Boundary layer.  The layer of air that flows next to an aerodynamic surface. Because of the design of the surface and local surface roughness, the boundary layer often has a random flow pattern, sometimes even flowing in a direction opposite to the direction of flight. A turbulent boundary layer causes a great deal of aerodynamic drag. 

Bourdon tube.  A pressure-indicating mechanism used in most oil pressure and hydraulic pressure gages. It consists of a sealed, curved tube with an elliptical cross section. Pressure inside the tube tries to straighten it, and as it straightens, it moves a pointer across a calibrated dial. Bourdon-tube pressure gauges are used to measure temperature by measuring the vapor pressure in a sealed container of a volatile liquid, such as methyl chloride, whose vapor pressure varies directly with its temperature. 

Brazing.  A method of thermally joining metal parts by wetting the surface with a molten nonferrous alloy. When the molten material cools and solidifies, it holds the pieces together. Brazing materials melt at a temperature higher than 800 °F, but lower than the melting temperature of the metal on which they are used. 

British thermal unit (BTU).  The amount of heat energy needed to raise the temperature of one pound of pure water 1 °F. 

Bucking bar. A heavy steel bar with smooth, hardened surfaces, or faces. The bucking bar is held against the end of the rivet shank when it is driven with a pneumatic rivet gun, and the shop head is formed against the bucking bar. 

Buffeting.  Turbulent movement of the air over an aerodynamic surface. 

Bulb angle.  An L-shaped metal extrusion having an enlarged, rounded edge that resembles a bulb on one of its legs. 

Bulkhead.  A structural partition that divides the fuselage of an aircraft into compartments, or bays. 

Bungee shock cord.  A cushioning material used with the nonshock absorbing landing gears installed on older aircraft. Bungee cord is made up of many small rubber bands encased in a loose-woven cotton braid. 

Burnish (verb).  To smooth the surface of metal that has been damaged by a deep scratch or gouge. The metal piled up at the edge of the damage is pushed back into the damage with a smooth, hard steel burnishing tool. 

Burr.  A sharp rough edge of a piece of metal left when the metal was sheared, punched, or drilled. 

Bus.  A point within an electrical system from which the individual circuits get their power. 

Buttock line.  A line used to locate a position to the right or left of the center line of an aircraft structure. 

Butyl.  Trade name for a synthetic rubber product made by the polymerization of isobutylene. Butyl withstands such potent chemicals as phosphate ester-base (Skydrol) hydraulic fluids. 

Cage (verb).  To lock the gimbals of a gyroscopic instrument so it will not be damaged by abrupt flight maneuvers or rough handling. 

Calendar month.  A measurement of time used by the FAA for inspection and certification purposes. One calendar month from a given day extends from that day until midnight of the last day of that month. 

Calibrated airspeed (CAS).  Indicated airspeed corrected for position error. See position error. 

Calorie. The amount of heat energy needed to raise the temperature of one gram of pure water 1 °C. 

Canted rate gyro.  A rate gyro whose gimbal axis is tilted so it can sense rotation of the aircraft about its roll axis as well as its yaw axis. 

Camber (wheel alignment).  The amount the wheels of an aircraft are tilted, or inclined, from the vertical. If the top of the wheel tilts outward, the camber is positive. If the top of the wheel tilts inward, the camber is negative. 

Canard.  A horizontal control surface mounted ahead of the wing to provide longitudinal stability and control. 

Cantilever wing.  A wing that is supported by its internal structure and requires no external supports. The wing spars are built in such a way that they carry all the bending and torsional loads. 

Cap strip.  The main top and bottom members of a wing rib. The cap strips give the rib its aerodynamic shape. 

Capacitance-type fuel quantity measuring system.  A popular type of electronic fuel quantity indicating system that has no moving parts in the fuel tank. The tank units are cylindrical capacitors, called probes, mounted across the tank, from top to bottom. The dielectric between the plates of the probes is either fuel or the air above the fuel, and the capacitance of the probe varies with the amount of fuel in the tank. The indicator is a servo-type instrument driven by the amplified output of a capacitance bridge. 

