Aviation Maintenance Terms beginning with B
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.
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