Bead seat area.  The flat surface on the inside of the rim of  an aircraft wheel on which the bead of the tire seats. Bearing strength (sheet metal characteristic).  The amount  of pull needed to cause a piece of sheet metal to tear at the  points at which it is held together with rivets. The bearing  strength of a material is affected by both its thickness and  the diameter of the rivet. Beehive spring.  A hardened-steel, coil-spring retainer used  to hold a rivet set in a pneumatic rivet gun. This spring gets  its name from its shape. It screws onto the end of the rivet  gun and allows the set to move back and forth, but prevents  it being driven from the gun. Bend allowance.  The amount of material actually used  to make a bend in a piece of sheet metal. Bend allowance  depends upon the thickness of the metal and the radius of  the bend, and is normally found in a bend allowance chart. Bend radius.  The radius of the inside of a bend. Bend tangent line.  A line made in a sheet metal layout that  indicates the point at which the bend starts. Bernoulli’s principle.  The basic principle that explains  the relation between kinetic energy and potential energy in  fluids that are in motion. When the total energy in a column  of moving fluid remains constant, any increase in the kinetic  energy of the fluid (its velocity) results in a corresponding  decrease in its potential energy (its pressure). Bezel.  The rim that holds the glass cover in the case of an  aircraft instrument. Bias-cut surface tape.  A fabric tape in which the threads  run at an angle of 45° to the length of the tape. Bias-cut tape  may be stretched around a compound curve such as a wing  tip bow without wrinkling. Bilge area.  A low portion in an aircraft structure in which  water and contaminants collect. The area under the cabin  floorboards is normally called the bilge. Bipolar transistor.  A solid-state component in which the  flow of current between its emitter and collector is controlled  by a much smaller flow of current into or out of its base.  Bipolar transistors may be of either the NPN or PNP type. BITE.  Built-in test equipment. Blade track.  The condition of a helicopter rotor in which each  blade follows the exact same path as the blade ahead of it. Black box.  A term used for any portion of an electrical or  electronic system that can be removed as a unit. A black box  does not have to be a physical box. Bladder-type fuel cell.  A plastic-impregnated fabric bag  supported in a portion of an aircraft structure so that it forms  a cell in which fuel is carried. Bleeder.  A material such as glass cloth or mat that is placed  over a composite lay-up to absorb the excess resin forced out  of the ply fibers when pressure is applied. Bleeding dope.  Dope whose pigments are soluble in the  solvents or thinners used in the finishing system. The color  will bleed up through the finished coats. Bleeding of brakes.  The maintenance procedure of removing  air entrapped in hydraulic fluid in the brakes. Fluid is bled  from the brake system until fluid with no bubbles flows out. Blimp.  A cigar-shaped, nonrigid lighter-than-air flying machine.  Blush.  A defect in a lacquer or dope finish caused by moisture  condensing on the surface before the finish dries. If the  humidity of the air is high, the evaporation of the solvents  cools the air enough to cause the moisture to condense. The  water condensed from the air mixes with the lacquer or dope  and forms a dull, porous, chalky-looking finish called blush.  A blushed finish is neither attractive nor protective. Bonding.  The process of electrically connecting all isolated  components to the aircraft structure. Bonding provides a  path for return current from electrical components, and a  low-impedance path to ground to minimize static electrical  charges. Shock-mounted components have bonding braids  connected across the shock mounts. Boost pump.  An electrically driven centrifugal pump  mounted in the bottom of the fuel tanks in large aircraft. Boost  pumps provide a positive flow of fuel under pressure to the  engine for starting and serve as an emergency backup in the  event an engine-driven pump should fail. They are also used  to transfer fuel from one tank to another and to pump fuel  overboard when it is being dumped. Boost pumps prevent  vapor locks by holding pressure on the fuel in the line to the  engine-driven pump. Centrifugal boost pumps have a small  agitator propeller on top of the impeller to force vapors from  the fuel before it leaves the tank. G-5