Aviation Maintenance Terms beginning with B
Ballast
A weight installed or carried in an aircraft to move the center of gravity to a location within its allowable limits.
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.
Basic empty weight
Standard empty weight plus optional equipment.
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.
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.
Borescope
A device that enables the inspector to see inside areas that could not otherwise be inspected without disassembly.
Boyle’s law
States that the volume of an enclosed dry gas varies inversely with its absolute pressure, provided the temperature remains constant.
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 required to change the temperature of 1 pound of water by 1 degree Fahrenheit.
Buoyancy
The upward force that any fluid exerts on a body submerged in it.
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
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-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.
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.
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.
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.
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
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 which holds the glass cover in the case of an aircraft instrument.
Bidirectional fibers
Fibers in a piece of composite material arranged to sustain loads in two directions.
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.
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.
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.
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.
Boost
A term for manifold pressure that has been increased above the ambient atmospheric pressure by a supercharger.
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
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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 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
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 electrons between the emitter and the collector.
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.
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.
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).
Bezel
The rim that holds the glass cover in the case of an aircraft instrument.
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.
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.
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.
Blade track
The condition of a helicopter rotor in which each blade follows the exact same path as the blade ahead of it.
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.
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.
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.
Blimp
A cigar-shaped, nonrigid lighter-than-air flying machine.
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.
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.
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.
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.
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.
British thermal unit (BTU)
The amount of heat energy needed to raise the temperature of one pound of pure water 1 °F.
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 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.
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.
Buttock line
A line used to locate a position to the right or left of the center line of an aircraft structure.
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.
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.
Brake horsepower
The power delivered at the propeller shaft (main drive or main output) of an aircraft 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.
Base metal
The metal to be welded, brazed, soldered, or cut.
Black light
Electromagnetic radiation in the near ultraviolet range of wavelength.
Blade station
A reference position on a blade that is a specified distance from the center of the hub.
Bond
The adhesion of one surface to another, with or without the use of an adhesive as a bonding agent.
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.
Borescope
A long, tubular optical instrument designed for remote visual inspection of surfaces.
Brashness
A condition of wood characterized by low resistance to shock and by an abrupt failure across the grain without splintering.
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
The joining of two pieces of metal by wetting their surface with molten alloy of copper, zinc, or tin.
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.
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.
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.
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.
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.
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.
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 track
The path established by the tip of each blade as it passes a fixed point in rotation.
Blade
One arm of a propeller from the hub to tip.