BITE.  Built-in test equipment. A troubleshooting system  installed in many modern electronic equipment. BITE  equipment monitors engine and airframe systems, and when a  fault is found, isolates it and provides maintenance personnel  with a code that identifies the LRU (line replaceable unit)  that contains the fault. Bell mouth inlet duct.  A form of convergent inlet-air duct  used to direct air into the compressor of a gas turbine engine.  It is extremely efficient, and is used where there is little ram  pressure available to force air into the engine. Bell mouth  ducts are used in engine test cells and on engines installed  in helicopters. Benzene.  A colorless, volatile, flammable, aromatic  hydrocarbon liquid which has the chemical formula C$_{6}$H$_{6}$.  Benzene, which is sometimes called benzoil, is used as a  solvent, a cleaning fluid, and a fuel for some special types  of reciprocating engines. Blade.  The component of a propeller that converts the  rotation of the propeller shaft into thrust. The blade of a  propeller corresponds to the wing of an airplane. Blending.  A method of repairing damaged compressor  and turbine blades. The damage is removed and the area is  cleaned out with a fine file to form a shallow depression with  generous radii. The file marks are then removed with a fine  abrasive stone so the surface of the repaired area will match  the surface of the rest of the blade. Bernoulli’s principle.  A physical principle that explains  the relationship between kinetic and potential energy in a  stream of moving fluid. When energy is neither added to  nor taken from the fluid, any increase in its velocity (kinetic  energy) will result in a corresponding decrease in its pressure  (potential energy). Blisk.  A turbine wheel machined from a single slab of steel.  The disk and blades are an integral unit. Blow-in doors.  Spring-loaded doors in the inlet duct of some  turbojet or turbofan engine installations that are opened by  differential air pressure when inlet air pressure drops below  that of the ambient air. Air flowing through the doors adds  to the normal inlet air passing through the engine and helps  prevent compressor stall. Beta control range (Beta mode).  The range of operation  of a turboprop powerplant used for in-flight approach and  ground handling of the engine and aircraft. Typically, the  Beta mode includes operations from 65% to 95% of the  engine’s rated rpm. Beta tube.  A tube in a Garrett TPE331 turboprop powerplant  that extends into the propeller pitch control to act as a follow- up device. It provides movement of the propeller blades in  proportion to movement of the power lever. BMEP.  Brake mean effective pressure. The average pressure  inside the cylinder of a reciprocating engine during the power  stroke. BMEP, measured in pounds per square inch, relates  to the torque produced by the engine and can be calculated  when you know the brake horsepower. Bezel.  The rim which holds the glass cover in the case of an  aircraft instrument. BHP.  Brake horsepower. The actual horsepower delivered  to the propeller shaft of a reciprocating or turboprop engine. Bidirectional fibers.  Fibers in a piece of composite material  arranged to sustain loads in two directions. Boost.  A term for manifold pressure that has been increased  above the ambient atmospheric pressure by a supercharger. Bootstrapping.  An action that is self-initiating or self- sustaining. In a turbocharger system, bootstrapping  describes a transient increase in engine power that causes the  turbocharger to speed up, which in turn causes the engine to  produce more power. 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 coefficients of expansion,  and as the temperature changes, the spiral either tightens or  loosens. A bimetallic hair spring is used in a thermocouple  temperature changes at the reference junction. Bore.  The diameter of a reciprocating engine cylinder. Borescope.  An inspection tool for viewing the inside of  a turbine engine without disassembling it. The instrument  consists of a light, mirror, and magnifying lens mounted  inside a small-diameter tube that is inserted into a turbine  engine through borescope inspection ports. Bimetallic strip.  A metal strip made of two different types  of metal fastened together side by side. When heated, the two  metals expand different amounts and the strip warps or bends.  G-4