Aviation Maintenance Terms beginning with D
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.
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