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process.
bay (structural component). A compartment in the structure of an aircraft. A bay is the portion of the
structure between adjacent bulkheads, frames, or struts.
bayonet exhaust stack (aircraft reciprocating engine component). The elongated and flattened end of
the exhaust stack used on an aircraft reciprocating engine. The gases leave the stack through a slot parallel
to its length.
Bayonet stacks decrease both exhaust back pressure and noise and prevent cold air from flowing
into the exhaust stack during such maneuvers as slips. This cold air could cause the valves to warp.
bayonet fueling nozzle. A type of nozzle used to fuel aircraft with a pressure, or single-point, fueling
system. The nozzle is connected to the fueling receptacle in the aircraft, and the handles are turned a
portion of a turn to lock it in place.
bayonet gage. A term for a dip stick, such as that used to measure the quantity of a liquid in a tank or a
reservoir.
bayonet thermocouple probe. A thermocouple pickup used to measure cylinder head temperature on an
air-cooled aircraft engine.
The bayonet probe fits into an adapter screwed into the cylinder head and a spring holds the end of
the probe tightly against the cylinder head.
B-battery. A source of high-voltage direct current needed for vacuum tubes. Vacuum tubes must have a
high positive DC voltage on their plates and screen grids to pull electrons across the vacuum from the
cathode. This high voltage is called B plus (B+) voltage.
BBC. See before bottom center.
BCD (binary coded decimal). A system used for changing decimal numbers into binary numbers. Each
digit from 0 through 9 is represented with four binary digits:
Decimal Binary Decimal Binary
0 = 0000 5 = 0101
1 = 0001 6 = 0110
2 = 0010 7 = 0111
3 = 0011 8 = 1000
4 = 0100 9 = 1001
The decimal number 126 for example, expressed in BCD, is: 0001 0010 0110.
B-check. See maintenance checks.
BDC. See bottom dead center.
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 67
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 (tubing). A rounded ridge formed near the end of a piece of rigid tubing used for carrying fluids in an
aircraft.
A hose is slipped over the end of the tube, and a hose clamp is installed between the end of the
hose and the bead. The bead keeps the tube from being pulled out of the hose.
bead (welding). The ridge of filler metal that sticks up above the surface of metal that has been welded.
The appearance of the bead usually indicates the condition of the weld. The bead of a good weld is
uniform in height, has a smooth, uniform ripple on its surface, and blends evenly into the base metal.
bead seat area of a wheel. The flat surface of an aircraft wheel on which the bead of the tire seats.
The bead seat area of wheels that have tubeless tires installed on them must be very smooth and
free of damage. Only then can the tire form an airtight seal with the wheel.
bead thermistor (fire detection system component). A component in a fire detection system that signals
the presence of a fire or overheat condition.
Beads of a thermistor material (a material, such as germanium, whose resistance changes with its
temperature) support a wire that forms part of the fire detection circuit. The bead-supported wire is
enclosed in a metal tube routed through the area to be protected.
If the tube is heated by a fire, the thermistor material changes from an insulator into a conductor
and completes an electrical circuit between the central wire and the tube. This completed circuit initiates
the fire-warning procedure by turning on the fire-warning light and sounding the fire-warning bell.
beaded coaxial cable (radio frequency transmission line). A form of coaxial transmission line in which
the inner conductor is centered in the outer conductor by a series of beads made of insulating material.
beam (radio navigation). The beam is a descriptive term which refers to an invisible path produced by
radio signals. An aircraft can follow the beam when going to or from a navigational fix.
The term beam became popular with the four-course, low-frequency radio range that projected
four clearly defined beams, or courses, from each of the transmitting stations. When a pilot was flying an
aircraft along one of these courses, he was said to be “on the beam.”
beam (structural member). A long, heavy, metal or wood member in any type of structure used to support
both bending and shear loads.
beam antenna (radio antenna). A radio transmitting antenna that concentrates its radiation into a narrow
　
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