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 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.