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British thermal unit. See Btu.
brittleness. A physical characteristic of a material that causes it to break, or fracture, without bending or
distorting in any appreciable amount. Brittleness is the opposite of malleability.
broaching (metalworking procedure). The process of removing excess metal by pushing or pulling a
cutting tool through a hole or the inside of a cylinder. The points inside a socket wrench are cut by
broaching.
broad-band antenna (radio antenna). A radio antenna capable of receiving or transmitting a broad band
of frequencies.
broadcast. The transmission of information not addressed to a specific station or person, and for which no
acknowledgment is expected.
broadcast (ICAO). A transmission of information relating to air navigation that is not addressed to a
specific station or stations.
broken-line graph. A type of graph used to show the way values change. The horizontal axis of the graph
represents time, and the vertical axis represents the changing value. Straight lines are used to connect points
that show the values at each plotted time.
bromine. A heavy, volatile, corrosive, reddish-brown, nonmetallic, liquid chemical element. Bromine’s
symbol is Br, its atomic number is 35, and its atomic weight is 79.904. Bromine is used as an antiknock
additive for gasoline and as a component in dyes.
bronze. A copper alloy containing tin and sometimes small amounts of zinc and phosphorus. Bronze’s low
coefficient of friction makes it useful for bearings and bushings.
brush (electrical component). The component in an electric motor or generator through which current
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flows into and out of the armature. Brushes used in generators are made of a carbon compound, and they
slide over the surface of the copper commutator bars. Current induced into the armature windings is
brought out of the windings through the brushes.
Starter motor bushes which must carry large amounts of current are often made of a copper alloy.
brush (paintbrush). A device used to apply paint to a surface. A paintbrush is made by attaching a bundle
of bristles to a wood or plastic handle. The bristles are made of animal hair or synthetic fibers.
The bristles are dipped into the paint and then moved over the surface to be painted. Good quality
brushes allow the paint to flow out smoothly over the surface.
brush guard (helicopter tail rotor protector). A protective frame at the tail of a single-rotor helicopter
used to protect the tail rotor from damage during ground operation.
BSFC (brake specific fuel consumption). A measure of the amount of fuel used for a given amount of
power developed by a heat engine.
BSFC is expressed in pounds of fuel burned per hour for each brake horsepower the engine is
developing.
BTC. See before top center.
Btu (British thermal unit). A unit of heat measurement. The amount of heat energy needed to raise the
temperature of 1 pound of pure water from 60° to 61°F.
bubble memory (computer memory). A method of digital data storage in which individual bits (0s and 1s)
are stored in the form of extremely small localized magnetic fields in a thin film of magnetic crystalline
material.
Bits of stored data can be moved about by the use of an external magnetic field.
bubble octant (navigation instrument). A type of celestial navigation instrument. A bubble level in the
octant provides an artificial horizon which allows the navigator to find the angle between a line tangent to
the earth’s surface (the horizon) and a line to the stars being used for navigation.
bubble. A small volume of gas entrapped in a liquid.
bucket wheel. A turbine wheel in a turbosupercharger or in a gas turbine engine.
bucket. Colloquial, or informal, term for a turbine blade.
bucking (rivet bucking). The process in which a shop head is formed on a solid aircraft rivet. The rivet is
inserted into holes drilled through the metal sheets being joined. The manufactured head of the rivet is
hammered with a pneumatic hammer through a rivet set that fits the shape of the head.
A hardened-steel bucking bar is held against the shank of the rivet, and the hammering causes the
end of the shank to expand and become shorter, forming the shop head that clamps the metal pieces
together.
bucking bar (sheet metalworking tool). A heavy steel block or bar with smooth, hardened surfaces, or
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faces. A bucking bar is held flat against the end of the shank of a solid rivet when the head is driven with a
rivet gun. Driving the rivet flattens the end of the shank against the bucking bar and forms the bucked, or
　
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