transmitter, the pilot receives an accurate indication of the airplane’s distance from the runway through the medium of a flashing light and an aural signal.

Mass

A measure of the amount of matter in an object. For the purpose of measuring the mass of air flowing through a turbine engine, the weight of the air, in pounds per second, is divided by the acceleration due to gravity (32.3 feet per second).

Mass

A measure of the quantity of matter in an object.

Master switch

A switch designed to control all electric power to all circuits in a system.

Master switch

A switch in an aircraft electrical system that can disconnect the battery from the bus and open the generator or alternator field circuit.

Matrix

The material used in composite construction to bond the fibers together and to transmit the forces into the fibers. Resins are the most widely used matrix materials.

Matrix (advanced composites)

The material that bonds the fibers together in an advanced composite structure. The matrix carries the stresses into the fibers.

Matter

Something that has mass, takes up space, and exists as a solid, liquid, or gas.

Matter

Any substance that has mass and takes up space.

Maximum engine overtorque

As it applies to turbopropeller and turboshaft engines incorporating free power turbines for all ratings except one engine inoperative (OEI) ratings of two minutes or less, is the maximum torque of the free power turbine rotor assembly, the inadvertent occurrence of which, for periods of up to 20 seconds, will not require rejection of the engine from service, or any maintenance action other than to correct the cause.

Maximum landing weight

The heaviest weight an aircraft can have when it lands. For large wide body commercial airplanes, it can be 100,000 pounds less than maximum takeoff weight, or even more.

Maximum ramp weight

The heaviest weight to which an aircraft can be loaded while it is sitting on the ground, sometimes referred to as the maximum taxi weight.

Maximum speed for stability characteristics, VFC /MFC

A speed that may not be less than a speed midway between maximum operating limit speed (VMO/MMO) and demonstrated flight diving speed (VDF/MDF), except that, for altitudes where the Mach number is the limiting factor, MFC need not exceed the Mach number at which effective speed warning occurs.

Maximum takeoff weight

The heaviest weight an aircraft can have when it starts the takeoff roll. The difference between this weight and the maximum ramp weight would equal the weight of the fuel that would be consumed prior to takeoff.

Maximum weight

The maximum authorized weight of the aircraft and its contents, and is indicated in the Aircraft Specifications or Type Certificate Data Sheet.

Maximum zero fuel weight

The heaviest weight an aircraft can be loaded to without having any usable fuel in the fuel tanks. Any weight loaded above this value must be in the form of fuel.

Mean aerodynamic chord (MAC)

The average distance from the leading edge to the trailing edge of the wing.

Mean camber

A line that is drawn midway between the upper and lower camber of an airfoil section. The mean camber determines the aerodynamic characteristics of the airfoil.

Mechanical advantage

A ratio of the resistance force to the effort force.

Mechanical energy

This includes all methods of producing increased motion of molecules such as friction, impact of bodies, or compression of gases.

Medium-bypass ratio engine

A turbofan engine whose bypass ratio is between 2:1 and 4:1.

Mercerize

A treatment given to cotton thread to make it strong and lustrous. The thread is stretched while it is soaked in a solution of caustic soda.

Methanol

Alcohol made from wood.

METO horsepower

The maximum power allowed to be continuously produced by an engine. Takeoff power is usually limited to a given amount of time, such as 1 minute or 5 minutes.

Micro-Mesh

A patented graduated series of cloth-backed cushioned seats that contain abrasive crystals. Micro-Mesh is used for polishing and restoring transparency to acrylic plastic windows and windshields.

Micro-organism

An organism, normally bacteria or fungus, or microscopic size.

Microballoons

Tiny, hollow spheres of glass or phenolic material used to add body to a resin.

Microbial contaminants

The scum that forms inside the fuel tanks of turbine-engine-powered aircraft that is caused by micro-organisms. These micro-organisms live in water that condenses from fuel, and they feed on the fuel. The scum they form clogs fuel filters, lines, and fuel controls and holds water in contact with the aluminum alloy structure, causing corrosion.

Microinches rms

A measure used for cylinder wall surface roughness. Twenty microinches rms means that the highest and lowest deviation from the average surface is 20 millionths of an inch.

Micron

A measurement used to identify the size of particles trapped by filters. One micron is a micro meter, or one millionth of a meter. It is 0.000039 inch.

Micron (“micro meter”)

A unit of linear measurement equal to one millionth of a meter, one thousandth of a millimeter, or 0.000039 inch. A micron is also called a micrometer.

Micronic filter

The registered trade name of a type of fluid filter whose filtering element is a specially treated cellulose paper formed into vertical convolutions, or wrinkles. Micronic filters prevent the passage of solids larger than about 10 microns, and are normally replaced with new filters rather than cleaned.

Microprocessor

A single silicon chip that contains the arithmetic and logic functions of a computer.

Microswitch

The registered trade name for a precision switch that uses a short throw of the control plunger to actuate the contacts. Microswitches are used primarily as limit switches to control electrical units automatically.

Mil

One thousandth of an inch (0.001 inch). Paint film thickness is usually measured in mils.

Mildew

A gray or white fungus growth that forms on organic materials. Mildew forms on cotton and linen aircraft fabric and destroys its strength.

Milliammeter

An instrument that measures electrical current in units of thousandths of an ampere.

Millibar

A unit of pressure in the metric system. One bar is a pressure of 14.5 psi, or 29.52 in. Hg. One millibar is one thousandth of a bar, or 0.01469 psi, or 0.02952 in. Hg.

Millivoltmeter

An electrical instrument that measures voltage in units of millivolts (thousandths of a volt).

Minor alteration

An alteration other than a major alteration.

Minor alteration

Any alteration that does not fit the definition of a major repair. See major repair.

Minor repair

A repair other than a major repair.

Mist coat

A very light coat of zinc chromate primer. It is so thin that the metal is still visible, but the primer makes pencil marks easy to see.

Mixed number

A combination of a whole number and a fraction. For example, 53⁄8 is a mixed number.

Mode Control Panel

The Mode Control Panel, (MCP) contains controls that allow aircrew to interface with the autopilot system. The MCP can be used to instruct the autopilot to perform tasks such as; hold a specific altitude, change altitudes at a specific rate, hold a specific heading, turn to a new heading, and or follow the directions of a flight management computer. The MCP is not the autopilot, it just controls the mode in which the autopilot operates.

Module (modular engine construction)

The method of construction for mast modern gas turbine engine. The engine is made of several modules, or units, that can be removed and replaced or serviced independent of the rest of the engine.

Moisture content of wood

Weight of the water contained in the wood usually expressed in percentage of the weight of the kiln-dry wood.

Moisture separator

A component in a high-pressure pneumatic system that removes most of the water vapor from the compressed air. When the compressed air is used, its pressure drops, and this pressure drop causes a drop in temperature. If any moisture were allowed to remain in the air, it would freeze and block the system.

Mold line

A line used in the development of a flat pattern for a formed piece of sheet metal. The mold line is an extension of the flat side of a part beyond the radius. The mold line dimension of a part is the dimension made to the intersection of mold lines and is the dimension the part would have if its corners had no radius.

Mold point

The intersection of two mold lines of a part. Mold line dimensions are made between mold points.

Molecule

The smallest particle of an element or compound that retains the chemical properties of the element or compound.

Moment

A force that causes or tries to cause an object to rotate. The value of a moment is the product of the weight of an object (or the force) multiplied by the distance between the center of gravity of the object (or the point of application of the force) and the fulcrum about which the object rotates.

Moment

In determining weight and balance, the moment is the product of a weight multiplied by its arm.

Momentum

A force caused by the inertia of a moving body as it tries to keep the object moving in the same direction, at the same speed.

Monel

An alloy of nickel, copper, and aluminum or silicon.

Monocoque

A single-shell type of aircraft structure in which all of the flight loads are carried in the outside skin of the structure.

Motor

(verb) The act of rotating a turbine engine using the starter, with the ignition system deactivated. An engine is motored to force air through it to purge fuel fumes.

MS flareless fittings

Designed primarily for highpressure (3,000 psi) hydraulic systems that may be subjected to severe vibration or fluctuating pressure. Using this type of fitting eliminates all tube flaring, yet provides a safe and strong, dependable tube connection. The fitting consists of three parts: a body, a sleeve, and a nut.

Multiconductor cable

Consists of two or more cables or wires, all of which are encased in an outer covering composed of synthetic rubber, fabric, or other material.

Multimeter

An electrical test instrument that consists of a single current-measuring meter and all of the needed components to allow the meter to be used to measure voltage, resistance, and current. Multimeters are available with either analog-or digital-type displays.

Multiple-can combustor

A combustor used in a gas turbine engine that consists of a series of individual burner cans, each made of an inner liner and an outer case. The individual cans are arranged around the periphery of a centrifugal compressor. Hot gases flow directly from the cans into the turbine.

Multiple-disc brakes

Aircraft brakes in which one set of discs is keyed to the axle and remains stationary. Between each stationary disc there is a rotating disc that is keyed to the inside of the wheel. When the brakes are applied, the stationary discs are forced together, clamping the rotating discs between them. The friction between the discs slows the aircraft.

Multiplication

The process of repeated addition

N1

A symbol representing the rotational speed of the low- pressure compressor in a dual-spool gas turbine engine.

N2

A symbol representing the rotational speed of the high- pressure compressor in a dual-spool gas turbine engine.

Nacelle

An enclosed compartment, normally in the leading edge of the wing, in which an aircraft engine is mounted.

Nailing strip

A method of applying pressure to the glue in a scarf joint repair in a plywood skin. A strip of thin plywood is nailed over the glued scarf joint with the nails extending into a supporting structure beneath the skin. The strip is installed over vinyl sheeting to prevent it sticking to the skin. When the glue is thoroughly dry, the nailing strip is broken away and the nails removed.

Nap of the fabric

The ends of the fibers in a fabric. The first coat of dope on cotton or linen fabric raises the nap, and the fiber ends stick up. These ends must be carefully removed by sanding to get a smooth finish.

Naphtha

A volatile, flammable liquid distilled from petroleum. It is used as a cleaning agent and solvent, and is present in some blended turbine-engine fuels.

Naphtha

A volatile and flammable hydrocarbon liquid used chiefly as a solvent or as a cleaning fluid.

Naturally aspirated engine

A reciprocating engine that depends upon atmospheric pressure to force the air/fuel mixture into the cylinders. Naturally aspirated engines are neither supercharged nor turbocharged.

Nautical Mile (NM)

A unit of length used in the fields of air and marine navigation. In this document, a nautical mile is always the international nautical mile of 1852 m exactly.

Navigation Accuracy Category for Position (NACP)

Used to indicate, with 95 percent certainty, the accuracy of the aircraft reported horizontal position. A NACP of 8 or greater is required by § 91.227.

Navigation Integrity Category (NIC)

A parameter that specifies an integrity containment radius. A NIC of 7 or greater is required by § 91.227.

Navigational Accuracy Category for Velocity (NACV)

Used to indicate, with 95 percent certainty, the accuracy of the aircraft reported horizontal velocity. A NACV of 1 or greater is required by § 91.227.

Negative number

A number that is less than zero.

Negative pressure relief valve (pressurization component)

A valve that opens anytime the outside air pressure is greater than the cabin pressure. It prevents the cabin altitude from ever becoming greater than the aircraft flight altitude.

Net thrust

The thrust produced by a turbojet or turbofan engine in which the acceleration factor is the difference between the velocity of the incoming air and the velocity of the exhaust gases leaving the engine.

Neutral axis (neutral plane)

A line through a piece of material that is bent. The material in the outside of the bend is stretched and that on the inside of the bend is shrunk. The material along the neutral plane is neither shrunk nor stretched.

Neutral flame

An oxyacetylene flame produced when the ratio of oxygen and acetylene is chemically correct and there is no excess of oxygen or carbon. A neutral flame has a rounded inner cone and no feather around it.

Neutral position

The position of the magnet in a magneto when its poles are between the pole shoes and no lines of flux are flowing through the magnetic circuit.

Newton

The unit of force needed to accelerate a mass of one kilogram one meter per second per second. One newton is equal to 1000,000 dynes, or 2.248 × 10-1 pound.

Nichrome

The registered trade name for an alloy of nickel and chromium. Nichrome wire is used for making electrical heater elements and precision wire-wound resistors. Nichrome’s resistance is approximately 65 times that of copper.

Nick

A sharp notch-like displacement of metal surface.

Nick

A sharp notch-like displacement of metal usually found on leading and trailing edges.

Nitriding

A method of case hardening steel. Steel is placed in a retort (a sealed, high-temperature furnace and heated to a specified temperature while surrounded by ammonia gas (NH3). The ammonia breaks down into nitrogen and hydrogen, and the nitrogen unites with some of the alloying elements in the steel to form an extremely hard surface. Nitriding hardens crankshaft bearing surfaces and cylinder walls in reciprocating engines. It takes place at a lower temperature than other forms of case hardening, and does not cause warping.

Noise (electrical)

An unwanted electrical signal within a piece of electronic equipment.

Nomex

A patented nylon material used to make the honeycomb core for certain types of sandwich materials.

Nomex braid

Nomex® is the trade name for a high-temperature polyamide thread that is braided over the larger sizes (# 8 gage and larger) of many of the military specification wires. It can be encountered in either an off-white or black/green color.

Nominal Loaded Radius

The allowable tolerance on the loaded radius, and the actual load radius for the test tires should be identified.

Nomogram

A graph that usually consists of three sets of data. Knowledge of any two sets of data enables the reader to determine the third set.

Nonenergizing brake

A brake that does not use the momentum of the aircraft to increase the friction.

Nonvolatile memory

Memory in a computer that is not lost when power to the computer is lost.

Normal category airplane

An aircraft that is certificated under 14 CFR part 23 that is not certificated under the acrobatic, utility, or commuter category.

Normal heptane

A hydrocarbon, C7H16, with a very low critical pressure and temperature. Normal heptane is used as the low reference in measuring the anti-detonation characteristics of a fuel.

Normal shock wave

A type of pressure wave that forms at right angles to a surface when air moves at the speed of sound.

Normal shock wave

A shock wave that forms ahead of a blunt object moving through the air at the speed of sound. The shock wave is normal (perpendicular) to the air approaching the object. Air passing through a normal shock wave is slowed to a subsonic speed and its static pressure is increased.

Normalizing

Reforming of the grain structure of a metal or alloy by proper heat treatment to relieve internal stresses.

Normalizing

The process of heating the part to the proper temperature, holding it at that temperature until it is uniformly heated, and then cooling it in still air.

Normalizing

Aprocess of strain-relieving steel that has been welded and left in a strained condition. The steel is heated to a specified temperature, usually red hot, and allowed to cool in still air to room temperature.

Nose-gear centering cam

Notch sensitivity

A measure of the loss of strength of a material caused by the presence of a notch, or a V-shaped cut.

Nozzle guide vanes

See turbine inlet guide vanes.

Nuclear energy

Energy stored in the nucleus of atoms is released during the process of nuclear fission in a nuclear reactor or atomic explosion.

Null position

The position of an ADF loop antenna when the signal being received is canceled in the two sides of the loop and the signal strength is the weakest.

Numerator

The upper part of a fraction (represented by the letter N in N⁄D)

O-ring

A widely used type of seal made in the form of a rubber ring with a round cross section. An O-ring seals in both directions, and it can be used as a packing or a gasket.

Oblique shock wave

A pressure wave that forms on a sharp- pointed object when air flows past it at a supersonic speed.

Oblique shock wave

A shock wave that forms on a sharp- pointed object moving through air at a speed greater than the speed of sound. Air passing through an oblique shock wave is slowed down, but not to a subsonic speed, and its static pressure is increased.

Oblique view

A view that is similar to an isometric view except with two of the three drawing axes always at right angles to each other.

Octane rating

A system used to rate the antidetonation characteristics of a reciprocating engine fuel. Fuel with an octane rating of 80 performs in a laboratory test engine the same as the fuel made of a mixture of 80% iso-octane and 20% heptanes.

Octane rating

A rating of the anti-detonation characteristics of a reciprocating engine fuel. It is based on the performance of the fuel in a special test engine. When a fuel is given a dual rating such as 80/87, the first number is its anti-detonating rating with a lean fuel-air mixture, and the higher number is its rating with a rich mixture.

Odometer

The portion of an automobile speedometer that indicates the distance traveled.

Offset throw (crankshaft design)

Crank arms on a reciprocating engine crankshaft. The arms, or throws, to which the connecting rods and pistons are attached are offset from the center of the crankshaft to move the pistons in and out of the cylinder. The amount of the offset determines the stroke of the engine.

Ohm

The standard unit used to measure resistance.

Ohmmeter

A current measuring instrument that provides its own source (self-excited) of power.

Ohm’s Law

Explains the relationship between voltage, current, and resistance in an electrical circuit, and states that current flow in an electrical circuit is directly proportional to the amount of voltage applied to the circuit.

Oil analysis

A method of measuring the contents in parts per million of various chemical elements in oil. A sample of the oil is burned in an electric arc, and the resulting light is analyzed with a spectroscope which identifies the chemical elements in the oil and gives an indication of the amount of each element. This type of oil analysis is called a spectrometric oil analysis program, or SOAP.

Oil dilution

A method of temporarily decreasing the viscosity of the lubricating oil to make it possible to start a reciprocating engine when the temperature is very low. Before shutting the engine down, enough gasoline from the fuel system is mixed with the lubricating oil in the engine to dilute it so the starter can turn the engine over when the oil is cold and viscous. When the engine starts and the oil warms up, the gasoline evaporates.

Oil-damped bearing

A type of roller bearing installation in a gas turbine engine in which the outer race is installed in an oil damper compartment whose inside diameter is a few thousandths of an inch larger than the outside diameter of the outer race. Oil under pressure fills the oil damper compartment and allows the bearing to compensate for sight misalignment and to absorb vibrations of the shaft.

Oleo shock absorber

A shock absorber used on aircraft landing gear. The initial landing impact is absorbed by oil transferring from one compartment in the shock strut into another compartment through a metering orifice. The shocks of taxiing are taken up by a cushion of compressed air.

On-condition maintenance

A maintenance program that closely monitors the operating condition of an engine and allows major repairs or replacements to be made when engine performance deteriorates to a specific level.

On-speed condition

The speed condition in which the engine is turning at the rpm for which the propeller governor is set.

One-hundred-hour inspection

An inspection required by 14 CFR part 91, section 91.409 for FAA-certificated aircraft operated for hire or used for flight instruction for hire. A 100-hour inspection is identical in content to an annual inspection, but can be conducted by an aviation maintenance technician who holds an Airframe and Powerplant rating, but does not have an Inspection Authorization. See 14 CFR part 43, Appendix D for list of the items that must be included in an annual or 100-hour inspection.

One-hundred-hour inspection

A complete inspection that is required for all aircraft operated for hire every 100 hours. An annual inspection must be conducted by an A&P mechanic with Inspection Authorization.

Open angle

An angle in which sheet metal is bent less than 90°.

Open assembly time

The period of time between the application of the glue and the assembly of the joint components.

Open circuit

An incomplete or broken electrical circuit.

Open wiring

An electrical wiring installation in which the wires are tied together in bundles and clamped to the aircraft structure rather than being enclosed in conduit.

Open-center selector valve

A type of selector valve that functions as an unloading valve as well as a selector valve. Open-center selector valves are installed in series, and when no unit is actuated, fluid from the pump flows through the centers of all the valves and returns to the reservoir. When a unit is selected for actuation, the center of the selector valve is shut off and the fluid from the pump goes through the selector valve into one side of the actuator. Fluid from the other side of the actuator returns to the valve and goes back to the reservoir through the other selector valves. When the actuation is completed, the selector valve is placed in its neutral position. Its center opens, and fluid from the pump flows straight through the valve.

Open-grained wood

Common classification of painters for woods with large pores, such as oak, ash, chestnut, and walnut. Also known as “coarsetextured.”

Open-hydraulic system

A fluid power system in which the selector valves are arranged in series with each other. Fluid flows from the pump through the center of the selector valves, back into the reservoir when no unit is being actuated.

Operating center of gravity range

The center of gravity for an aircraft loaded and ready for flight.

Operating cycle

One complete series of events in the operation of a turbine engine that consists of starting the engine, taking off, landing, and shutting the engine down.

Operational check

An operational test to determine whether a system or component is functioning properly in all aspects in conformance with minimum acceptable manufacture design specifications.

Operational control

With respect to a flight, means the exercise of authority over initiating, conducting or terminating a flight.

Optical fiber

Any filament or fiber made of dielectric materials that guides light whether or not it is used to transmit signals.

Optoelectronic device

An electronic device that produces, modulates, or senses electromagnetic radiation in the ultraviolet, visible light, or infrared portions of the energy spectrum.

Orifice

Opening through which gas or air flows. It is usually the final opening controlled by a valve.

Orifice check valve

A component in a hydraulic or pneumatic system that allows unrestricted flow in one direction, and restricted flow in the opposite direction.

Ornithopter

A heavier-than-air flying machine that produces lift by flapping its wings. No practical ornithopter has been built.

Orthographic projection

A method of showing all six possible views of an object: front, top, bottom, rear, right side, and left side.

Oscilloscope

An electrical instrument that displays on the face of a cathode-ray tube the waveform of the electrical signal it is measuring.

Otto cycle

The constant-volume cycle of energy transformation used by reciprocating engines. A mixture of fuel and air is drawn into the cylinder as the piston moves to the bottom of its stroke. The mixture is compressed as the piston moves upward in the cylinder, and when the piston is near the top of its stroke, the mixture is electrically ignited and burns. The burning mixture heats and expands the air inside the cylinder and forces the piston down, performing useful work. The piston then moves back up, forcing the burned gases out of the cylinder.

Outflow valve (pressurization component)

A valve in the cabin of a pressurized aircraft that controls the cabin pressure by opening to relieve all pressure above that for which the cabin pressure control is set.

Outside calipers

Used for measuring outside dimensions, such as the diameter of a piece of round stock

Overboost

A condition of excessive manifold pressure in a reciprocating engine. Overboosting occurs when the supercharger is operated at too high a speed.

Overhaul

Complete teardown and reassembly associated with major repair or maintenance. The terms overhaul and reconditioning are synonymous for fixed pitch propellers.

Overpressure

The tire is required to withstand, for at least 3 seconds without rupture, a minimum test inflation pressure of at least 3.0 times the rated inflation pressure at ambient temperature.

Overrunning clutch

A type of clutch that couples an input shaft with an output shaft. When the input shaft is driven, the output shaft rotates with it. When the output shaft is driven, the output shaft rotates with it. But when the output shaft is driven, the input shaft does not turn.

Overspeed condition

A speed condition in which the engine is turning at an rpm higher than that for which the propeller governor is set.

Overvoltage protector

A component in an aircraft electrical system that opens the alternator field circuit any time the alternator output voltage is too high.

Own-ship

From the perspective of a flight crew, or of the ASSAP and CDTI functions used by that flight crew, the own-ship is the ASA participant (aircraft or vehicle) that carries that flight crew and those ASSAP and CDTI functions.

Oxidizing

Combining oxygen with any other substance. For example, a metal is oxidized when the metal is burned, i. e. , oxygen is combined with all the metal or parts of it oxidizing flame.

Oxidizing flame

An oxy-fuel gas flame having an oxidizing effect due to excess oxygen.

Oxidizing flame

An oxyacetylene flame in which there is an excess of oxygen. The inner cone is pointed and often a hissing sound is heard.

Oxygen cutting

Cutting metal using the oxygen jet which is added to an oxygen-acetylene flame.

Oxygen regulator

Manually-adjustable device used to reduce cylinder pressure to torch pressure and to keep the pressure constant. They are never to be used as fuel gas regulators.

Ozone

An unstable form of oxygen produced when an electric spark passes through the air. Ozone is harmful to rubber products.

P-lead

Primary lead. The wire that connects the primary winding of a magneto to the ignition switch. The magneto is turned off by grounding its P-lead.

Packing

A seal between two parts where there is relative motion.

Paint

A covering applied to an object or structure to protect it and improve its appearance. Paint consists of a pigment suspended in a vehicle such as oil or water. When the vehicle dries by evaporation or curing, the pigment is left as a film on the surface.

Parabolic reflector

A reflector whose surface is made in the form of a parabola.

Parallel circuit

A circuit in which two or more electrical resistances or loads are connected across the same voltage source.

Parallel circuit

A method of connecting electrical components so that each component is in a path between the terminals of the source of electrical energy.

Paralleling circuit

A circuit in a multi-engine aircraft electrical system that controls a flow of control current which is used to keep the generators or alternators sharing the electrical load equally. The relay opens automatically to shut off the flow of paralleling current any time the output of either alternator or generator drops to zero.

Paralleling relay

A relay in multi-engine aircraft electrical system that controls a flow of control current which is used to keep the generators or alternators sharing the electrical load equally. The relay opens automatically to shut off the flow of paralleling current any time the output of either alternator or generator drops to zero.

Parasite drag

A form of aerodynamic drag caused by friction between the air and the surface over which it is flowing.

Parent metal

The metal being welded. This term is used to distinguish between the metal being welded and the welding rod.

Partial pressure

The percentage of the total pressure of a mixture of gases produced by each of the individual gases in the mixture.

Parting film

A layer of thin plastic material placed between a composite lay-up and the heating blanket. It prevents the blanket from sticking to the fabric.

Pascal

The unit of pressure produced when one newton of force acts uniformly over an area of one square meter. One pascal is equal to 14.503 ∙ 10-5 (0.00014503) psi. The kilopascal (kPa) is easier to manipulate. 1 kPa = 1,000 Pa = 0.14503 psi.

Pascal’s Law

The law that states that pressure applied anywhere to a body of fluid causes a force to be transmitted equally in all directions; the force acts at right angles to any surface in contact with the fluid.

Pascal’s Law

A basic law of fluid power which states that the pressure in an enclosed container is transmitted equally and undiminished to all points of the container, and the force acts at right angles to the enclosing walls.

Peak voltage

The voltage of AC electricity that is measured from zero voltage to the peak of either alternation.

Peel ply

A layer of resin-free material used to protect a laminate for later secondary bonding (sometimes referred to as a release film).

Penetrant dwell time

The length of time a part is left in the penetrant when preparing it for inspection by the fluorescent or dye penetrant method. The hotter the part and the longer the penetrant dwell time, the smaller the fault that will be detected.

Percentage

Used to express a number as a fraction of 100. Using the percentage sign, %, 90 percent is expressed as 90%.

Performance number

The rating of antidetonation characteristics of a reciprocating engine fuel that is better than the high rating reference fuel, iso-octane. Performance numbers are greater than 100.

Performance number

The anti-detonation rating of a fuel that has a higher critical pressure and temperature than iso-octane (a rating of 100). Iso-octane that has been treated with varying amounts of tetraethyl lead is used as the reference fuel.

Permanent magnet

A piece of hardened steel that has been exposed to a strong magnetizing force which has aligned the spin axes of the electrons surrounding its atoms. The high retentivity of the material causes the electrons to retain their magnetic orientation.

Permanent-mold casting

A casting made in a reusable metal mold. The walls of permanent-mold castings can be made thinner than similar walls made by sand casting.

Permeability

Used to refer to the ease with which a magnetic flux can be established in a given magnetic circuit.

Permeability

A measure of the ease with which lines of magnetic flux can pass through a material.

Person

An individual, firm, partnership, corporation, company, association, joint-stock association, or governmental entity. It includes a trustee, receiver, assignee, or similar representative of any of them.

Perspective view

A drawing that shows a three-dimensional object (portraying height, width, and depth) as it appears to an observer. It most closely resembles the way an object would look in a photograph.

Petrolatum-zinc dust compound

A special abrasive compound used inside an aluminum wire terminal being swaged onto a piece of aluminum electrical wire. When the terminal is compressed, the zinc dust abrades the oxides from the wire, and the petrolatum prevents oxygen reaching the wire so no more oxides can form.

Petroleum fractions

The various components of a hydrocarbon fuel that are separated by boiling them off at different temperatures in the process of fractional distillation.

Phantom line

Composed of one long and two short evenly spaced dashes, indicates an alternate position of parts of the object or the relative position of a missing part.

Phase sequence, or phase rotation

The sequence with which the output phases of a three-phase generator are connected to the load. Reversing the phase sequence of a generator from A-B-C to A-C-B prevents the generator from being synchronized with the others on the bus.

Phased array antenna

A complex antenna which consists of a number of elements. A beam of energy is formed by the superimposition of the signals radiating from the elements. The direction of the beam can be changed by varying the relative phase of the signals applied to each of the elements.

Phenolic plastic

A plastic material made of a thermosetting phenol-formaldehyde resin, reinforced with cloth or paper. Phenolic plastic materials are used for electrical insulators and for chemical-resistant table tops.

Pi (π) filter

An electronic filter used to prevent radio frequency energy produced in the ignition exciter from feeding back into the aircraft electrical system. The filter is made of an inductor with a capacitor on its input and output. The name is derived from the resemblance of the three components on a schematic diagram to the Greek letter pi (π).

Pickling

The treatment of a metal surface by an acid to remove surface corrosion.

Pictorial drawing

A drawing that is similar to a photograph. It shows an object as it appears to the eye, but it is not satisfactory for showing complex forms and shapes.

pieces of thin wood together

The grain of all pieces runs in the same direction.

Pilot hole

A small hole punched or drilled in a piece of sheet metal to locate a rivet hole.

Pilot in command

The person who: (1) Has final authority and responsibility for the operation and safety of the flight; (2) Has been designated as pilot in command before or during the flight; and (3) Holds the appropriate category, class, and type rating, if appropriate, for the conduct of the flight.

Pin knot cluster

A group of knots, all having a diameter of less than approximately 1⁄16 inch.

Pinion

A small gear that meshes with and drives a larger gear.

Pinion

A small gear that meshes with a larger gear, a sector of a gear, or a toothed rack.

Pinked-edge tape

Cloth tape whose edges have small V-shaped notches cut along their length. The pinked edges prevent the tape from raveling.

Pinking shears

Shears used to cut aircraft fabric with a series of small notches along the cut edge.

Piston

A sliding plug in an actuating cylinder used to convert pressure into force and then into work.

Piston (reciprocating engine component)

The movable plug inside the cylinder of a reciprocating engine. The piston moves in and out to compress the air/fuel mixture and to transmit the force from the expanding gas in the cylinder to the crankshaft.

Piston pin

See wrist pin.

Pitch

The distance, in inches, that a propeller section will move forward in one revolution, or the distance a nut will advance in one revolution of the screw in a single thread.

Pitch

The theoretical distance that the propeller blade reference station would move forward if it were moving along a helix with an angle equal to the blade section angle.

Pitch (aircraft maneuver)

Rotation of an aircraft about its lateral axis.

Pitch (aircraft maneuver)

Rotation of an aircraft about its lateral axis.

Pitch (rivet layout dimension)

The distance between the centers of adjacent rivets installed in the same row.

Pitch (rivet)

The distance between the centers of adjacent rivets installed in the small row.

Pitch (thread dimension)

The linear distance, measured parallel to the length of a threaded fastener, between corresponding points on two adjacent threads.

Pitch angle

The angle between the chordline of a propeller blade and the plane of rotation. See blade angle.

Pitch angle (helicopter rotor blade)

The angle between the chord line of a rotor blade and the reference plane of the main rotor hub, or the plane of rotation of the rotor.

Pitch angle (propeller specification)

The angle between the chord line of a propeller blade and the plane of rotation.

Pitch axis (aircraft axis)

The lateral axis of an aircraft that extends from wing tip to wing tip and passes through the center of gravity. This is the axis about which the aircraft pitches.

Pitch distribution

The gradual twist in the propeller blade from shank to tip.

Pitch distribution

The gradual change in pitch angle of a propeller blade from the root to the tip.

Pitch distribution (propeller specification)

The gradual twist in the propeller blade from shank to tip.

Pitch pocket (wood defect)

Pockets of pitch that appear in the growth rings of a piece of wood.

Pitch setting

The propeller blade setting as determined by the blade angle measured in a manner, and at a radius, specified by the instruction manual for the propeller.

Pitot pressure

Ram air pressure used to measure airspeed. The pitot tube faces directly into the air flowing around the aircraft. It stops the air and measures its pressure.

Pitted

Small irregular shaped cavities in the surface of the parent material usually caused by corrosion, chipping, or heavy electrical discharge.

Pitting

The formation of pockets of corrosion products on the surface of a metal.

Pitting

Small irregularly shaped cavities from which material has been removed by erosion or corrosion. Corrosive pitting is usually accompanied by a deposit such as rust, a corrosion byproduct, formed by a corrosive agent on the base metal.

Plain-weave fabric

Fabric in which each warp thread passes over one fill thread and under the next. Plain-weave fabric typically has the same strength in both warp and fill directions.

Plan position indicator (PPI)

A type of radar scope that shows both the direction and distance of the target from the radar antenna. Some radar antenna rotate and their PPI scopes are circular. Other antenna oscillate and their PPI scopes are fan shaped.

Plane of rotation

The plane in which a propeller blade rotates. The plane of rotation is perpendicular to the propeller shaft.

Planer

A woodworking power tool used to smooth the surfaces of a piece of wood.

Planetary gears

A type of large-ratio reduction gearing. A series of small planetary gears are mounted on a spider attached to the output shaft. The planetary gears rotate between a fixed sun gear and a driven ring gear.

Plastic

An organic substance of large molecular weight which is solid in its finished state and, at some stage during its manufacture or its processing into a finished article, can be shaped by flow.

Plastic media blasting (PMB)

A method of removing paint from an aircraft surface by dry-blasting it with tiny plastic beads.

Plasticizer

A constituent in dope or lacquer that gives its film flexibility and resilience.

Plastics

The generic name for any of the organic materials produced by polymerization. Plastics can be shaped by molding or drawing.

Plenum

An enclosed chamber in which air can be held at a pressure higher than that of the surrounding air.

Plenum chamber

An enclosed chamber in which air can be held at a pressure slightly higher than that of the surrounding air. Plenum chambers are used to stabilize the pressure of the air before it enters a double entry centrifugal compressor.

Plumb bob

A heavy metal object, cylinder- or coneshaped, with a sharp point at one end that is suspended by a string to produce a vertical reference line useful in aircraft measurements.

Ply rating

The rating of an aircraft tire that indicates its relative strength. The ply rating does not indicate the actual number of plies of fabric in the tire; it indicates the number of piles of cotton fabric needed to produce the same strength as the actual piles.

Plywood

A wood product made by gluing several pieces of thin wood veneer together. The grain of the wood in each layer runs at 90° or 45° to the grain of the layer next to it.

Pneumatics

The system of fluid power which transmits force by the use of a compressible fluid.

Pole shoe

Inward extensions from the field frame of a generator around which the field coils are wound.

Polyester braid

A plastic braiding thread, when used as the outer surface of a wire, provides a clothlike appearance.

Polyester fibers

A synthetic fiber made by the polymerization process in which tiny molecules are united to form a long chain of molecules. Polyester fibers are woven into fabrics that are known by their trade names of Dacron, Fortrel, and Kodel. Polyester film and sheet are known as Mylar and Celenar.

Polyester resin

A thermosetting resin used as a matrix for much of the fiberglass used in composite construction.

Polyimide varnish

A liquid form of polyimide that is applied to the outer surface of a wire through the process of repeated dipping through the varnish bath with subsequent heat curing. The successive layers rarely reach a total buildup of 1 mil.

Polyimidetape

A plastic film (commonly referred to by the trade name, KAPTON). The tape has a dark brown color, and is frequently coated with a polyimide varnish that has a very distinct mustard yellow color. At times, the spiral edge of the outermost tape is apparent under the varnish topcoat. It may be used for wire insulation. Total polyimide tape insulated wire constructions are inactive for new design on military aircraft and are subject to the procedures defined in FAA Advisory Circular AC 29-2A Change 2 Paragraph 29.1359 in Civil Aircraft.

Polymerization

Basic processes for making large (high-polymer) molecules from small ones, normally without chemical change; can be by addition, condensation, rearrangement, or other methods.

Polyurethane enamel

A hard, chemically resistant finish used on aircraft. Polyurethane enamel is resistant to damage from all types of hydraulic fluid.

Polyvinyl chloride

A thermoplastic resin used in the manufacture of transparent tubing for electrical insulation and fluid lines which are subject to low pressures.

Poppet valve

A T-shaped valve with a circular head. Poppet valves are used to cover the intake and exhaust openings in the cylinder head of a reciprocating engine. The valves are held closed by one or more coil springs and are opened by a cam lobe or a rocker arm pushing on the end of the valve stem.

Porcelain

A hard, white, translucent ceramic material that was used as the insulator in some of the early aircraft spark plugs.

Porosity

Cavity-type discontinuities in metal formed by gas entrapment during solidification.

Portable oxygen concentrator

A medical device that separates oxygen from other gasses in ambient air and dispenses this concentrated oxygen to the user.

Position error

The error in pitot-static instruments caused by the static ports not sensing true static air pressure. Position error changes with airspeed and is usually greatest at low airspeeds.

Position Source

The on-board avionics equipment that provides the latitude, longitude, geometric altitude, velocity, position and velocity accuracy metrics, and position integrity metric. Additionally, the position source may provide the vertical rate parameters.

Positive number

A number that is greater than zero.

Positive-displacement pump

A fluid pump that moves a specific volume of fluid each time it rotates. Spur-gear pumps, gerotor pumps, and vane pumps are all positive- displacement pumps.

Pot life

The length of time a resin will remain workable after the catalyst has been added. If a catalyzed material is not used within its usable pot life, it must be discarded and a new batch mixed up.

Potential difference

A difference in electrical pressure.

Potential energy

Energy that is stored.

Potential energy

Energy possessed in an object because of its position, chemical composition, shape, or configuration.

Potentiometer

A variable tapped resistor that can be used as a voltage divider.

Potentiometer

A variable resistor having connections to both ends of the resistance element and to the wiper that moves across the resistance.

Power

Power is the time rate at which work is done or energy is transferred.

Power

The time rate of doing work. Power is found by dividing the amount of work done, measured in floor-pounds, by the time in seconds or minutes used to do the work. Power may be expressed in foot-pounds of work per minute or in horsepower. One horsepower is 33,000 foot-pounds of work done in one minute, or 550 foot pounds of work done in one second.

Power

The time rate of doing work. Power is force multiplied by distance (work) divided by time.

Power (exponent)

A shorthand method of indicating how many times a number, called the base, is multiplied by itself. For example, in the number 43, 3 is the power, or exponent, and 4 is the base. That is, 43 is equal to 4 × 4 × 4 = 64.

Power brakes

Aircraft brakes that use the main hydraulic system to supply fluid for the brake actuation. Aircraft that require a large amount of fluid for their brake actuation normally use power brakes, and the volume of fluid sent to the brakes is increased by the use of deboosters.

Power control valve

A hand-operated hydraulic pump unloading valve. When the valve is open, fluid flows from the pump to the reservoir with little opposition. To actuate a unit, turn the selector valve, and manually close the power control valve. Pressurized fluid flows to the unit, and when it is completely actuated, the power control valve automatically opens.

Power-assurance check

A test run made of a gas turbine engine to determine how its performance compares with its precious performance as new or freshly overhauled.

Powered parachute

A powered aircraft comprised of a flexible or semi-rigid wing connected to a fuselage so that the wing is not in position for flight until the aircraft is in motion. The fuselage of a powered parachute contains the aircraft engine, a seat for each occupant and is attached to the aircraft’s landing gear.

Powered-lift

A heavier-than-air aircraft capable of vertical takeoff, vertical landing, and low speed flight that depends principally on engine-driven lift devices or engine thrust for lift during these flight regimes and on nonrotating airfoil(s) for lift during horizontal flight.

Powerplant

The complete installation of an aircraft engine, propeller, and all accessories needed for its proper function.

Powers of ten

Also called scientific notation. It is a shorthand method of depicting very large or very small numbers.

Pre-ignition

Ignition of the air/fuel mixture inside the cylinder of an engine before the time for normal ignition. Pre-ignition is often caused by incandescent objects inside the cylinder.

Precession

The characteristic of a gyroscope that causes a force to be felt, not at the point of application, but at a point 90° in the direction of rotation from that point.

Precipitation heat treatment

A method of increasing the strength of heat-treated aluminum alloy. After the aluminum alloy has been solution-heat-treated by heating and quenching, it is returned to the oven and heated to a temperature lower than that used for the initial heat treatment. It is held at this temperature for a specified period of time, and then removed from the oven and allowed to cool slowly.

Preflight inspection

A required inspection to determine the condition of the aircraft for the flight to be conducted. It is conducted by the pilot-in-command.

Prepreg

A mat, a fabric, or covering impregnated with resin that is ready for lay up and curing.

Prepreg

Preimpregnated fabric. A type of composite material in which the reinforcing fibers are encapsulated in an uncured resin. Prepreg materials are cut to size and shape and laid up with the correct ply orientation, and the entire component is cured with heat and pressure.

Prepreg (preimpregnated fabric)

A type of composite material in which the reinforcing fibers are encapsulated in an uncured resin. Prepreg materials must be kept refrigerated to prevent them from curing before they are used.

Press-to-test light fixture

An indicator light fixture whose lens can be pressed in to complete a circuit that tests the filament of the light bulb.

Pressure

The amount of force acting on a specific amount of surface area, typically measured in pounds per square inch or psi.

Pressure

A measure of force applied uniformly over a given unit of surface area.

Pressure

Force per unit area. Hydraulic and pneumatic pressure are normally given in units of pounds per square inch (psi).

Pressure altitude

The altitude in standard air at which the pressure is the same as that of the existing air. Pressure altitude is read on an altimeter when the barometric scale is set to the standard sea level pressure of 29.92 inches of mercury.

Pressure altitude

The altitude in standard atmosphere at which the pressure is the same as the existing pressure.

Pressure Altitude

Altitude reported by a barometric pressure altimeter without corrections for local pressure settings.

Pressure carburetor

A carburetor installed on some aircraft reciprocating engines that uses the pressure difference between air inside the venture and ram air entering the carburetor to produce a fuel-metering force. Pressure carburetors have generally been replaced with continuous- flow fuel injection systems.

Pressure cooling

A method of air cooling a reciprocating engine in which the cylinders are enclosed in tight-fitting shrouds. The cowling is divided into two compartments by baffles and seals, with half of each cylinder in each compartment. Ram air is directed into one compartment, and the pressure in the other is decreased by air flowing over a flared exit or adjustable cowl flaps. The pressure difference across the cylinders causes cooling air to be drawn through the fins to remove the unwanted heat.

Pressure manifold (hydraulic system component)

The portion of a fluid power system from which the selector valves receive their pressurized fluid.

Pressure plate (brake component)

A strong, heavy plate used in a multiple-disc brake. The pressure plate receives the force from the brake cylinders and transmits this force to the discs.

Pressure reducing valve (oxygen system component)

A valve used in an oxygen system to change high cylinder pressure to low system pressure.

Pressure relief valve (oxygen system component)

A valve in an oxygen system that relieves the pressure if the pressure reducing valve should fail.

Pressure vessel

The strengthened portion of an aircraft structure that is sealed and pressurized in flight.

Pressure-demand oxygen system

A type of oxygen system used by aircraft that fly at very high altitude. This system functions as a diluter-demand system until, at about 40,000 feet, the output to the mask is pressurized enough to force the needed oxygen into the lungs, rather than depending on the low pressure produced when the wearer of the mask inhales to pull in the oxygen. (See diluter-demand oxygen system.) Pressure fueling. The method of fueling used by almost all transport aircraft. The fuel is put into the aircraft through a single underwing fueling port. The fuel tanks are filled to the desired quantity and in the sequence selected by the person conducting the fueling operation. Pressure fueling saves servicing time by using a single point to fuel the entire aircraft, and it reduces the chances for fuel contamination.

Pressure-injection carburetor

A multibarrel pressure carburetor used on large radial and V-engines. Fuel is metered on the basis of air mass flowing into the engine and is sprayed under pressure into the eye, or center, of the internal supercharger impeller.

Prevailing torque

The torque required to turn a threaded fastener before it contacts the surface it is intended to hold.

Preventive maintenance

Simple or minor preservation operations and the replacement of small standard parts not involving complex assembly operations.

Preventive maintenance

Simple or minor preservation operations and the replacement of small standard parts not involving complex assembly operations.

Primary Surveillance Radar (PSR)

A radar sensor that listens for the echoes of pulses that it transmits to illuminate aircraft targets. PSR sensors, in contrast to secondary surveillance radar (SSR) sensors, do not depend on the carriage of transponders on board the aircraft targets.

Primary winding

The winding in a magneto or ignition coil that is between the source of voltage and the breaker points. The primary winding is normally made of comparatively large diameter wire, and has a small number of turns, typically about 200.

Primer (finishing system component)

A component in a finishing system that provides a good bond between the surface and the material used for the topcoats.

Process Specifications

Documents approved by the Administrator containing information for performing specialized maintenance, such as specifications. Air carriers or conmercial operators having a continuous airworthiness maintenance program under FAR Parts 121, 127, or 135, are required to include a process specification for retreading their tires in their program. Part 125 operators have inspection programs that require each operator to include a process specification in its manual for retreading its tires.

Product

The result of multiplication.

Profile drag

Aerodynamic drag produced by skin friction. Profile drag is a form of parasite drag.

Profile tip (compressor blade tip)

The tip of an axial-flow compressor bladed whose thickness is reduced to give it a higher resonant frequency so it will not be subject to the vibrations that would affect a blade with a squared tip. The profile tip also provides a more aerodynamically efficient shape for the high velocity air that is moved by the blade. Profile tips often touch the housing and make a squealing noise as the engine is shut down. For this reason profile tips are often called squealer tips.

Profilometer

A precision measuring instrument used to measure the depth of the hone marks in the surface of a cylinder wall.

Progressive inspection

An inspection that may be used in place of an annual or 100-hour inspection. It has the same scope as an annual inspection, but it may be performed in increments so the aircraft will not have to be out of service for a lengthy period of time.

Progressive inspection

Breaking down the large task of conducting a major inspection into smaller tasks which can be accomplished periodically without taking the aircraft out of service for an extended period of time.

Prony brake

An instrument used to measure the amount of horsepower an engine is delivering to its output shaft. The engine is operated at a specific rpm, and a brake is applied to its output shaft. The amount of torque applied to the brake is measured, and this, with the rpm, is converted into brake horsepower.

Propeller

A rotating airfoil that consists of two or more blades attached to a central hub which is mounted on the engine crankshaft.

Propeller

A device for propelling an aircraft that has blades on an engine-driven shaft and that, when rotated, produces by its action on the air, a thrust approximately perpendicular to its plane of rotation. It includes control components normally supplied by its manufacturer, but does not include main and auxiliary rotors or rotating airfoils of engines.

Propeller

A device for propelling an aircraft that has blades on an engine-driven shaft and that, when rotated, produces by its action on the air, a thrust approximately perpendicular to its plane of rotation. It includes associated control components normally supplied by its manufacturer.

Propeller

Propeller end

The end of a reciprocating engine to which the propeller is attached.

Propeller, Constant speed

A propeller in which a governor is used to automatically provide constant revolutions per minute as the pilot selects the propeller pitch setting.

Propeller, Controllable pitch

A propeller with blades that may be altered continuously to any desired angle during flight.

Propeller, Fixed-Pitch

A one-piece propeller with fixed blade angles.

Propeller, Full feathering

A propeller with blades that can be rotated to a high positive blade angle to stop rotation (windmilling). This feature is common on multi-engine aircraft, because it allows an engine to be shut down and rotation stopped to reduce drag and asymmetric control forces.

Propeller, reversing

A propeller in which blades can be rotated to a “negative” blade angle where they will provide a rearward thrust to slow down, stop, or move the aircraft backward.

PropFan engine

The registered trade name by Hamilton Standard of an ultra-high-bypass turbine engine. See UHB engine.

Proportion

A proportion is a statement of equality between two or more ratios. The example of A is to B as C is to D can be represented A:B = C:D or A/B = C/D.

Propulsive efficiency

A measure of the effectiveness with which an aircraft engine converts the fuel it burns into useful thrust. It is the ratio of the thrust horsepower produced by a propeller to the torque horsepower of the shaft turning the propeller. The nearer the speed of the aircraft is to the speed of the exhaust jet or propeller wake, the less kinetic energy is lost in the jet or wake, and the higher the propulsive efficiency.

Protractor

A device for measuring angles.

Pulsating DC

Direct current whose voltage periodically changes, but whose electrons flow in the same direction all of the time.

Pulse-jet engine

A type of air-breathing reaction engine used during World War II to power jet-propelled missiles. Fuel is sprayed into the combustion chamber and ignited. As the heated air expands, it closes the one-way shutter valve in the front of the engine and exits the engine through the nozzle at the rear. As soon as the pressure inside the combustion chamber decreases, air enters through the shutter valve and more fuel is ignited. The thrust is produced in a series of pulses.

Pump control valve

A control valve in a hydraulic system that allows the pilot to manually direct the output of the hydraulic pump back to the reservoir when no unit is being actuated.

Pureclad

A registered trade name for clad aluminum alloy.

Purge (air conditioning system operation)

To remove all of the moisture and air from a cooling system by flushing the system with a dry gaseous refrigerant.

Push fit

A fit between pieces in a mechanical assembly that is close enough to require the parts to be pushed together. A push fit is looser than a press fit, but closer than a free fit.

Pusher engine

An engine installed with the propeller facing the rear of the aircraft. Thrust produced by the propeller mounted on a pusher engine pushes rather than pulls the aircraft.

Pusher powerplant

A powerplant whose propeller is mounted at the rear of the airplane and pushes, rather than pulls, the airplane through the air.

Pusher propeller

A propeller installed on an aircraft engine so that it faces the rear of the aircraft. Thrust from the propeller pushes rather than pulls the aircraft.

Pythagorean Theorem

An equation used to find the length of a third side of any right triangle when the lengths of two sides are known. The Pythagorean Theorem states that a2 + b2 = c2 . The square of the hypotenuse (side opposite the right angle) is equal to the sum of the squares of the other two sides (a and b)

Quartersawed wood

Wood sawed from a tree in such a way that the annual rings cross the plank at an angle greater than 45°.

Quenching

The rapid cooling of metal in the heat-treatment process.

Quick-disconnect fitting

A hydraulic line fitting that seals the line when the fitting is disconnected. Quick-disconnect fittings are used on the lines connected to the engine-driven hydraulic pump. They allow the pump to be disconnected and an auxiliary hydraulic power system connected to perform checks requiring hydraulic power while the aircraft is in the hangar.

Quill shaft

A type of shaft used to couple parts of an engine that are subject to torsional loads. A quill shaft is a long, hardened steel shaft with splines on each end. One end splines into the drive shaft and the other end splines into the device being driven. Torsional vibrations are absorbed by the quill shaft twisting.

Quotient

The result of dividing two numbers.

Rack-and-pinion actuator

A form of rotary actuator where the fluid acts on a piston on which a rack of gear teeth is cut. As the piston moves, it rotates a pinion gear which is mated with the teeth cut in the rack.

Radar (radio detecting and ranging)

Radio equipment that utilizes reflected pulse signals to locate and determine the distance to any reflecting object within its range.

Radial

A directional line radiating outward from a radio facility, usually a VOR. When an aircraft is flying outbound on the 330° from the station.

Radial bearing load

The load on a bearing perpendicular to the shaft on which the bearing is mounted. Centrifugal loads are radial loads.

Radial engine (static radial)

A form of reciprocating engine in which the cylinders radiate out from a small central crankcase. The pistons in the cylinders drive a central crankshaft which in turn drives the propeller.

Radial tire

A pneumatic tire in which the ply cords extend to the beads and are laid substantially at 90 degrees to the centerline of the tire. Additional stabilization is provided by essentially inextensible belting in the tread area.

Radial-inflow turbine

A turbine, similar in appearance to a centrifugal compressor rotor. Radial-inflow turbines are used to drive the compressor in reciprocating engine turbochargers and some of the smaller APU turbine engines. Hot gases flow into the turbine from its outside rim, then radially inward through the vanes and out of the turbine at its center.

Radiant energy

Electromagnetic waves of certain frequencies produce heat when they are absorbed by the bodies they strike such as x-rays, light rays, and infrared rays

Radiation

The continuous emission of energy from the surface of all bodies.

Radiation

See electromagnetic radiation.

Radical sign

The symbol √, used to indicate the root of a number.

Radiographic inspection

Inspection using radiography to locate defects or flaws in airframe structures or engines with little or no disassembly.

Radius (circle)

Equal to one-half the diameter of a circle.

Radius dimpling

A process of preparing a hole in sheet metal for flush riveting. A cone-shaped male die forces the edges of the rivet hole into the depression in a female die. Radius dimpling forms a round-edged depression into which the rivet head fits.

Radome

A nonmetallic cover used to protect the antenna assembly of a radar system.

Ram air

Air whose pressure has been increased by the forward motion of the aircraft. Ram air pressure is the same as pitot pressure.

Ram drag

The loss of thrust produced by a turbojet or turbofan engine caused by the increase of velocity of air entering the engine. Ram drag is the difference between gross thrust and net thrust.

Ram pressure

Pressure produced when a moving fluid is stopped.

Ram-recovery speed

The speed of an aircraft at which the ram effect caused by the forward movement increases the air pressure at the compressor inlet so that it is the same as that of the ambient air.

Ramjet engine

The simplest type of air-breathing reaction engine. Air entering the front of the engine at a high velocity has fuel sprayed into it and ignited. A barrier formed by the incoming air forces the expanding gases to leave through the nozzle at the rear. The energy added by the burning fuel accelerates the air and produces a forward thrust. Ramjet engines are used in some military unmanned aircraft that are initially boosted to a speed high enough for the engine to function.

Range markings

Colored marks on an instrument dial that identify certain ranges of operation as specified in the aircraft maintenance or flight manual and listed in the appropriate aircraft Type Certificate Data Sheets or Aircraft Specifications. Color coding directs attention to approaching operating difficulties. Airspeed indicators and most pressure and temperature indicators are marked to show the various ranges of operation. These ranges and colors are the most generally used: Red radial line, do not exceed. Green arc, normal operating range. Yellow arc, caution range. Blue radial line, used on airspeed indicators to show best single- engine rate of climb speed. White arc, used on airspeed indicators to show flap operating range.

Range Reference

The CDTI feature of displaying range rings or other range markings at specified radii from the own-ship symbol.

Rated 2-minute OEI Power

With respect to rotorcraft turbine engines, the approved brake horsepower developed under static conditions at specified altitudes and temperatures within the operating limitations established for the engine underpart 33 of this chapter, for continuation of one flight operation after the failure or shutdown of one engine in multiengine rotorcraft, for up to three periods of use no longer than 2 minutes each in any one flight, and followed by mandatory inspection and prescribed maintenance action.

Rated 21⁄2-minute OEI power

With respect to rotorcraft turbine engines, the approved brake horsepower developed under static conditions at specified altitudes and temperatures within the operating limitations established for the engine underpart 33 of this chapterfor periods of use no longer than 21⁄2 minutes each after the failure or shutdown of one engine of a multiengine rotorcraft.

Rated 30-minute OEI power

With respect to rotorcraft turbine engines, the approved brake horsepower developed under static conditions at specified altitudes and temperatures within the operating limitations established for the engine underpart 33 of this chapter, and limited in use to one period of use no longer than 30 minutes after the failure or shutdown of one engine of a multiengine rotorcraft.

Rated 30-second OEI Power

With respect to rotorcraft turbine engines, the approved brake horsepower developed under static conditions at specified altitudes and temperatures within the operating limitations established for the engine underpart 33 of this chapter, for continuation of one flight operation after the failure or shutdown of one engine in multiengine rotorcraft, for up to three periods of use no longer than 30 seconds each in any one flight, and followed by mandatory inspection and prescribed maintenance action.

Rated continuous OEI power

With respect to rotorcraft turbine engines, the approved brake horsepower developed under static conditions at specified altitudes and temperatures within the operating limitations established for the engine underpart 33 of this chapter, and limited in use to the time required to complete the flight after the failure or shutdown of one engine of a multiengine rotorcraft.

Rated maximum continuous augmented thrust

With respect to turbojet engine type certification, the approved jet thrust that is developed statically or in flight, in standard atmosphere at a specified altitude, with fluid injection or with the burning of fuel in a separate combustion chamber, within the engine operating limitations established underPart 33 of this chapter, and approved for unrestricted periods of use.

Rated maximum continuous power

With respect to reciprocating, turbopropeller, and turboshaft engines, the approved brake horsepower that is developed statically or in flight, in standard atmosphere at a specified altitude, within the engine operating limitations established under part 33, and approved for unrestricted periods of use.

Rated maximum continuous thrust

With respect to turbojet engine type certification, the approved jet thrust that is developed statically or in flight, in standard atmosphere at a specified altitude, without fluid injection and without the burning of fuel in a separate combustion chamber, within the engine operating limitations established underpart 33 of this chapter, and approved for unrestricted periods of use.

Rated takeoff augmented thrust

With respect to turbojet engine type certification, the approved jet thrust that is developed statically under standard sea level conditions, with fluid injection or with the burning of fuel in a separate combustion chamber, within the engine operating limitations established underpart 33 of this chapter, and limited in use to periods of not over 5 minutes for takeoff operation.

Rated takeoff power

With respect to reciprocating, turbopropeller, and turboshaft engine type certification, the approved brake horsepower that is developed statically under standard sea level conditions, within the engine operating limitations established under part 33, and limited in use to periods of not over 5 minutes for takeoff operation.

Rated takeoff thrust

With respect to turbojet engine type certification, the approved jet thrust that is developed statically under standard sea level conditions, without fluid injection and without the burning of fuel in a separate combustion chamber, within the engine operating limitations established underpart 33 of this chapter, and limited in use to periods of not over 5 minutes for takeoff operation.

Rating

A statement that, as a part of a certificate, sets forth special conditions, privileges, or limitations.

Ratio

The comparison of two numbers or quantities.

Ratiometer indicator

An analog temperature measuring instrument in which the pointer deflection is proportional to the ratio between the current flowing in an internal reference circuit and that flowing through the temperature-sensing probe.

Reach (spark plug specification)

The length of the threads on the shell of a spark plug.

Reaction engine

A form of heat engine that produces thrust by heating a mass of air inside the engine and discharging it at a high velocity through a specially shaped nozzle. The amount of thrust is determined by the mass of the air and the amount it is accelerated.

Reactive power

Wattless power in an AC circuit. It is the power consumed in the inductive and capacitive reactances. Reactive power is expressed in volt-amps reactive (var) or in kilovolt-amps reactive (kvar).

Reamed fir

The fit of a shaft in a hole in which the hole is drilled undersize and cut with a reamer to the correct diameter. Reamed holes have smooth walls and a consistent diameter.

Reamers

Tools made of either carbon tool steel or high- speed steel that are used to smooth and enlarge holes to exact size.

Rebreather oxygen mask

A type of oxygen mask used with a continuous flow oxygen system. Oxygen continuously flows into the bottom of the loose-fitting rebreather bag on the mask. The wearer of the mask exhales into the top of the bag. The first air exhaled contains some oxygen, and this air goes into the bag first. The last air to leave the lungs contains little oxygen, and it is forced out of the bag as the bag is filled with fresh oxygen. Each time the wearer of the mask inhales, the air first exhaled, along with fresh oxygen, is taken into the lungs.

Rebuilt engine

A used engine that has been completely disassembled, inspected, repaired as necessary, and reassembled, tested, and approved in the same manner and to the same tolerances and limits as a new engine, using either new or used parts. However, all parts used must conform to all production drawings, tolerances, and limits for new parts, or be of approved oversize or undersize dimensions for a new engine. According to 14 CFR part 91, section 91.421, a rebuilt engine is considered to have no precious operating history and may be issued a zero-time logbook. Only the engine manufacturer can rebuild an engine and issue a zero- time record.

Recap and Recapping

Synonymous with the terms retread and retreading.

Receiver Autonomous Integrity Monitoring (RAIM)

Any algorithm that verifies the integrity of the position output using GPS measurements, or GPS measurements and barometric aiding, is considered a RAIM algorithm. An algorithm that uses additional information (such as a multi-sensor system with inertial reference system) to verify the integrity of the position output may be acceptable as a RAIM equivalent. Within this AC, the term RAIM is a synonym for aircraft-based augmentation system (ABAS) and is used to refer to both RAIM and RAIM-equivalent algorithms.

Receiver-dryer

The component in a vapor-cycle cooling system that serves as a reservoir for the liquid refrigerant. The receiver-dryer contains a desiccant that absorbs any moisture that may be in the system.

Reciprocating engine

A type of heat engine that changes the reciprocating (back-and-forth) motion of pistons inside the cylinders into rotary motion of a crank-shaft.

Reconditioning

The repair of major or minor blade damage caused by erosion or striking small objects during normal operation. Reconditioning consists of penetrant inspection, refinishing, and re-balancing. It is accomplished on an irregular basis as necessary and required.

Rectangle

A plane surface with four sides whose opposite sides are parallel and whose angles are all right angles.

Rectification (arc welding condition)

A condition in AC- electric arc welding in which oxides on the surface of the metal act as a rectifier and prevent electrons flowing from the metal to the electrode during the half cycle when the electrode is positive.

Rectifier

A device for converting alternating current to direct current.

Rectifier

A device for converting alternating current to direct current.

Rectifier

A device that allows electrons to flow in one direction while preventing their flow in the opposite direction. Rectifiers are used to change AC into DC.

Reducing flame

See carburizing flame.

Reed valve

A thin, leaf-type valve mounted in the valve plate of an air conditioning compressor to control the flow of refrigerant gases into and out of the compressor cylinders.

Reference Aircraft

An aircraft within the ITP geometric criteria for usage as reference when requesting an ITP clearance.

Reheat system

The British name for an afterburner. See afterburner.

Reid vapor pressure

The amount of pressure that must be exerted on a liquid to keep it from vaporizing. Reid vapor pressure is measured at 100°F.

Reinforcing tape

A narrow strip of woven fabric material placed over the fabric as it is being attached to the aircraft structure with rib lacing cord. This tape carries a large amount of the load and prevents the fabric tearing at the stitches.

Reinforcing tape

A narrow woven cotton or polyester tape used over aircraft fabric to reinforce it at the stitching attachments.

Rejuvenator

A finishing material used to restore resilience to an old dope film. Rejuvenator contains strong solvents to open the dried-out film and plasticizers to restore resilience to the old dope.

Relative humidity

The ratio of the amount of water vapor actually present in the atmosphere to the amount that would be present if the air were saturated at the prevailing temperature and pressure.

Relative wind

The direction the wind strikes an airfoil.

Relay

An electrical component which uses a small amount of current flowing through a coil to produce a magnetic pull to close a set of contacts through which a large amount of current can flow. The core in a relay coil is fixed.

Relay

An electrically-operated remote-control switch.

Reliability

The ability of an aircraft engine to perform its designed functions under widely varying operating conditions.

Relief hole

A hole drilled at the point at which two bend lines meet in a piece of sheet metal. This hole spreads the stresses caused by the bends and prevents the metal cracking.

Relief valve

A pressure-control valve that relieves any pressure over the amount for which it is set. They are damage- preventing units used in both hydraulic and pneumatic systems. In an aircraft hydraulic system, pressure relief valves prevent damaging high pressures that could be caused by a malfunctioning pressure regulator, or by thermal expansion of fluid trapped in portions of the system.

Remainder

The leftover number in the process of division.

Repair

A maintenance procedure in which a damaged component is restored to its original condition, or at least to a condition that allows it to fulfill its design function.

Repair

The correction, on an irregular basis as necessary, of minor or major damage caused by a physical mishap. The amount, degree, and extent of damage determine whether or not the propeller can be repaired as a minor repair in the field by a mechanic.

Repair

The restoration of an aircraft component to its previous state.

Repair station

A maintenance facility certificated under 14 CFR part 145 to perform maintenance functions.

Residual magnetic particle inspection

A form of magnetic particle inspection for small steel parts that have a high degree of retentivity. The part is magnetized, removed, and inspected away from the magnetizing machine.

Residual magnetism

The magnetism that remains in the field frame of a generator when no current is flowing in the field coils.

Residual voltage

The voltage produced in a generator armature when the armature is rotated in the residual magnetism.

Resin

Vast profusion of natural and increasingly, synthetic materials used as adhesives, fillers, binders and for insulation.

Resistance

The opposition a device or material offers to the flow or current.

Resistance

The opposition a device or material offers to the flow or current.

Resistor spark plug

A shielded spark plug with a resistor between the ignition lead terminal and the center electrode. The resistor stops the flow of secondary current when its voltage drops to a specified value. The resistor prevents capacitive afterfiring.

Resonance

The increase in amplitude of vibrations of an electric or mechanical system exposed to a periodic force whose frequency is equal or very close to the natural frequency of the system.

Resonance Method (Ringing) of Ultrasonic inspection

A method of detecting material thickness or indications of internal damage by injecting variable frequency ultrasonic energy into a material. A specific frequency of energy will produce the clearest indication of damage in a given thickness of material. When the equipment is calibrated for a specific thickness, and this thickness changes, an aural or visual alert is given.

Resonant frequency

The frequency of a source of vibration that is exactly the same as the natural vibration frequency of the structure.

Resonate

A mechanical system is said to resonate when its natural vibration frequency is exactly the same as the frequency of the force applied. When an object resonates at a particular frequency, the amplitude in its vibration will increase immensely as that frequency is reached and will be less on either side of that frequency.

Restrictor

A fluid power system component that controls the rate of actuator movement by restricting the flow of fluid into or out of the actuator.

Retard breaker points

A set of breaker points in certain aircraft magnetos that are used to provide a late (retarded) spark for starting the engine.

Retarded sparks

The timing of the firing of the spark plugs used to start a reciprocating engine. The sparks for starting occur later in terms of crankshaft rotation than those used for normal operation. Retarding the sparks prevent the engine from kicking back when it is being started.

Retarder (finishing system component)

Dope thinner that contains certain additives that slow its rate of evaporation enough to prevent dope blushing.

Retentivity

The ability of a material to hold its magnetism after the magnetizing field has been removed.

Retentivity

The ability of a magnetizable material to retain the alignment of the magnetic domains after the magnetizing force has been removed. Hard steel normally has a high retentivity, while soft iron and electrical steel both have very low retentivity.

Retread

The replacement of the tread rubber on an aircraft tire.

Retread

The methods of restoring a used tire by renewing the tread alone, or by renewing the tread plus one or both sidewalls. It refers as well to the process of extending new sidewall material to cover the bead area of the tire. Repairs are included in the retreading of tires.

Retreating blade

The blade on a helicopter rotor whose tip is moving in the direction opposite to that in which the helicopter is moving.

Retreating blade stall

The stall of a helicopter rotor disc that occurs near the tip of the retreating blade. A retreating blade stall occurs when the flight airspeed is high and the retreating blade airspeed is low. This results in a high angle of attack, causing the stall.

Return manifold

The portion of a fluid power system through which the fluid is returned to the reservoir.

Reverse polarity welding

DC-electric arc welding in which the electrode is positive with respect to the work.

Reverse-flow combustor

A type of combustor in which the air from the compressor enters the combustor outer case and reverses its direction as it flows into the inner liner. It again reverses its direction as it flows into the inner liner. It again reverses its direction before it flows through the turbine. Reverse-flow combustors are used where engine length is critical.

Rheostat

A variable resistor used to vary the amount of current flowing in a circuit.

Rib

Part of primary structure, whose purpose is to maintain profile of airfoil and support fabric or thin wood covering.

Rib thread

A series of circumferential grooves cut into the tread of a tire. This tread pattern provides superior traction and directional stability on hard-surfaced runways.

Ribbon direction

The direction in a piece of honeycomb material that is parallel to the length of the strips of material that make up the core.

Rich blowout

A condition in which the fire in a gas turbine engine goes out because the air/fuel mixture ratio is too rich to sustain combustion.

Rich mixture

A air/fuel mixture that contains less than 15 parts of air to 1 part of fuel, by weight.

Riffle file

A hand file with its teeth formed on a curved surface that resembles a spoon.

Rigid conduit

Aluminum alloy tubing used to house electrical wires in areas where they are subject to mechanical damage.

Rigidity in space

The characteristic of a gyroscope that prevents its axis of rotation tilting as the earth rotates. This characteristic is used for attitude gyro instruments.

Rime ice

A rough ice that forms on aircraft flying through visible moisture, such as a cloud, when the temperature is below freezing. Rime ice disturbs the smooth airflow as well as adding weight.

Rivet cutters

Special cutting pliers that resemble diagonal cutters except that the jaws are ground in such a way that they cut the rivet shank, or stem, off square.

Rivet set

A tool used to drive aircraft solid rivets. It is a piece of hardened steel with a recess the shape of the rivet head in one end. The other end fits into the rivet gun.

Rms

Root mean square. A dimension that is the square root of the average of an infinite number of varying values. An rms dimension is used to indicate the allowable surface roughness of a reciprocating engine cylinder wall.

Rocker arm

A pivoted arm on the cylinder head of a reciprocating engine. The pushrod forces one end of the rocker arm up, and as the other end moves down, it forces the poppet valve off of its seat.

Rocker box

The enclosed part of a reciprocating engine cylinder that houses the rocker arm and valve mechanism.

Rocket

An aircraft propelled by ejected expanding gases generated in the engine from self-contained propellants and not dependent on the intake of outside substances. It includes any part which becomes separated during the operation.

Rocket engine

A form of reaction engine whose fuel and oxidizer contain all of the oxygen needed for the release of heat energy. The released heat expands the gases which are ejected at a high velocity from a nozzle at the rear of the rocket. Because rocket engines carry their own oxygen, they can operate in outer space where there is no atmosphere.

Rocking shaft

A shaft used in the mechanism of a pressure measuring instrument to change the direction of movement by 90° and to amplify the amount of movement.

Roll (aircraft maneuver)

Rotation of an aircraft about its longitudinal axis.

Root

A number that when multiplied by itself a specified number of times will produce a given number. The two most commonly used roots are the square root and the cube root.

Roots-type air compressor

A positive-displacement air pump that uses two intermeshing figure-8-shaped rotors to move the air.

Rosette weld

A method of securing one metal tube inside another by welding. Small holes are drilled in the outer tube and the inner tube is welded to it around the circumference of the holes.

Rotary actuator

A fluid power actuator whose output is rotational. A hydraulic motor is a rotary actuator.

Rotary radial engine

A form of reciprocating engine used in some early aircraft. The crankshaft is rigidly attached to the airframe, and the propeller, crankcase, and cylinders all revolve as a unit.

Rotating combustion (RC) engine

A form of internal combustion engine in which a rounded, triangular-shaped rotor with sliding seals at the apexes forms the combustion space inside an hourglass-shaped chamber. Expanding gases from the burning air/fuel mixture push the rotor around and turn a geared drive shaft in its center. The RC engine was conceived in Germany by Felix Wankel in 1955.

Rotorcraft

A heavier-than-air aircraft that depends principally for its support in flight on the lift generated by one or more rotors.

Rotorcraft-load combination

The combination of a rotorcraft and an external-load, including the external- load attaching means. Rotorcraft-load combinations are designated as Class A, Class B, Class C, and Class D, as follows: (1) Class A rotorcraft-load combination means one in which the external load cannot move freely, cannot be jettisoned, and does not extend below the landing gear. (2) Class B rotorcraft-load combination means one in which the external load is jettisonable and is lifted free of land or water during the rotorcraft operation. (3) Class C rotorcraft-load combination means one in which the external load is jettisonable and remains in contact with land or water during the rotorcraft operation. (4) Class D rotorcraft-load combination means one in which the external-load is other than a Class A, B, or C and has been specifically approved by the Administrator for that operation.

Routine inspection

A visual examination or check of an item in which no disassembly is required

Roving

A lightly twisted roll or strand of fibers.

Ruddervators

The two movable surfaces on a V-tail empennage. When these two surfaces are moved together with the in-and-out movement of the control yoke, they act as elevators, and when they are moved differentially with the rudder pedals, they act as the rudder.

Run in

A time of controlled operation of a new or freshly overhauled engine that allows the moving parts to wear together.

Run up

A procedure in which an aircraft engine is operated on the ground to determine its condition and performance.

Runout

A measure of the amount a shaft, flange, or disc is bent or fails to run true. Runout is normally measured with a dial indicator.

Sacrificial corrosion

A method of corrosion protection in which a surface is plated with a metal less noble than itself. Any corrosion will attack the plating rather than the base metal.

Saddle gusset

A piece of plywood glued to an aircraft structural member. The saddle gusset has a cutout to hold a backing block or strip tightly against the skin to allow a nailing strip to be used to apply pressure to a glued joint in the skin.

Safety cage

A reinforced structure that has been specifically designed to protect service personnel from the effects of any explosive event that may occur during the pressurization of a tire/wheel assembly.

Safety gap

A location in a magneto that allows a spark to jump to ground from the secondary circuit before the voltage rises high enough to damage the secondary insulation.

Sailplane

A high-performance glider.

Sand casting

A method of molding metal parts in a mold made of sand. A pattern that duplicates the part to be molded is made of wood and is covered with a special casting sand that contains a resin to bind it. The mold is separated along a special parting line, and the pattern is removed. The mold is put back together, and molten metal is poured into the cavity. When the metal cools, the sand is broken away from the molded part. Sand casting is less expensive than permanent-mold casting.

Sandwich construction

A structural panel concept consisting in its simplest form of two relatively thin, parallel sheets (face sheets) of structural material bonded to and separated by a relatively thick, lightweight core. High strength-to-weight ratios are obtained with sandwiched materials.

Sandwich material

A type of composite structural material in which a core material is bonded between face sheets of metal or resin-impregnated fabric.

Satellite-Based Augmentation System (SBAS)

A wide coverage augmentation system in which the user receives augmentation information from a satellite-based transmitter. In the United States, this is referred to as Wide Area Augmentation System (WAAS).

Satin-weave fabric

Fabric in which the warp threads pass under one fill thread and over several others. Satin-weave fabrics are used when the lay-up must be made over complex shapes.

Saybolt Seconds Universal (SSU) viscosity

A measurement of viscosity (resistance to flow)of a lubricating oil. The number of seconds needed for 60 milliliters of oil at a specified temperature to flow through a calibrated orifice. The viscosity number used for commercial aviation engine lubricating oil relates closely to the SSU viscosity of the oil at 210°F.

Scarf joint

A joint in a wood structure in which the ends to be joined are cut in a long taper, normally about 12:1, and fastened together by gluing. A glued scarf joint makes a strong splice because the joint is made along the side of the wood fibers rather than along their ends.

Scarf joint

A joint made by cutting away similar angular segments of two adherents and bonding the adherents with cut areas fitted together.

Scavenge subsystem

The subsystem in the lubrication system of a gas turbine engine that collects oil after it has lubricated the bearings and gears and returns it to the oil tank.

Schematic diagram

A diagram of an electrical system in which the system components are represented by symbols rather than drawings or pictures of the actual devices.

Schematic diagram

A diagram that locates components with respect to each other within a system.

Schrader valve

A type of service valve used in an air conditioning system. This is a spring-loaded valve much like the valve used to put air into a tire.

Scientific notation

Used as a type of shorthand to express very large or very small numbers. For example, to express 1,250,000,000,000 in scientific notation is 1.25 × 1012.

Scimitar shape

The shape of the blades of the propellers mounted on UHB engines. The name is derived from the shape of a curved Asian sword that has its edge on the convex side. See UHB engine.

Scissors

A name commonly used for torque links. See torque links.

Score

Groove-like indentations from rubbing by a hard, rough surface. See gouge.

Score

A surface tear or break on a surface that has a depth and length ranging between a scratch and a gouge.

Scramjet

Supersonic combustion ramjet. A special type of ramjet engine whose fuel can be ignited while the vehicle is mobbing at a supersonic speed.

Scratch

A superficial small cut on a surface.

Scrim cloth

Scrim cloth can be used in repair applications or for reinforcement of other types of materials including fiberglass, concrete and some plastics. When fully cured, the scrim cloth will add reinforcement and mimic the expansion and contraction of the surrounding substrate.

Scuffing

Severe damage to moving parts caused when one metal part moves across another without sufficient lubricant between them. Enough heat is generated by friction to cause the high points of the surfaces to weld together; continued movement tears, or scuffs, the metal.

Scupper

A recess around the filler neck of an aircraft fuel tank. Any fuel spilled when the tank is being serviced collects in the scupper and drains to the ground through a drain line rather than flowing into the aircraft structure.

Sea level engine

A reciprocating engine whose rated takeoff power can be produced only at sea level.

Sea level engine

A reciprocating aircraft engine having a rated takeoff power that is producible only at sea level.

Sea level pressure

The atmospheric pressure at sea level. Average sea level pressure is 29.92 inches of mercury, or 1013.25 millibars.

Sea-level boosted engine

A reciprocating engine that has had its sea-level rated horsepower increased by supercharging. This is the same as a ground-boosted engine.

Secondary Surveillance Radar (SSR)

A radar sensor that listens to replies sent by transponders carried on board airborne targets. SSR sensors, in contrast to primary surveillance radar (PSR) sensors, require the aircraft under surveillance to carry a transponder.

Secondary winding

The winding in a magneto or ignition coil that connects to the distributor rotor. The secondary winding is normally made of very small diameter wire and has a large number of turns, typically about 20,000.

Sectional view

A view obtained by cutting away part of an object to show the shape and construction at the cutting plane.

Sector gear

A part of a gear wheel containing the hub and a portion of the rim with teeth.

See Dutch roll

A yaw damper senses yaw with a rate gyro and moves the rudder an amount proportional to the rate of yaw, but in the opposite direction.

Segmented-rotor brake

A heavy-duty, multiple-disc brake used on large, high-speed aircraft. Stators that are surfaced with a material that retains its friction characteristics at high temperatures are keyed to the axle. Rotors which are keyed into the wheels mesh with the stators. The rotors are made in segments to allow for cooling and for their large amounts of expansion.

Selcal system

Selective calling system. Each aircraft operated by an airline is assigned a particular four-tone audio combination for identification purposes. A ground station keys the signal whenever contact with that particular aircraft is desired. The signal is decoded by the airborne selcal decoder and the crew alerted by the selcal warning system.

Selected Traffic

Traffic for which additional information is requested by the flight crew.

Selective Availability (SA)

A protection technique employed by the Department of Defense that degraded GPS accuracy. Selective availability was discontinued on May 1, 2000.

Selector valve

A flow control valve used in hydraulic systems that directs pressurized fluid into one side of an actuator, and at the same time directs return fluid from the other side of the actuator back to the reservoir. There are two basic types of selector valves: open-center valves and closed-center valves. The four-port closed-center valve is the most frequently used type. See closed-center selector valve and open-center selector valve.

Self-accelerating speed

The speed attained by a gas turbine engine during start-up that allows it to accelerate to its normal idling speed without assistance from the starter.

Selsyn system

A DC synchro system used in remote indicating instruments. The rotor in the indicator is a permanent magnet and the stator is a tapped toroidal coil The transmitter is a circular potentiometer with DC power fed into its wiper which is moved by the object being monitored. The transmitter is connected to the indicator in such a way that rotation of the transmitter shaft varies the current in the sections of the indicator toroidal coil. The magnet in the indicator on which the pointer is mounted locks with the magnetic field produced by the coils and follows the rotation of the transmitter shaft.

Selvage edge

The woven edge of fabric used to prevent the material unraveling during normal handling. The selvage edge, which runs the length of the fabric parallel to the warp threads, is usually removed from materials used in composite construction.

Semiconductor

Any device based on either preferred conduction through a solid in one direction, as in rectifiers, or on a variation in conduction characteristics through a partially conductive material, as in a transistor.

Semiconductor device

Any device based on either preferred conduction through a solid in one direction, as in rectifiers; or on a variation in conduction characteristics through a partially conductive material, as in a transistor.

Semiconductor diode

A two-element electrical component that allows current to pass through it in one direction, but blocks its passage in the opposite direction. A diode acts in an electrical system in the same way a check valve acts in a hydraulic system.

Semiconductor transducer

A piezoelectric crystal that converts input energy of one form, such as pressure, into output energy of another, such as an electrical signal.

Semimonocoque structure

A form of aircraft stressed skin structure. Most of the strength of a semimonocoque structure is in the skin, but the skin is supported on a substructure of formers and stringers that give the skin its shape and increase its rigidity.

Sensible heat

Heat that is added to a liquid causing a change in its temperature but not its physical state.

Sensitivity

A measure of the signal strength needed to produce a distortion-free output in a radio receiver.

Sensor

A measurement device. An air data sensor measures atmospheric pressure and temperature, to estimate pressure altitude, and pressure altitude rate, airspeed, etc. A primary surveillance radar sensor measures its antenna direction and the times of returns of echoes of pulses that it transmits to determine the ranges and bearings of airborne targets. A secondary surveillance radar sensor measures its antenna direction and the times of returns of replies from airborne transponders to estimate the ranges and bearings of airborne targets carrying those transponders.

Separation

Failure of adhesion between components or tearing within a single tire component.

Separation

The minimum distance between aircraft/vehicles allowed by regulations. Separation requirements vary by factors such as radar coverage (none, single, composite), domain (terminal, en route, oceanic), and flight rules (instrument or visual).

Sequence valve

A valve in a hydraulic system that requires a certain action to be completed before another action can begin. Sequence valves are used to assure that the hydraulically actuated wheel-well doors are completely open before pressure is directed to the landing gear to lower it.

Series circuit

A method of connecting electrical components in such a way that all the current flows through each of the components. There is only one path for current to flow.

Series circuit

The most basic electrical circuit in which there is only one possible path for current to flow. Current must pass through the circuit components, the battery and the resistor, one after the other, or “in series.” Series-parallel DC circuits. A grouping of parallel resistors connected in series with other resistors.

Series-parallel circuit

An electrical circuit in which some of the components are connected in parallel and others are connected in series.

Series-wound motor

An electric motor with field coils connected in series with the armature.

Service pressure

The inflation pressure needed to support the maximum operating load for a wheel position. Service pressure is measured with the assembly under load.

Serviceable limits

Limits included in a reciprocating engine overhaul manual. If a part measures outside of the new-parts limits, but within the serviceable limits, it will not likely wear to the point of causing engine failure within the next TBO interval.

Servo

An electrical or hydraulic actuator connected into a flight control system. A small force on the flight deck control is amplified by the servo and provides a large force to move the control surface.

Servo amplifier

An electronic amplifier in an autopilot system that increases the signal from the autopilot enough that it can operate the servos that move the control surfaces.

Servo system

A type of automatic control system in which part of the output is fed back into the input.

Servo tab

A small movable tab built into the trailing edge of a primary control surface of an airplane. The flight deck controls move the tab in such a direction that it produces an aerodynamic force moving the surface on which it is mounted.

Setback

The distance the jaws of a brake must be set back from the mold line to form a bend. Setback for a 90° bend is equal to the inside radius of the bend plus the thickness of the metal being bent. For a bend other than 90°, a K-factor must be used. See also K-factor.

Severe wind and moisture problem (SWAMP) areas

Areas such as wheel wells, wing folds, and near wing flaps, and areas directly exposed to extended weather conditions are considered SWAMP areas on aircraft.

Shaft horsepower

The horsepower actually available at a rotating shaft.

Shake (wood defect)

Longitudinal cracks in a piece of wood, usually between two annual rings.

Shank

The portion of the blade from the butt face to the first blade station.

Shear section

A necked-down section of the drive shaft of a constant-displacement engine-driven fluid pump. If the pump should seize, the shear section will break and prevent the pump from being destroyed or the engine from being damaged. Some pumps use a shear pin rather than a shear section.

Shear strength

The strength of a riveted joint in a sheet metal structure in which the rivets shear before the metal tears at the rivet holes.

Shelf life

The length of time a product is good when it remains in its original unopened container.

Shielded wire

Electrical wire enclosed in a braided metal jacket. Electromagnetic energy radiated from the wire is trapped by the braid and is carried to ground.

Shielding

The electrically conductive covering placed around an electrical component to intercept and conduct to ground any electromagnetic energy radiated from the device.

Shimmy

Abnormal, and often violent, vibration of the nose wheel of an airplane. Shimmying is usually caused by looseness of the nose wheel support mechanism or an unbalanced wheel.

Shimmy damper

A small hydraulic shock absorber installed between the nose wheel fork and the nose wheel cylinder attached to the aircraft structure.

Shock mounts

Resilient mounting pads used to protect electronic equipment by absorbing low-frequency, high amplitude vibrations.

Shock wave

A pressure wave formed in the air by a flight vehicle moving at a speed greater than the speed of sound. As the vehicle passes through the air, it produces sound waves that spread out in all directions. But since the vehicle is flying faster than these waves are moving, they build up and form a pressure wave at the front and rear of the vehicle. As the air passes through a shock wave it slows down, its static pressure increases, and its total energy decreases.

Shop head

The head of a rivet which is formed when the shank is upset.

Short circuit

A low-resistance connection between two points in an electric circuit.

Show-type finish

The type of finish put on fabric-covered aircraft intended for show. This finish is usually made up of many coats of dope, with much sanding and rubbing of the surface between coats.

Shower of Sparks ignition system

A patented ignition system for reciprocating engines. An induction vibrator sends pulsating DC into a set of retard breaker points on one of the magnetos. This provides a hot and retarded spark for starting the engine.

Shunt winding

Field coils in an electric motor or generator that are connected in parallel with the armature.

Shuttle valve

An automatic selector valve mounted on critical components such as landing gear actuation cylinders and brake cylinders. For normal operation, system fluid flows into the actuator through the shuttle valve, but if normal system pressure is lost, emergency system pressure forces the shuttle over and emergency fluid flows into the actuator.

Sidestick controller

A flight deck flight control used on some of the fly-by-wire equipped airplanes. The stick is mounted rigidly on the side console of the flight deck, and pressures exerted on the stick by the pilot produce electrical signals that are sent to the computer that flies the airplane.

Sidewall

The rubber side area of the tire located between the shoulder and the beads that covers the underlying structure and protects the carcass from damage.

Sidewall vents

Specific perforations in the lower sidewalls of tubeless tires, usually located above the rim centering rib, and defined by paint marks. These perforations release detrimental internal casing pressure.

Sight glass (air conditioning system component)

A small window in the high side of a vapor-cycle cooling system. Liquid refrigerant flows past the sight glass, and if the charge of refrigerant is low, bubbles will be seen. A fully charged system has no bubbles in the refrigerant.

Sight line

A line drawn on a sheet metal layout that is one bend radius from the bend-tangent line. The sight line is lined up directly below the nose of the radius bar in a cornice brake. When the metal is clamped in this position, the bend tangent line is in the correct position for the start of the bend.

Signed numbers

A signed number can be either a positive or negative number. A positive number is a number that is greater than zero. A negative number is a number that is less than zero.

Silicon controlled rectifier (SCR)

A semiconductor electron control device. An SCR blocks current flow in both directions until a pulse of positive voltage is applied to its gate. It then conducts in its forward direction, while continuing to block current in its reverse direction.

Silicone rubber

An elastomeric material made from silicone elastomers. Silicone rubber is compatible with fluids that attack other natural or synthetic rubbers.

Silicone rubber

A high temperature (200 °C) plastic insulation that has a substantial silicone content.

Simple Action

A flight crew action that may be performed within a short period of time and without requiring significant concentration that would distract from the main aviation tasks (e.g., a button press). Please refer to AC 25-11( ) for more details.

Sine

A trigonometric function comparing two sides of a right triangle as follows: Sine = opposite side hypotenuse Sine wave. A continuous waveform with a constant frequency and amplitude.

Single-acting actuator

A linear hydraulic or pneumatic actuator that uses fluid power for movement in one direction and a spring force for its return.

Single-action hand pump

A hand-operated fluid pump that moves fluid only during one stroke of the pump handle. One stroke pulls the fluid into the pump and the other forces the fluid out.

Single-disc brakes

Aircraft brakes in which a single steel disc rotates with the wheel between two brake-lining blocks. When the brake is applied, the disc is clamped tightly between the lining blocks, and the friction slows the aircraft.

Single-servo brakes

Brakes that uses the momentum of the aircraft rolling forward to help apply the brakes by wedging the brake shoe against the brake drum.

Single-shaft turbine engine

A turboprop engine in which the propeller reduction gears are driven by the same shaft that drives the compressor for the gas generator.

Single-spool gas-turbine engine

A type of axial-flow- compressor gas turbine engine that has only one rotating element.

Sintered metal

A porous material made by fusing powdered metal under heat and pressure.

Sketch

A simple rough drawing that is made rapidly and without much detail.

Skin radiator

A type of radiator used on some early liquid- cooled racing airplanes. The radiator was made of two thin sheets of brass, slightly separated so the heated coolant could flow between them. Skin radiators were mounted on the surface of the wing, on the sides of the fuselage, or on the floats of seaplanes. Air flowing over the smooth surface of the radiator removed heat from the coolant.

Skydrol hydraulic fluid

The registered trade name for a synthetic, nonflammable, phosphate ester-base hydraulic fluid used in modern high-temperature hydraulic systems.

Slat

A secondary control on an aircraft that allows it to fly at a high angle of attack without stalling. A slat is a section of leading edge of wing mounted on curved tracks that move into and out of the wing on rollers.

Slide caliper

Often used to measure the length of an object. It provides greater accuracy than a ruler.

Slip (propeller specification)

The difference between the geometric and effective pitch of a propeller.

Slip ring

A smooth, continuous ring of brass or copper mounted on the rotor shaft of an electrical generator or alternator. Brushes riding on the smooth surface of the slip ring carry current into and out of the rotor coil.

Slip roll former

A shop tool used to form large radius curves on sheet metal.

Slippage mark

A paint mark extending across the edge of an aircraft wheel onto a tube-type tire. When this mark is broken, it indicates the tire has slipped on the wheel, and there is a good reason to believe the tube has been damaged.

Slipstream area

For the purpose of rib stitch spacing, the slipstream area is considered to be the diameter of the propeller plus one wing rib on each side.

Slot (aerodynamic device)

A fixed, nozzle-like opening near the leading edge of an airplane wing ahead of the aileron. A slot acts as a duct to force high-energy air down on the upper surface of the wing when the airplane is flying at a high angle of attack. The slot, which is located ahead of the aileron, causes the inboard portion of the wing to stall first, allowing the aileron to remain effective throughout the stall.

Slow-blow fuse

A special type of electrical circuit protection device that allows a momentary flow of excess current, but opens the circuit if the excessive flow is sustained.

Slow-blow fuse

An electrical fuse that allows a large amount of current to flow for a short length of time but melts to open the circuit if more than its rated current flows for a longer period.

Sludge

A heavy contaminant that forms in an aircraft engine lubricating oil because of oxidation and chemical decomposition of the oil.

Sludge plugs

Spool-shaped sheet metal plugs installed in the hollow throws of some engine crankshafts.

Slug

The unit of mass equal to that which experiences an acceleration of one foot per second, per second when a force of one pound acts on it. It is equal to 32.174 pounds, or 14.5939 kilograms, of mass. Also called a G-pound.

Small aircraft

Aircraft of 12,500 pounds or less, maximum certificated takeoff weight.

Small unmanned aircraft

An unmanned aircraft weighing less than 55 pounds on takeoff, including everything that is on board or otherwise attached to the aircraft. Small unmanned aircraft system (small UAS)means a small unmanned aircraft and its associated elements (including communication links and the components that control the small unmanned aircraft) that are required for the safe and efficient operation of the small unmanned aircraft in the national airspace system.

Smoke detector

A device that warns the flight crew of the presence of smoke in cargo and/or baggage compartments. Some smoke detectors are of the visual type, others are photoelectric or ionization devices.

Snubber

A device in a hydraulic or pneumatic component that absorbs shock and/or vibration. A snubber is installed in the line to a hydraulic pressure gauge to prevent the pointer fluctuating.

Softwood

Wood from a tree that bears cones and has needles rather than leaves.

Soldering

A method of thermally joining metal parts with a molten nonferrous alloy that melts at a temperature below 800 °F. The molten alloy is pulled up between close-fitting parts by capillary action. When the alloy cools and hardens, it forms a strong, leak-proof connection.

Soldering

A group of welding processes that produces coalescence of materials by heating them to the soldering temperature and by using a filler metal having a liquidus not exceeding 450 °C (840 °F) and below the solidus of the base metals. The filler metal is distributed between the closelyfitted surfaces of the joint by capillary action.

Solenoid

An electrical component using a small amount of current flowing through a coil to produce a magnetic force that pulls an iron core into the center of the coil. The core may be attached to a set of heavy-duty electrical contacts, or it may be used to move a valve or other mechanical device.

Solenoid

A tubular coil for the production of a magnetic field; electromagnet with a core which is able to move in and out.

Solenoid

A loop of wire, often wrapped around a metal core, which produces a magnetic field when an electrical current is passed through it.

Solidity (helicopter rotor characteristic)

The solidity of a helicopter rotor system is the ratio of the total blade area to the disc area.

Solution heat treatment

A type of heat treatment in which the metal is heated in a furnace until it has a uniform temperature throughout. It is then removed and quenched in cold water. When the metal is hot, the alloying elements enter into a solid solution with the base metal to become part of its basic structure. When the metal is quenched, these elements are locked into place.

Sonic venturi

A sonic venturi in a line between a turbine engine or turbocharger and a pressurization system. When the air flowing through the sonic venturi reaches the speed of sound, a shock wave forms across the throat of the sonic venturi and limits the flow. A sonic venturi is also called a flow limiter.

Sound suppressor

The airframe component that replaces the turbine engine tail pipe. It reduces the distance the sounds made by the exhaust gases propagate by converting low- frequency vibrations.

Source Integrity Level (SIL)

The probability of the reported horizontal position exceeding the radius of containment defined by the NIC without alerting, assuming the avionics has no faults. A SIL of 3 is required by § 91.227.

Spacing

A distance maintained from another aircraft for specific operations.

Spar

Main spanwise structural member(s) of an aircraft wing or rotorcraft rotor. A wing may have one or two made into a single strong box to which secondary leading and trailing structures are added.

Specific gravity

The ratio of the density of a material to the density of pure water.

Specific gravity

The ratio of the mass of a solid or liquid to the mass of an equal volume of water.

Specific heat

The number of BTUs of heat energy needed to change the temperature of one pound of a substance 1 °F.

Specific heat

The quantity of heat necessary to increase the temperature of a unit of the mass of a substance 1 °C. The specific heat of a substance is the ratio of its specific heat capacity to the specific heat capacity of water.

Specific weight

The ratio of the weight of an aircraft engine to the brake horsepower it develops.

Speed brakes

A secondary control of an airplane that produces drag without causing a change in the pitch attitude of the airplane. Speed brakes allow an airplane to make a steep descent without building up excessive forward airspeed.

Speed of sound

The speed of sound at sea level under standard temperature and pressure conditions is 1,108 feet per second or 658 knots.

Spike knot

A knot that runs through the depth of a beam perpendicular to the annual rings. Spike knots appear most frequently in quartersawed wood.

Spin

A flight maneuver in which an airplane descends in a corkscrew fashion. One wing is stalled and the other is producing lift.

Spiral grain

A type of growth in wood which the fibers take a spiral course about the bole of a tree instead of the normal vertical course. The spiral may extend right-handed or left-handed around the tree trunk.

Spirit level

A curved glass tube partially filled with a liquid, but with a bubble in it. When the device in which the tube is mounted is level, the bubble will be in the center of the tube.

Spirit level

A leveling instrument placed on or against a specified place on the aircraft. Spirit levels have vials that are full of liquid, except for a small air bubble. When the air bubble is centered between the two black lines, a level condition is indicated.

Splayed patch (wood structure repair)

A type of patch made in an aircraft plywood structure in which the edges of the patch are tapered for approximately five times the thickness of the plywood. A splayed patch is not recommended for use on plywood less than 1⁄10 inch thick.

Spline

Parallel slots cut in the periphery of a shaft, parallel to its length. Matching slots, cut into the hub or wheel that fits on the shaft, lock the shaft into the device to transmit torque.

Split (wood defect)

A longitudinal crack in a piece of wood caused by externally induced stress.

Split bus

A type of electrical bus that allows all of the voltage-sensitive avionic equipment to be isolated from the rest of the aircraft electrical system when the engine is being started or when the ground-power unit is connected.

Split-rocker switch

An electrical switch whose operating rocker is split so one half of the switch can be opened without affecting the other half. Split-rocker switches are used as aircraft master switches. The battery can be turned on without turning on the alternator, but the alternator cannot be turned on without also turning on the battery. The alternator can be turned off without turning off the battery, but the battery cannot be turned off without also turning off the alternator.

Spoilers

Flight controls that are raised up from the upper surface of a wing to destroy, or spoil, lift. Flight spoilers are used in conjunction with the ailerons to decrease lift and increase drag on the descending wing. Ground spoilers are used to produce a great amount of drag to slow the airplane on its landing roll.

Spongy brakes

Hydraulic brakes whose pedal has a spongy feel because of air trapped in the fluid.

Spontaneous combustion

Self-ignition of a material caused by heat produced in the material as it combines with oxygen from the air.

Sprag clutch

A freewheeling, nonreversible clutch that allows torque to be applied to a driven unit in one direction only.

Springback

A condition in the rigging of an aircraft engine control in which the stop at the engine is reached before the stop in the flight deck. The flight deck control moves slightly after the stop in the engine is reached, and when it is released, it springs back slightly.

Springwood

The portion of an annual ring in a piece of wood formed principally during the first part of the growing season, the spring of the year. Springwood is softer, more porous, and lighter than the summerwood.

Spur-gear pump

A form of constant-displacement fluid pump that uses two meshing spur-gears mounted in a close fitting housing. Fluid is taken into the housing where it fills the space between the teeth of the gears and is carried around the housing as the gears rotate. On the discharge side of the pump, the teeth of the two gears mesh, and the fluid is forced out of the pump.

Square

A four-sided plane figure whose sides are all the same length, whose opposite sides are parallel, and whose angles are all right angles.

Square root

A non-negative number that must be multiplied by itself to equal a given number.

Squat switch

An electrical switch actuated by the landing gear scissors on the oleo strut. When no weight is on the landing gear, the oleo piston is extended and the switch is in one position; but when weight is on the gear, the oleo strut compresses and the switch changes its position.

Squat switch

An electrical switch actuated by the landing gear scissors on the oleo strut. When no weight is on the landing gear, the oleo piston is extended and the switch is in one position, but when weight is on the gear, the oleo strut compresses and the switch changes its position. Squat switches are used in antiskid brake systems, landing gear safety circuits, and cabin pressurization systems.

Squealer tip (compressor blade tip)

See profile tip.

Squeeze film bearings

Another name for oil-damped bearings. See oil-damped bearings.

Squib

An explosive device in the discharge valve of a high-rate-discharge container of fire-extinguishing agent. The squib drives a cutter into the seal in the container to discharge the agent.

Stabilator

A flight control on the empennage of an airplane that acts as both a stabilizer and an elevator. The entire horizontal tail surface pivots and is moved as a unit.

Stability

The characteristic of an aircraft that causes it to return to its original flight condition after it has been disturbed.

Stabilons

Small wing-like horizontal surfaces mounted on the aft fuselage to improve longitudinal stability of airplanes that have an exceptionally wide center of gravity range.

Stage length

The distance between landing points in airline operation.

Stage of a compressor

One disc of rotor blades and the following set of stator vanes in an axial-flow compressor.

Staggered timing

Ignition timing that causes the spark plug nearest the exhaust valve to fire a few degrees of crankshaft rotation before the spark plug nearest the intake valve.

Stagnation point

The point on the leading edge of a wing at which the airflow separates, with some flowing over the top of the wing and the rest below the wing.

Stall

A flight condition in which an angle of attack is reached at which the air ceases to flow smoothly over the upper surface of an airfoil. The air becomes turbulent and lift is lost.

Stall strip

A fixed device employed on the leading edge of fixed-wing aircraft to initiate flow separation at chosen locations on the wing during high-angle of attack flight, so as to improve the controllability of the aircraft when it enters stall.

Standard day conditions

Conditions that have been decided upon by the ICAO for comparing all aircraft and engine performance. The most basic standard day conditions are: temperature, 15°C or 59°F; altitude, mean sea level; pressure, 29.92 inches of mercury.

Standard empty weight

The weight of the airframe, engines, all permanently installed equipment, and unusable fuel. Depending upon the part of the Federal regulations under which the aircraft was certificated, either the undrainable oil or full reservoir of oil is included.

Standard J-1

A World War I training airplane powered by a Curtiss OX-5 engine.

Standard weights

Values used in weight and balance calculations if specific weight for an item is unknown. The following are examples: • Aviation gasoline 6 pounds per gallon • Crew and passengers 170 pounds per person • Lubricating oil 7.5 pounds per gallon • Turbine fuel 6.7 pounds per gallon • Water 8.35 pounds per gallon Static stability. The initial response that an airplane displays after its equilibrium is disrupted.

Standpipe

A pipe which protrudes upward from the base of an oil tank and through which oil used for normal engine lubrication is drawn. In the event of a catastrophic leak when all oil available to the engine-driven pump is lost overboard, enough oil is available from an outlet below the standpipe to feather the propeller.

Standpipe

A pipe sticking up in a tank or reservoir that allows part of the tank to be used as a reserve, or standby, source of fluid.

Starter-generator

A single-component starter and generator used on many smaller gas-turbine engines. It is used to start the engine, and when the engine is running, its circuitry is shifted so that it acts as a generator.

Starter-generator

Asingle-component starter and generator used on many of the smaller gas-turbine engines. It is used as a starter, and when the engine is running, its circuitry is shifted so that it acts as a generator.

State Vector

An aircraft’s current horizontal position, vertical position, horizontal velocity, vertical velocity, and navigational accuracy and integrity.

Static

Still, not moving.

Static air pressure

Pressure of the ambient air surrounding the aircraft. Static pressure does not take into consideration any air movement.

Static dischargers

Devices connected to the trailing edges of control surfaces to discharge static electricity harmlessly into the air. They discharge the static charges before they can build up high enough to cause radio receiver interference.

Static pressure

The pressure of an unmoving fluid.

Static rpm

The number of revolutions per minute an aircraft engine can produce when the aircraft is not moving.

Static stability

The characteristic of an aircraft that causes it to return to straight and level flight after it has been disturbed from that condition.

Stations

Locations perpendicular to the blade center line at which dimensions are checked.

Stator

The part of an AC generator or motor which contains the stationary winding.

Steam cooling

A method of liquid cooling in which the coolant, normally water, is allowed to absorb enough heat that it boils. The steam gives up its heat when it condenses back into a liquid.

Stellite

A nonferrous alloy of cobalt, chromium, and tungsten. Stellite is hard, water resistant, and corrosion resistant, and it does not soften until its temperature is extremely high. Stellite is welded to the faces of many reciprocating engine exhaust valves that operate at very high temperatures.

Stepping motor

A precision electric motor whose output shaft position is changed in steps by pulses from the control device. Stepping motors can make high-torque changes in small angular increments to their output shaft.

Stoddard solvent

A petroleum product, similar to naphtha, used as a solvent and a cleaning fluid.

Stoichiometric mixture

The air/fuel mixture ratio that, when burned, leaves no uncombined oxygen nor any free carbon. It releases the maximum amount of heat, and therefore produces the highest exhaust gas temperature. A stoichiometric mixture of gasoline and air contains 15 pounds of air for 1 pound of gasoline.

Stop drilling

A method of stopping the growth of a crack in a piece of metal or transparent plastic by drilling a small hole at the end of the crack. The stresses are spread out all around the circumference of the hole rather than concentrated at the end of the crack.

Straight polarity welding

DC-electric arc welding in which the electrode is negative with respect to the work.

Straight-through combustor

A combustor in a gas turbine engine through which the air from the compressor to the turbine flows in an essentially straight line.

Strain

A deformation or physical change in a material caused by a stress.

Strain

A deformity or change in an object due to stress.

Stratosphere

The upper part of the Earth’s atmosphere. The stratosphere extends upward from the tropopause, which is approximately 36,000 feet above the surface of the Earth, to approximately 85,000 feet. The temperature of the air in the stratosphere remains constant at -56.5°C (-69.7°F).

Stress

A force within an object that tries to prevent an outside force from changing its shape.

Stress

A force set up within an object that tries to prevent an outside force from changing its shape.

Stress

The internal resistance of an object to external forces attempting to strain or deform that object. Measured in pounds per square foot or pounds per square inch (psi).

Stress corrosion

Corrosion of the intergranular type that forms within metals subject to tensile stresses which tend to separate the grain boundaries.

Stress corrosion

Occurs as the result of the combined effect of sustained tensile stresses and a corrosive environment.

Stress riser

A location where the cross-sectional area of the part changes abruptly. Stresses concentrate at such a location and failure is likely. A scratch, gouge, or tool mark in the surface of a highly stressed part can change the area enough to concentrate the stresses and become a stress riser.

Stressed skin structure

A type of aircraft structure in which all or most of the stresses are carried in the outside skin. A stressed skin structure has a minimum of internal structure.

Stringer

A part of an aircraft structure used to give the fuselage its shape and, in some types of structure, to provide a small part of fuselage strength. Formers give the fuselage its cross-sectional shape and stringers fill in the shape between the formers.

Stroboscopic tachometer

A tachometer used to measure the speed of any rotating device without physical contact. A highly accurate variable-frequency oscillator triggers a high-intensity strobe light. When the lamp is flashing at the same frequency the device is rotating, the device appears to stand still.

Stroboscopic tachometer

A tachometer used to measure the speed of any rotating device without physical contact. A highly accurate variable-frequency oscillator triggers a high-intensity strobe light.

Stroke

The distance the piston moves inside the cylinder.

Sublimation

A process in which a solid material changes directly into a vapor without passing through the liquid stage.

Subsonic flight

Flight at an airspeed in which all air flowing over the aircraft is moving at a speed below the speed of sound.

Subtraction

The process where the value of one number is taken from the value of another.

Sum

The resulting answer in the addition process.

Summerwood

The less porous, usually harder portion of an annual ring that forms in the latter part of the growing season, the summer of the year.

Sump

A low point in an aircraft fuel tank in which water and other contaminants can collect and be held until they can be drained out.

Sump (aircraft engine component)

A low point in an aircraft engine in which lubricating oil collects and is stored or transferred to an external oil tank. A removable sump attached to the bottom of the crankcase of a reciprocating engine is often called an oil pan.

Sump (fuel tank component)

A low point in an aircraft fuel tank in which water and other contaminants collect and are held until they can be drained out.

Super heterodyne circuit

A sensitive radio receiver circuit in which a local oscillator produces a frequency that is a specific difference from the received signal frequency. The desired signal and the output from the oscillator are mixed, and they produce a single, constant intermediate frequency This IF is amplified, demodulated, and detected to produce the audio frequency that is used to drive the speaker.

Supercharged engine

A reciprocating engine that uses a mechanically driven compressor to increase the air pressure before it enters the engine cylinders.

Supercharger

An air compressor used to increase the pressure of the air being taken into the cylinders of a reciprocating engine.

Supercooled water

Water in its liquid form at a temperature well below its natural freezing temperature. When supercooled water is disturbed, it immediately freezes.

Superheat

Heat energy that is added to a refrigerant after it changes from a liquid to a vapor.

Supersonic flight

Flight at an airspeed in which all air flowing over the aircraft is moving at a speed greater than the speed of sound.

Supplemental restraint system

Any device that is not installed on the aircraft pursuant to an FAA approval, used to secure an individual inside an aircraft when that person is not properly secured by an FAA- approved safety belt and, if installed, shoulder harness, or an approved child restraint system. It consists of a harness secured around the torso of the individual using the supplemental restraint system and a lanyard that connects the harness to an FAA-approved airframe attachment point inside the aircraft.

Supplemental Type Certificate (STC)

An approval issued by the FAA for a modification to a type certificated airframe, engine, or component. More than one STC can be issued for the same basic alteration, but each holder must prove to the FAA that the alteration meets all the requirements of the original type certificate.

Supplemental Type Certificates (STC)

A document issued by the FAA approving a product (aircraft, engine, or propeller) modification.

Surface corrosion

Caused by either direct chemical or electrochemical attack, it appears as a general roughening, etching, or pitting of the surface of a metal, frequently accompanied by a powdery deposit of corrosion products.

Surface roughness

The condition of the surface of a reciprocating engine cylinder wall that has been honed to make it hold lubricating oil. Surface roughness is measured in micro-inches rms.

Surface tape

Pinked-edge strips of fabric doped over all seams, rib stitching, and edges of fabric covering (also called finishing tape).

Surface tape

Strips of aircraft fabric that are doped over all seams and places where the fabric is stitched to the aircraft structure. Surface tape is also doped over the wing leading edges where abrasive wear occurs. The edges of surface tape are pink, or notched, to keep them from raveling before the dope is applied.

Surfactant

A surface active agent, or partially soluble contaminant, which is a by-product of fuel processing or of fuel additives. Surfactants adhere to other contaminants and cause them to drop out of the fuel and settle to the bottom of the fuel tank as sludge.

Surge

A condition of unstable airflow, through the compressor of a gas turbine engine, in which the compressor blades have an excessive angle of attack. Surge usually affects an entire stage of compression.

Surveyor’s transit

An instrument consisting of a telescope mounted on a flat, graduated, circular plate on a tripod. The plate can be adjusted so it is level, and its graduations oriented to magnetic north. When an object is viewed through the telescope, its azimuth and elevation may be determined.

Swaged Fittings

These fittings create a permanent connection that is virtually maintenance free. Swaged fittings are used to join hydraulic lines in areas where routine disconnections are not required and are often used with titanium and corrosion resistant steel tubing

Swashplate

The component in a helicopter control system that consists basically of two bearing races with ball bearings between them. The lower, or nonrotating, race is tilted by the cyclic control, and the upper, or rotating, race has arms which connect to the control horns on the rotor blades. Movement of the cyclic pitch control is transmitted to the rotor blades through the swashplate. Movement of the collective pitch control raises or lowers the entire swashplate assembly to change the pitch of all the blades at the same time.

Switch

A device for opening or closing an electrical circuit.

Symmetrical airfoil

An airfoil that has the same shape on both sides of its chord line, or center line.

Symmetry check

A check of an airframe to determine that the wings and tail are symmetrical about the longitudinal axis.

Synchro system

A remote instrument indicating system. A synchro transmitter is actuated by the device whose movement is to be measured, and it is connected electrically with wires to a synchro indicator whose pointer follows the movement of the shaft of the transmitter.

Synthetic oil

Oil made by chemical synthesis of a mineral, animal, or vegetable base. Synthetic oils have appropriate additives that give them such characteristics as low volatility, low pour point, high viscosity index, good lubricating qualities, low coke and lacquer formation, and low foaming.

System Design Assurance (SDA)

The failure condition that the position transmission chain is designed to support. An SDA of 2 or greater is required by § 91.227. Refer to A.2.29 for more information.

System-pressure regulator (hydraulic system component)

A type of hydraulic system-pressure control valve. When the system pressure is low, as it is when some unit is actuated, the output of the constant-delivery pump is directed into the system. When the actuation is completed and the pressure builds up to a specified kick-out pressure, the pressure regulator shifts. A check valve seals the system off and the pressure is maintained by the accumulator. The pump is unloaded and its output is directed back into the reservoir with very little opposition. The pump output pressure drops, but the volume of flow remains the same. When the system pressure drops to the specified kick-in pressure, the regulator again shifts and directs fluid into the system. Spool-type and balanced-pressure-type system pressure regulators are completely automatic in their operation and require no attention on the part of the flight crew.

Tachometer

An instrument that measures the rotational speed of an object.

Tack coat

A coat of finishing material sprayed on the surface and allowed to dry until the solvents evaporate. As soon as the solvents evaporate, a wet full-bodied coat of material is sprayed over it.

Tack rag

A clean, lintless rag, slightly damp with thinner. A tack rag is used to wipe a surface to prepare it to receive a coat of finishing material.

Tack weld

A method of holding parts together before they are permanently welded. The parts are assembled, and small spots of weld are placed at strategic locations to hold them in position.

Tacky

Slightly sticky to the touch.

Tail pipe

The portion of the exhaust system of a gas turbine engine through which the gases leave. The tail pipe is often called the exhaust duct, or exhaust pipe.

Tailets

Small vertical surfaces mounted underside of the horizontal stabilizer of some airplanes to increase the directional stability.

Takeoff Cycles

For these cycles the loads, speeds, and distance are required to conform to either Figure 1 or Figure 2.

Takeoff power

(1) With respect to reciprocating engines, means the brake horsepower that is developed under standard sea level conditions, and under the maximum conditions of crankshaft rotational speed and engine manifold pressure approved for the normal takeoff, and limited in continuous use to the period of time shown in the approved engine specification; and (2) With respect to turbine engines, means the brake horsepower that is developed under static conditions at a specified altitude and atmospheric temperature, and under the maximum conditions of rotor shaft rotational speed and gas temperature approved for the normal takeoff, and limited in continuous use to the period of time shown in the approved engine specification.

Takeoff safety speed

A referenced airspeed obtained after lift-off at which the required one-engine- inoperative climb performance can be achieved.

Takeoff thrust

With respect to turbine engines, means the jet thrust that is developed under static conditions at a specific altitude and atmospheric temperature under the maximum conditions of rotorshaft rotational speed and gas temperature approved for the normal takeoff, and limited in continuous use to the period of time shown in the approved engine specification.

Takeoff warning system

An aural warning system that provides audio warning signals when the thrust levers are advanced for takeoff if the stabilizer, flaps, or speed brakes are in an unsafe condition for takeoff.

Tandem wing configuration

A configuration having two wings of similar span, mounted in tandem.

Tang

A tapered shank sticking out from the blade of a knife or a file. The handle of a knife or file is mounted on the tang.

Tangent (tan)

A trigonometric function comparing two sides of a right triangle as follows: Tan = opposite side adjacent side Tap. Instrument used to cut threads on the inside of a hole.

Tape

A tape or a “narrow fabric” is loosely defined as a material that ranges in width from 1/4 inch to 12 inches.

Tare weight

The weight of any chocks or devices used to hold an aircraft on scales when it is weighed. The tare weight must be subtracted from the scale reading to get the net weight of the aircraft.

Taxi Cycles

The tire at rated load is required to withstand at least 8 taxi cycles on a dynamometer for a minimum speed of 40 mph and a minimum roll distance of 35,000 feet.

TCAS Potential Threat

Traffic detected by TCAS equipment on board the own-ship, that has met the Potential Threat classification criteria for a TCAS TA and does not meet the Threat Classification criteria for a TCAS RA (RTCA/DO-185B § 1.8). If the ASAS own- ship CDTI display is also used as a TCAS TA display, then information about TCAS potential threats will be conveyed to the CDTI via the ASSAP function.

TCAS Proximate Traffic

Traffic, detected by TCAS equipment on board the own-ship, that is within 1200 feet vertically and 6 NM horizontally of the own-ship (RTCA/DO-185B § 1.8). If the ASA system own-ship CDTI display is also used as a TCAS display, then information about TCAS proximate traffic will be conveyed to the CDTI, possibly via the ASSAP function.

TCAS-Only Traffic

A traffic element about which TCAS has provided surveillance information, but which the ASSAP function has not correlated with traffic from other surveillance sources such as ADS-B, ADS-R, or TIS-B.

Teflon

The registered trade name for a fluorocarbon resin used to make hydraulic and pneumatic seals, hoses, and backup rings.

Tempered glass

Glass that has been heat-treated to increase its strength. Tempered glass is used in bird-proof, heated windshields for high-speed aircraft.

Tempering

Process that reduces the brittleness imparted by hardening and produces definite physical properties within the steel. Tempering always follows, never precedes, the hardening operation.

Terminal strips

A group of threaded studs mounted in a strip of insulating plastic. Electrical wires with crimped-on terminals are placed over the studs and secured with nuts.

Terminal VOR

A low-powered VOR that is normally located on an airport.

Test club

A wide-blade, short-diameter propeller used on a reciprocating engine when it is run in a test cell. A test club applies a specific load to the engine and forces the maximum amount of air through the engine cooling fins.

Test Inflation Pressure

The pressure required at an identified ambient temperature to obtain the same loaded radius against the flywheel of the dynamometer as the radius for flat surface as defined in paragraph 7c (2) of this AC. Adjustment to the test inflation pressure may not be made to compensate for increases due to temperature rise Occurring during the tests.

Testing

Tires operating at ground speeds greater than 160 mph are required to be tested on a dynamometer. Tires operating at ground speeds of 160 mph or less may, as an option, be tested on the dynamometer in accordance with paragraph 7c (7)(111).

Tetraethyl lead (TEL)

A heavy, oily, poisonous liquid, Pb(C2H5)4 , that is mixed into aviation gasoline to increase its critical pressure and temperature.

Therapeutic mask adapter

A calibrated orifice in the mask adapter for a continuous-flow oxygen system that increases the flow of oxygen to a mask being used by a passenger who is known to have a heart or respiratory problem.

Thermal dimpling

See hot dimpling.

Thermal efficiency

The ratio of the amount of useful work produced by a heat engine, to the amount of work that could be done by all of the heat energy available in the fuel burned.

Thermal expansion

The increase in size of a material as temperature increases.

Thermal expansion coefficient

A number that relates to the change in the physical dimensions of a material as the temperature of the material changes. The thermal expansion coefficient of aluminum is approximately twice that of steel.

Thermal relief valve

A relief valve in a hydraulic system that relieves pressure that builds up in an isolated part of the system because of heat. Thermal relief valves are set at a higher pressure than the system pressure relief valve.

Thermal shock

The sudden change in engine operating temperature that occurs when engine power is suddenly reduced at the same time the airspeed, thus the cooling, is increased. Thermal shock occurs when an aircraft is required to rapidly descend to a lower altitude.

Thermistor

A semiconductor material whose electrical resistance varies with its temperature.

Thermistor

A special form of electrical resistor whose resistance varies with its temperature.

Thermistor material

A material with a negative temperature coefficient that causes its resistance to decrease as its temperature increases.

Thermocouple

A device used to generate an electrical current. A thermocouple is made of two dissimilar metal wires whose ends are welded together to form a loop. A voltage exists in the loop proportional to the difference in temperature of the junctions at which the wires are joined. The amount of current flowing in the loop is determined by the types of metals used for the wires, the temperature difference between the junctions, and the resistance of the wires.

Thermocouple

Device to convert heat energy into electrical energy.

Thermocouple

A loop consisting of two kinds of wire, joined at the hot, or measuring, junction and at the cold junction in the instrument. The voltage difference between the two junctions is proportional to the temperature difference between the junctions. In order for the current to be meaningful, the resistance of the thermocouple is critical, and the leads are designed for a specific installation. Their length should not be altered. Thermocouples used to measure cylinder head temperature are usually made of iron and constantan, and thermocouples that measure exhaust gas temperature for turbine engines are made of chromel and alumel.

Thermocouple fire-detection system

A fire-detection system that works on the principle of the rate-of-temperature rise. Thermocouples are installed around the area to be protected, and one thermocouple is surrounded by thermal insulation that prevents its temperature changing rapidly. In the event of a fire, the temperature of all the thermocouples except the protected one will rise immediately and a fire warning will be initiated. In the case of a general overheat condition, the temperature of all the thermocouples will rise uniformly and there will be no fire warning.

Thermoplastic material

A material that can be repeatedly softened by an increase in the temperature and hardened by a decrease in the temperature with no accompanying chemical change. For example, a puddle of tar on the road in the summer during the heat of day: the tar is soft and fluid; however, when cooler in the evening, it becomes solid again.

Thermoplastic resin

Atype of plastic material that becomes soft when heated and hardens when cooled.

Thermoset material

A material which becomes substantially infusible and insoluble when cured by the application of heat or by chemical means. A material that will undergo, or has undergone, a chemical reaction (different from a thermoplastics physical reaction) by the action of heat, catalysts, ultraviolet light, etc. Once the plastic becomes hard, additional heat will not change it back into a liquid as would be the case with a thermoplastic.

Thermosetting resin

A plastic resin that, once it has been hardened by heat, cannot be softened by heating again.

Thermosetting resin

A type of plastic material that, when once hardened by heat, cannot be softened by being heated again.

Thermostatic expansion valve (TEV)

The component in a vapor-cycle cooling system that meters the refrigerant into the evaporator. The amount of refrigerant metered by the TEV is determined by the temperature and pressure of the refrigerant as it leaves the evaporator coils. The TEV changes the refrigerant from a high-pressure liquid into a low-pressure liquid.

Thermostatic valve

A temperature-sensitive valve that controls the temperature of oil in an aircraft engine. When the oil is cold, the valve shifts and directs the oil through the oil cooler.

Thermoswitch

An electrical switch that closes a circuit when it is exposed to a specified high temperature.

Thixotropic agents

Materials, such as microballoons, added to a resin to give it body and increase its workability.

Three-dimensional cam

A drum-shaped cam in a hydro- mechanical fuel control whose outer surface is ground so that followers riding on the surface, as the cam is moved up and down and rotated, can move mechanical linkages to control the fuel according to a preprogrammed schedule.

Throttle

The control in an aircraft that regulates the power or thrust the pilot wants the engine to produce.

Throw (crankshaft design)

See offset throw.

Thrust

The aerodynamic force produced by a propeller or turbojet engine as it forces a mass of air to the rear, behind the aircraft. A propeller produces its thrust by accelerating a large mass of air by a relatively small amount. A turbojet engine produces its thrust by accelerating a smaller mass of air by a much larger amount.

Thrust horsepower

The horsepower equivalent of the thrust produced by a turbojet engine. Thrust horsepower is found by multiplying the net thrust of the engine, measured in pounds, by the speed of the aircraft, measured in miles per hour, and then dividing this by 375.

Time in service

With respect to maintenance time records, the time from the moment an aircraft leaves the surface of the earth until it touches it at the next point of landing.

Time of Applicability

The time that a particular measurement or parameter is (or was) relevant.

Time-Rite indicator

A patented piston-position indicator used to find the position of the piston in the cylinder of a reciprocating engine. The body of the Time-Rite indicator screws into a spark plug hole, and as the piston moves outward in the cylinder, it contacts the arm of the indicator. A pointer contacted by the arm moves across a calibrated scale to show the location of the piston in degrees of crankshaft rotation before top center.

Timing light

An indicator light used when timing magnetos to an engine to indicate when the breaker points open. Some timing lights incorporate an oscillator or buzzer that changes its pitch when the points open.

Tip

The portion of the blade outermost from the axis of propeller rotation.

Tip

Part of the torch at the end where the gas burns, producing the high-temperature flame.

Tire (Pneumatic)

A complex engineered structure made of rubber and cord (of textiles or steel) that provide a resilient protective enclosure for the inflating gas.

Tire pressure

Indicating System (Installed on an Aircraft) A system installed on an aircraft used for tire pressure checks.

Tire Test Load

The tire is required to be forced against the dynamometer flywheel at not less than the rated load of the tire during the entire roll distance of the test.

Toe-in

A condition of landing gear alignment in which the front of the tires are closer together than the rear. When the aircraft rolls forward, the wheels try to move closer together.

Toe-out

A condition of landing gear alignment in which the front of the tires are further apart than the rear. When the aircraft rolls forward, the wheels try to move farther apart.

Toggle

A T-shaped handle fitted onto the end of a cable used to engage a simple starter with an overrunning clutch.

Tolerance

The sum of the plus and minus allowance figures.

Top overhaul

An overhaul of the cylinders of an aircraft engine. The valves, pistons, and cylinders are overhauled, but the crankcase is not opened.

Toroidal coil

An electrical coil wound around a ring-shaped core of highly permeable material.

Torque

The tendency of a force to cause or change rotational motion of a body.

Torque

A force that produces or tries to produce rotation.

Torque

A force that produces or tries to produce rotation.

Torque links

The hinged link between the piston and cylinder of an oleo-type landing gear shock absorber. The torque links allow the piston to move freely in and out of the landing gear cylinder, but prevent it rotating. The torque links can be adjusted to achieve and maintain the correct wheel alignment. Torque links are also called scissors and nutcrackers.

Torque tube

A tube in an aircraft control system that transmits a torsional force from the operating control to the control surface.

Torsion rod

A device in a spring tab to which the control horn is attached. For normal operation, the torsion rod acts as a fixed attachment point, but when the control surface loads are high, the torsion rod twists and allows the control horn to deflect the spring tab.

Total air pressure

The pressure a column of moving air will have if it is stopped.

Total air temperature

The temperature a column of moving air will have if it is stopped.

Total Latency

The total time between when the position is measured by the position source (GNSS TOM for GNSS systems) and when the position is transmitted from the aircraft (ADS-B time of transmission).

Total pressure

The pressure a column of moving fluid would have if it were stopped from its motion. Total pressure is the sum of dynamic pressure and static pressure.

Total temperature

The temperature of moving fluid that has been stopped from its motion. Total temperature is the sum of static temperature and the temperature rise caused by the ram effect as the fluid was stopped.

Townend ring

A type of ring cowling used over a single-row radial engine. The cross section of the ring is in the form of an airfoil that produces enough forward thrust to compensate for the cooling drag of the engine. In the United States, townend rings are often called speed rings.

Track

The path followed by a blade segment of a propeller or helicopter rotor in one rotation.

Track

(1) A sequence of reports from the ASSAP function that all pertain to the same traffic target. (2) Within the ASSAP function, a sequence of estimates of traffic target state that all pertain to the same traffic element.

Track Angle

See Ground Track Angle.

Track State

See State Vector.

Tracking

See blade track.

Tractor engine

An engine installed with the propeller facing the front of the aircraft. Thrust produced by the propeller mounted on a tractor engine pulls the aircraft through the air.

Tractor powerplant

An airplane powerplant in which the propeller is mounted in the front, and its thrust pulls the airplane rather than pushes it.

tractor propeller

A propeller mounted on an airplane in such a way that its thrust pulls the aircraft.

Traffic

All aircraft/vehicles that are within the operational vicinity of own-ship.

Traffic Collision Avoidance System

Collision Avoidance systems which rely on transponder interrogations and replies of other airborne aircraft.

Traffic Collision Avoidance System I

TCAS I is the first generation of collision avoidance technology. TCAS I systems are able to monitor the traffic situation around an aircraft and offer information on the approximate bearing and altitude of other aircraft. It can also generate collision warnings in the form of a "Traffic Advisory" (TA). The TA warns the pilot that another aircraft is in near vicinity, announcing "Traffic, traffic", but does not offer any suggested remedy Traffic Collision Avoidance System II.

Traffic Element

An aircraft or vehicle.

Traffic Information Service - Broadcast (TIS-B)

TIS-B is a ground broadcast service provided from an ADS-B ground system network over the UAT and 1090ES links that provides position, velocity, and other information on traffic detected by a secondary surveillance radar, but is not transmitting an ADS-B position.

Traffic Information Service – Broadcast (TIS-B)

A surveillance service that broadcasts traffic information derived from one or more ground surveillance sources to suitably equipped aircraft or surface vehicles, with the intention of supporting ASA applications.

Traffic Selection

Manual process of flight crew selecting a traffic element.

Traffic Symbol

A depiction on the CDTI display of an aircraft or vehicle other than the own-ship.

Traffic-To-Follow

A term used for CAVS which is used to refer to the aircraft preceding ownship.

Trailing edge

The thin edge at the rear of a propeller blade.

Trailing edge

The rear edge of the blade where the air leaves the blade.

Trammel (verb)

To square up the Pratt truss used in an airplane wing. Trammel points are set on the trammel bar so they measure the distance between the center of the front spar, at the inboard compression strut, and at the center of the rear spar at the next compression strut outboard. The drag and antidrug wires are adjusted until the distance between the center of the rear spar at the inboard compression strut and the center of the front spar at the next outboard compression strut is exactly the same as that between the first points measured.

Trammel bar

A wood or metal bar on which trammel points are mounted to compare distances.

Trammel points

A set of sharp-pointed pins that protrude from the sides of a trammel bar.

Transceiver

A unit serving as both a receiver and a transmitter.

Transducer

A device that changes energy from one form to another. Commonly used transducers change mechanical movement or pressures into electrical signals.

Transducer

A device that changes energy from one form to another. Commonly used transducers change mechanical movement or pressures into electrical signals.

Transformer

A device that changes electrical energy of a given voltage into electrical energy at a different voltage level. It consists of two coils that are not electrically connected, but arranged so that the magnetic field surrounding one coil cuts through the other coil.

Transformer

An electrical component used to change the voltage and current in an AC circuit.

Transformer

A device for raising or lowering AC voltage.

Transformer rectifier

A component in a large aircraft electrical system used to reduce the AC voltage and change it into DC for charging the battery and for operating DC equipment in the aircraft.

Transistor

A three-terminal device primarily used to amplify signals and control current within a circuit.

Translational lift

The additional lift produced by a helicopter rotor as the helicopter changes from hovering to forward flight.

Transmitter

An electronic system designed to produce modulated RF carrier waves to be radiated by an antenna; also, an electric device used to collect quantitative information at one point and send it to a remote indicator electrically.

Transonic flight

Flight at an airspeed in which some air flowing over the aircraft is moving at a speed below the speed of sound, and other air is moving at a speed greater than the speed of sound.

Transonic range

Flight at Mach numbers between 0.8 and 1.2. In this range, some air passing over the aircraft is subsonic, and some is supersonic.

Transponder

An airborne receiver-transmitter designed to aid air traffic control personnel in tracking aircraft during flight.

Transponder

A piece of equipment carried on board an aircraft to support the surveillance of that aircraft by secondary surveillance radar sensors. A transponder receives interrogation signals on 1030 MHz and replies on the 1090 MHz downlink frequency.

Transverse crack

A crack at an angle that is not parallel to the principle direction of the outer ply, breaker, or belt.

Transverse pitch

See gauge.

Trapezoid

A four-sided figure with one pair of parallel sides.

Tread

The expendable rubber-wearing surface of any pneumatic tire. It contains the groove pattern designed to facilitate water removal from the contact patch, and may, or may not, possess integral fabric materials.

Trend monitoring

A system for comparing engine performance parameters with a baseline of these same parameters established when the engine was new or newly overhauled. Parameters such as EGT, rpm, fuel flow, and oil consumption are monitored on every flight, and the baseline is plotted. Any deviation from a normal increase or decrease warns the technician of an impending problem.

Triangle

A three-sided figure in which the sum of the three angles equal 180°.

Triangle

A three-sided, closed plane figure. The sum of the three angles in a triangle is always equal to 180°.

Tricresyl phosphate (TCP)

A colorless, combustible compound, (CH3C6H4O)3PO, that is used as a plasticizer in aircraft dope and an additive in gasoline and lubricating oil. TCP aids in scavenging lead deposits left in the cylinders when leaded fuel is burned.

Tricresyl phosphate (TCP)

A chemical compound, (CH3C6H4O)3PO, used in aviation gasoline to assist in scavenging the lead deposits left from the tetraethyl lead.

Trigonometry

The study of the relationships between the angles and sides of a triangle.

Trim tab

A small control tab mounted on the trailing edge of a movable control surface. The tab may be adjusted to provide an aerodynamic force to hold the surface on which it is mounted deflected in order to trim the airplane for hands- off flight at a specified airspeed.

Trimmed flight

A flight condition in which the aerodynamic forces acting on the control surfaces are balanced and the aircraft is able to fly straight and level with no control input.

Trip-free circuit breaker

A circuit breaker that opens a circuit any time an excessive amount of current flows, regardless of the position of the circuit breaker’s operating handle.

Troubleshooting

A procedure used in aircraft maintenance in which the operation of a malfunctioning system is analyzed to find the reason for the malfunction and to find a method for returning the system to its condition of normal operation.

True airspeed

The airspeed of an aircraft relative to undisturbed air. True airspeed is equal to equivalent airspeed multiplied by (ρ0/ρ)1⁄2.

True airspeed (TAS)

Airspeed shown on the airspeed indicator (indicated airspeed) corrected for position error and nonstandard air temperature and pressure.

True power

The power dissipated in the resistance of a circuit, or the power actually used in the circuit.

Trunnion

Projections from the cylinder of a retractable landing gear strut about which the strut pivots retract.

Truss-type structure

A type of structure made up of longitudinal beams and cross braces. Compression loads between the main beams are carried by rigid cross braces. Tension loads are carried by stays, or wires, that go from one main beam to the other and cross between the cross braces.

Tube

A gas-tight rubber device placed inside tube-type tire casings for the purpose of containing the inflation gas. It is provided with an integral valve assembly.

Tube-Type tires

Tires requiring tubes for inflation retention.

Tubeless tires

Tires not requiring tubes. These tires are constructed with an innerliner.

Turbine

A wheel fitted with vanes, or buckets, radiating outward from its circumference. The reactive or aerodynamic force caused by the fluid flowing through the vanes is converted into mechanical power that spins the shaft on which the wheel is mounted.

Turbine

A rotary device actuated by impulse or reaction of a fluid flowing through vanes or blades that are arranges around a central shaft.

Turbine engine

See gas turbine engine.

Turbine inlet guide vanes

A series of stator vanes immediately ahead of the first-stage turbine. The function of the inlet guide vanes is to divert the hot gases in the proper direction to enter the turbine, and to provide a series of convergent ducts which increase the velocity of the gases.

Turbine nozzle

Another name for turbine inlet guide vanes.

Turbo-compound engine

A reciprocating engine that has power recovery turbines in its exhaust system. The power extracted from the exhaust by these turbines is directed into the engine crankshaft through a fluid coupling.

Turbocharger

An exhaust-driven air compressor used to increase the power of a reciprocating engine. A turbocharger uses a small radial inflow turbine in the exhaust system to drive a centrifugal-type air compressor on the turbine shaft. The compressed air is directed into the engine cylinders to increase power.

Turbofan engine

A type of gas turbine engine that has a set of lengthened blades on the low-pressure compressor or low-pressure turbine. Air moved by these special blades bypasses the core engine and produces between 30% and 75% of the total thrust.

Turbojet engine

A gas turbine engine that produces thrust by accelerating the air flowing through it. A minimum of energy is extracted by the turbine, with the majority used to produce an exhaust velocity much greater than the inlet velocity. The amount of thrust produced by the engine is determined by the amount the air is accelerated as it flows through the engine.

Turboprop engine

A turbine engine in which several stages of turbines are used to extract as much energy as possible. The turbines drive reduction gears which in turn drive a propeller.

Turboshaft engine

A turbine engine in which several stages of turbines are used to extract as much energy as possible. The turbines drive shafts which are used to drive helicopter rotors, generators, or pumps.

Turbosupercharger

A centrifugal air compressor driven by exhaust gases flowing through a turbine. The compressed air is used to increase the power produced by a reciprocating engine at altitude.

Turn and slip indicator

A rate gyroscopic flight instrument that gives the pilot an indication of the rate of rotation of the aircraft about its vertical axis. A ball in a curved glass tube shows the pilot the relationship between the centrifugal force and the force of gravity. This indicates whether or not the angle of bank is proper for the rate of turn. The turn and slip indicator shows the trim condition of the aircraft and serves as an emergency source of bank information in case the attitude gyro fails. Turn and slip indicators were formerly called needle and ball and turn and bank indicators.

Turnbuckle

A component in an aircraft control system used to adjust cable tension. A turnbuckle consists of a brass tubular barrel with right-hand threads in one end and left-hand in the other end. Control cable terminals screw into the two ends of the barrel, and turning the barrel pulls the terminals together, shortening the cable.

Twist drill

A metal cutting tool turned in a drill press or handheld drill motor. A twist drill has a straight shank and spiraled flutes. The cutting edge is ground on the end of the spiraled flutes.

Twist rope

A stripe of paint on flexible hose that runs the length of the hose. If this stripe spirals around the hose after it is installed, it indicates the hose was twisted when it was installed. Twist stripes are also called lay lines.

Two-spool engine

See dual-spool gas turbine engine.

Two-stroke cycle

A constant-volume cycle of energy transformation that completes its operating cycle in two strikes of the piston, one up and one down. When the piston moves up, fuel is pulled into the crankcase, and at the same time the air/fuel mixture inside the cylinder is compressed. When the piston is near the top of its stroke, a spark plug ignites the compressed air/fuel mixture, and the burning and expanding gases force the piston down. Near the bottom of the stroke, the piston uncovers an exhaust port and the burned gases leave the cylinder. When the piston moves further down, it uncovers the intake port, and a fresh charge of fuel and air are forced from the crankcase into the cylinder.

Two-terminal spot-type fire detection system

A fire detection system that uses individual thermoswitches installed around the inside of the area to be protected. These thermoswitches are wired in parallel between two separate circuits. A short or an open circuit can exist in either circuit without causing a fire warning.

Type

(3) As used with respect to the certification of aircraft engines means those engines which are similar in design. For example, JT8D and JT8D-7 are engines of the same type, and JT9D-3A and JT9D-7 are engines of the same type.

Type Certificate Data Sheet (TCDS)

The FAA issues a type certificate when a new aircraft, engine, propeller, etc., is found to meet safety standards set forth by the FAA. The TCDS lists the specifications, conditions and limitations under which airworthiness requirements were met for the specified product, such as engine make and model, fuel type, engine limits, airspeed limits, maximum weight, minimum crew, etc

Type Certificate Data Sheets (TCDS)

The official specifications of an aircraft, engine, or propeller issued by the Federal Aviation Administration. The TCDS lists pertinent specifications for the device, and it is the responsibility of the mechanic and/or inspector to ensure, on each inspection, that the device meets these specifications.

Ultimate tensile strength

The tensile strength required to cause a material to break or to continue to deform under a decreasing load.

Ultrasonic inspection

Uses high frequency sound energy to conduct examinations and make measurements. Ultrasonic inspection can be used for flaw detection/evaluation, dimensional measurements, and material characterization.

Ultraviolet-blocking dope

Dope that contains aluminum powder or some other pigment that blocks the passage of ultraviolet rays of the sun. The coat of dope protects the organic fabrics and clear dope from deterioration by these rays.

Unbonding

Adhesive or cohesive failure between laminates. Compare definitions of adhesive, cohesive debond, and disbond.

Uncompensated Latency

Any latency in the ADS-B system that is not compensated through extrapolation. Uncompensated latency can be represented as the difference between the time of applicability of the broadcast position and the actual time of transmission.

Undamped oscillation

Oscillation that continues with an unchanging amplitude once it has started.

Undercutting

A rubber fracture that progresses from the tread surface, or in a groove, in a direction parallel to the ply structure.

Underslung rotor

A helicopter rotor whose center of gravity is below the point at which it is attached to the mast.

Underspeed condition

A speed condition in which the engine is turning at an RPM lower than that for which the propeller governor is set.

Unidirectional fabric

Fabric in which all the threads run in the same direction. These threads are often bound with a few fibers run at right angles, just enough to hold the yarns together and prevent their bunching.

Unidirectional fibers

Fibers in a piece of composite material arranged so that they sustain loads in only one direction.

Universal Access Transceiver (UAT)

A wideband multipurpose data link intended to operate globally on a single channel with a channel signaling rate of just over 1Mbps. By design, UAT supports multiple broadcast services including FIS-B and TIS-B in addition to ADS-B. This is accomplished using a hybrid medium access approach that incorporates both time-slotted and random unslotted access.

Unloading valve

This is another name for system pressure regulator. See system pressure regulator.

Unmanned aircraft

An aircraft operated without the possibility of direct human intervention from within or on the aircraft.

Unmanned aircraft system

An unmanned aircraft and its associated elements (including communication links and the components that control the unmanned aircraft) that are required for the safe and efficient operation of the unmanned aircraft in the airspace of the United States.

Updraft carburetor

A carburetor that mounts on the bottom of a reciprocating engine. Air entering the engine flows upward through the carburetor.

Upper-deck pressure

The absolute pressure of air at the inlet to the fuel metering system of a turbocharged engine. Upper-deck pressure is the same as the turbocharger discharge pressure.

Useful load

Fuel, any other fluids that are not part of empty weight, passengers, baggage, pilot, copilot, and crewmembers. It is determined by subtracting the empty weight from the maximum allowable gross weight

Utility finish

The finish of an aircraft that gives the necessary tautness and fill to the fabric and the necessary protection to the metal, but does not have the glossy appearance of a show-type finish.

V-blocks

A fixture that allows a shaft to be centered and rotated to measure any out-of-round condition.

V-engine

A form of reciprocating engine in which the cylinders are arranged in two banks. The banks are separated by an angle of between 45° and 90°. Pistons in two cylinders, one in each bank, are connected to each throw of the crankshaft.

Valence electrons

Electrons in the outer shell, or ring, around the nucleus of an atom. It is the valence electrons that give an atom its electrical characteristics and are the electrons that may be pulled loose from an atom to cause electrical current.

Valve overlap

The portion of the operating cycle of a four- stroke-cycle reciprocating engine during which both the intake and exhaust valves are off of their seats at the same time.

Vapor lock

A condition of fuel starvation that can occur in a reciprocating engine fuel system. If the fuel in the line between the tank and carburetor is heated enough for the fuel to vaporize, a bubble will form in the line. If the vapor pressure of the bubble is high enough, it will block the fuel and keep it from flowing to the engine.

Vapor lock

A condition in which vapors form in the fuel lines and block the flow of fuel to the carburetor.

Vapor pressure

The portion of atmospheric pressure that is exerted by the moisture in the air (expressed in tenths of an inch of mercury).

Vapor pressure

The amount of pressure needed above a liquid to prevent it from evaporating.

Vapor pressure

The pressure of the vapor above a liquid needed to prevent the liquid evaporating. Vapor pressure is always specified at a specific temperature.

Vaporize

The changing of a liquid into a vapor.

Variable displacement pump

A fluid pump whose output is controlled by the demands of the system. These pumps normally have a built-in system pressure regulator. When the demands of the system are low, the pump moves very little fluid, but when the demands are high, the pump moves a lot of fluid. Most variable displacement pumps used in aircraft hydraulic systems are piston-type pumps.

Varnish (aircraft finishing material)

A material used to produce an attractive and protective coating on wood or metal. Varnish is made of a resin dissolved in a solvent and thinned until it has the proper viscosity to spray or brush. The varnish is spread evenly over the surface to be coated, and when the solvents evaporate, a tough film is left.

Varsol

A petroleum product similar to naphtha used as a solvent and cleaning fluid.

Vectored-thrust engine

A turbojet or turbofan engine with the fan and/or exhaust nozzles mounted in such a way that they may be rotated in flight to produce forward, vertically upward, or rearward thrust.

Velocity

A vector quantity that expresses both the speed an object is moving and the direction in which it is moving.

Velocity

The rate of change of position. Horizontal velocity is the horizontal component of velocity and vertical velocity is the vertical component of velocity.

Velocity turbine

A turbine driven by forces produced by the velocity, rather than the pressure, of gases flowing through the vanes.

Veneer

Thin sheets of wood “peeled” from a log. A wide- blade knife held against the surface of the log peels away the veneer as the log is rotated in the cutter. Veneer is used for making plywood. Several sheets of veneer are glued together, with the grain of each sheet placed at 45° or 90° to the grain of the sheets next to it.

Venture

A specially shaped restrictor in a tube designed to speed up the flow of fluid passing through it. According to Bernoulli’s principal, any time the flow of fluid speeds up without losing or gaining any energy from the outside, the pressure of the fluid decreases.

Vernier coupling

A timing coupling used with base- mounted magnetos. The vernier coupling allows the timing to be adjusted in increments of considerably less than one degree.

Vertical axis

An imaginary line, passing vertically through the center of gravity of an airplane.

Vertical fin

The fixed vertical surface in the empennage of an airplane. The vertical fin acts as a weathervane to give the airplane directional stability.

Vertical tape instrument

A tall rectangular instrument that displays the quantity of the parameter being measured by a movable strip of colored tape. The presentation resembles a vertical bar graph.

Very high frequency (VHF)

A frequency between 30 and 300 MHz VHF omnirange (VOR).

VHF Omnirange (VOR)

An electronic air navigation system that provides accurate direction information in relation to a certain ground station.

Vibration loop

A loop in a rigid fluid line used to prevent vibration from concentrating stresses that could cause the line to break.

Vibrator-type voltage regulator

A type of voltage regulator used with a generator or alternator that intermittently places a resistance in the field circuit to control the voltage. A set of vibrating contacts puts the resistor in the circuit and takes it out several times a second.

Videoscope

A type of borescope.

Viscosimeter

An instrument used to measure the viscosity of a liquid. The time required for a given volume of liquid at a specified temperature to flow through a calibrated orifice is used to indicate the viscosity of the liquid.

Viscosity

The resistance of a fluid to flow. Viscosity refers to the “stiffness” of the fluid, or its internal friction.

Viscosity

The resistance of a fluid to flow. Viscosity is the stiffness of the fluid, or its internal friction.

Viscosity cup

A specially shaped cup with an accurately sized hole in its bottom. The cup is submerged in the liquid to completely fill it. It is then lifted from the liquid and the time in seconds is measured from the beginning of the flow through the hole until the first break in this flow. The viscosity of the liquid relates to this time.

Viscosity index (VI)

A measure of change in viscosity of an oil as it changes temperature. The higher the viscosity index, the less the viscosity changes.

Viscosity index improver

An additive used to produce a multi-viscosity lubricating oil. The polymer additive expands as temperature increases and contracts as temperature decreases. VI improvers cause viscosity to increase as oil heats and decrease as it cools.

Visible light

Electromagnetic radiation that has a wavelength in the range from about 3,900 to 7,700 angstroms and that may be seen by the unaided human eye.

Visual check

Utilizing acceptable methods, techniques, and practices to determine physical condition and safety item.

Visual Separation on Approach (VSA)

The CDTI is used to assist the flight crew in acquiring and maintaining visual contact during visual separation on approach. The CDTI is also used in conjunction with visual, out-the-window contact to follow the preceding aircraft during the approach. The application is expected to improve both the safety and the performance of visual separation on approach. It may allow for the continuation of visual separation on approach when they otherwise would have to be suspended due to the difficulty of visually acquiring and tracking the other preceding aircraft.

Vixen file

A metal-cutting hand file that has curved teeth across its faces. Vixen files are used to remove large amounts of soft metal.

Volatile liquid

A liquid that easily changes into a vapor.

Volatile memory

Computer memory that is lost when the power to the computer is turned off.

Volatility

The characteristic of a liquid that relates to its ability to vaporize or change into a gas.

Volt

The basic unit of electrical potential or electromotive force. A potential of one volt appears across a resistance of one ohm when a current of one ampere flows through that resistance.

Volt

Unit of potential, potential difference, or electrical pressure.

Voltage regulator

Device used in connection with generators to keep the voltage constant as load or speed is changed.

Voltmeter

A current-measuring instrument, designed to indicate voltage by measuring the current flow through a resistance of known value

Voltmeter multiplier

A precision resistor in series with a voltmeter mechanism used to extend the range of the basic meter or to allow a single meter to measure several ranges of voltage.

Volume

The amount of space within a three-dimensional solid

Volumetric efficiency

The ratio of the volume of the charge of the fuel and air inside the cylinder of a reciprocating engine to the total physical volume of the cylinder.

Von Ohain, Dr Hans Pabst

The designer and developer of the first turbojet engine to power an airplane. His HeS3b engine was built in Germany by the Heinkel Company and it flew in a Heinkel He178 airplane on August 27, 1939.

Vortex

A whirling mass of air that sucks everything near it toward its center.

Vortex (plural vortices)

A whirling motion in a fluid.

Vortex dissipator

A high-velocity stream of compressor bleed air blown from a nozzle into an area where vortices are likely to form. Vortex dissipaters destroy the vortices that would otherwise suck debris from the ground into engines mounted in pods that are low to the ground.

Vortex generator

Small, low-aspect-ratio airfoils installed in pairs on the upper surface of a wing, on both sides of the vertical fin just ahead of the rudder, and on the underside of the vertical stabilizers of some airplanes. Their function is to pull high-energy air down to the surface to energize the boundary layer and prevent airflow separation until the surface reaches a higher angle of attack.

Wake

The high-velocity stream of turbulent air behind an operating aircraft engine.

Wankel engine

See rotating combustion (RC) engine.

Warning

The level or category of alert for conditions that require immediate flight crew awareness and immediate flight crew response.

Warp

Threads in a fabric that run the length of the woven material as it comes from the mill.

Warp clock

An alignment indicator included in a structural repair manual to show the orientation of the piles of a composite material. The ply direction is shown in relation to a reference direction.

Warp threads

Threads that run the length of the roll of fabric, parallel to the selvage edge. Warp threads are often stronger than fill threads.

Warp tracers

Threads of a different color from the warp threads that are woven into a material to identify the direction of the warp threads.

Wash in

A twist in an airplane wing that increases its angle of incidence near the tip.

Wash out

A twist in an airplane wing that decreases its angle of incidence near the tip.

Waste gate

A controllable butterfly valve in the exhaust pipe of a reciprocating engine equipped with an exhaust-driven turbocharger. When the waste gate is open, exhaust gases leave the engine through the exhaust pipe, and when it is closed, they leave through the turbine.

Waterline (WL)

A horizontal reference plane used to locate vertical positions on an aircraft. Positions are usually given in inches above or below the waterline.

Watt

A unit of power equal to one joule per second.

Watt

The basic unit of electrical power. One watt is equal to 1⁄746 horsepower.

Watt

The unit of power; equal to a joule per second.

Watt

The basic unit of power in the metric system. One watt is the amount of power needed to do one joule (0.7376 foot- pound of work) in one second. One watt is 1⁄746 horsepower.

Wattmeter

An instrument for measuring electrical power.

Waveguide

A hollow, typically rectangular, metallic tube designed to carry electromagnetic energy at extremely high frequencies.

Wavy-grained wood

Wood in which the fibers collectively take the form of waves or undulations.

Way point

A phantom location created in certain electronic navigation systems by measuring direction and distance from a VORTAC station or by latitude and longitude coordinates from Loran or GPS.

Web of a spar

The part of a spar between the caps.

Weft threads

See fill threads.

Weighing points

Locations on an aircraft that the manufacturer designates for the placement of scales when weighing aircraft.

Weight

A measure of the pull of gravity acting on the mass of an object.

Weight-shift-control aircraft

A powered aircraft with a framed pivoting wing and a fuselage controllable only in pitch and roll by the pilot’s ability to change the aircraft’s center of gravity with respect to the wing. Flight control of the aircraft depends on the wing’s ability to flexibly deform rather than the use of control surfaces.

Welding

A materials-joining process used in making welds.

Welding rod

A form of welding filler metal, normally packaged in straight lengths.

Welding torch

The device used in gas welding.

Wet-sump engine

An engine that carries its lubricating oil supply in a reservoir that is part of the engine itself.

Wet-sump lubrication system

A lubrication system in which the oil supply is carried within the engine itself. Return oil drains into the oil reservoir by gravity.

Wet-type vacuum pump

An engine-driven air pump that uses steel vanes. These pumps are lubricated by engine oil drawn in through holes in the pump base. The oil passes through the pump and is exhausted with the air. Wet-type pumps must have oil separators in their discharge line to trap the oil and return it to the engine crankcase.

Whittle, Sir Frank

The British Royal Air Force flying officer who in 1929 filed a patent application for a turbojet engine. Whittle’s engine first flew in a Gloster E.28 on May 15, 1941. The first jet flight in America was made on October 2, 1942, in a Bell XP-59A that was powered by two Whittle-type General Electric I-A engines.

Whole numbers

The numbers: 0, 1, 2, 3, 4, 5, and so on.

Wide Area Augmentation System (WAAS)

The U.S. implementation of SBAS.

Windmilling propeller

A propeller that is rotated by air flowing over the blades rather than powered by the engine.

Wing fences

Vertical vanes that extend chordwise across the upper surface of an airplane wing to prevent spanwise airflow.

Wing heavy

An out-of-trim flight condition in which an airplane flies hands off, with one wing low.

Winglet or tip fin

An out-of-plane surface extending from a lifting surface. The surface may or may not have control surfaces.

Wire bundle

A compact group of electrical wires held together with special wrapping devices or with waxed string. These bundles are secured to the aircraft structure with special clamps.

Wiring diagrams

A diagram that shows the electrical wiring and circuitry, coded for identification, of all the electrical appliances and devices used on aircraft.

Wood decay

Disintegration of wood substance through the action of wood-destroying fungi.

Wood decay - incipient

The early stage of decay in which the disintegration has not proceeded far enough to soften or otherwise perceptibly impair the hardness of the wood.

Wood decay - typical or advanced

The stage of decay in which the disintegration is readily recognized because the wood has become punky, soft and spongy, stringy, pitted, or crumbly.

Woof threads

See fill threads.

Work

The amount of energy transferred by a force

Work

The product of force times distance.

Work

The product of a force times the distance the force is moved.

Worm gear

A helical gear mounted on a shaft. The worm meshes with a spur gear whose teeth are cut at an angle to its face. A worm gear is an irreversible mechanism. The rotation of the shaft, on which the worm gear locks the spur gear so its shaft cannot be rotated.

Wrist pin

The hardened steel pin that attaches a piston to the small end of a connecting rod.

X-ray

A radiographic test method used to detect internal defects in a weld.

XL-Polyalkene

An insulation material based on the polyolefin family that has its normally thermomelt characteristic altered by the radiation cross-linking process to that of a nonmelt, therm-set material.

Yaw

Rotation of an aircraft about its vertical axis.

Yaw

Rotation of an aircraft about its vertical axis.

Yaw damper

An automatic flight control system that counteracts the rolling and yawing produced by Dutch roll.

Yield strength

The amount of stress needed to permanently deform a material.

Zener diode

A special type of solid-state diode designed to have a specific breakdown voltage and to operate with current flowing through it in its reverse direction.

Zeppelin

The name of large, rigid, lighter-than-air ships built by the Zeppelin Company in Germany prior to and during World War I.

Zero fuel weight

The weight of an aircraft without fuel.

Zero-center ammeter

An ammeter in a light aircraft electrical system located between the battery and the main bus. This ammeter shows the current flowing into or out of the battery.

Zero-lash valve lifter

A hydraulic valve lifter that maintains zero clearance in the valve actuating mechanism.

Zone numbers

On drawings, these are similar to the numbers and letters printed on the borders of a map, used for locating a particular point in the drawing

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