bilge area. The lowest part of an aircraft structure in which water and contaminants collect, and corrosion is likely to form. The area under the cabin floor is normally called the bilge.
bill of materials (aircraft drawing). A list of all of the materials used in the component or assembly shown in an aircraft drawing.
billet (metal manufacturing). A bar of semifinished steel. A billet is rolled from an ingot, and it generally has a square cross section.
bimetallic circuit breaker (electrical component). A circuit breaker in which one of the current-carrying contacts is mounted on a bimetallic strip. If more than the rated current flows through a bimetallic circuit breaker, the bimetallic strip will heat up and warp. When it warps, it snaps the contacts apart and opens the circuit.
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 coefficient of expansion, and as the temperature changes, the spiral either tightens or loosens.
The pointer of an analog thermocouple-type temperature indicator is mounted on a bimetallic hairspring to compensate its indication for changes in temperature of the reference junction.
bimetallic strip. A strip made of two different types of metal fastened together, side-by-side. When heated, these two metals expand differently, causing the strip to warp, or bend. Bimetallic strips are used in some types of thermometers and circuit breakers.
binary coded decimal. See BCD.
binary number system. The number system whose base is two. It uses only two values, zero and one. Each position represents a value twice that of the position to its right. For example:
643216 8 4 2 1
0 0 0 0 0 0 1 = 1
0 0 0 0 0 1 0 = 2
0 0 0 0 1 0 0 = 4
0 0 0 1 0 0 0 = 8
0 0 1 0 0 0 0 = 16

0 1 0 0 0 0 0 = 32
1 0 0 0 0 0 0 = 64
The binary number system is useful when working with digital electronics, because the two basic conditions of electricity, on and off, can be used to represent the two bits of the binary number system. When the system is on, the bit one is represented, and when it is off, zero is represented.
binding post (electrical equipment). A connector used to fasten a conductor to an electrical test panel. Most binding posts have an insulated nut that can be turned with the fingers to grip the wire. Some binding posts have, in addition to the nut, holes for a banana plug.
binoculars. A form of hand-held optical instrument used to observe distant objects. Binoculars have a set of magnifying lenses for each eye, and prisms are used to get a high degree of magnification in a short physical length.
binomial. An algebraic expression that contains two terms. 4X + 2Y is a binomial expression.
bioastronautics. The branch of science dealing with the medical and biological aspects of astronautics.
biochemistry. The branch of chemistry dealing with chemical compounds and processes involved with living organisms.