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. 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. 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. Canard.  A horizontal control surface mounted ahead of the  wing to provide longitudinal stability and control. 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. 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. Calorie.  The amount of heat energy needed to raise the  temperature of one gram of pure water 1 °C. Calibrated airspeed (CAS).  Indicated airspeed corrected  for position error. See position error. 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. Cage (verb).  To lock the gimbals of a gyroscopic instrument  so it will not be damaged by abrupt flight maneuvers or  rough handling. 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. Buttock line.  A line used to locate a position to the right or  left of the center line of an aircraft structure. Bus.  A point within an electrical system from which the  individual circuits get their power.  G-6