Butterfly valve.  A flat, disk-shaped valve used to control  the flow of fluid in a round pipe or tube. When the butterfly  valve is across the tube, the flow is shut off, and when it is  parallel with the tube, the obstruction caused by the valve  is minimum, and the flow is at its greatest. Butterfly-type  throttle valves are used to control the airflow through the  fuel metering system. Boss.  An enlarged area in a casting or machined part. A  boss provides additional strength to the part where holes for  mounting or attaching parts are drilled. Bottom. (verb)  A condition in the installation of a propeller  on a splined shaft when either the front or rear cone contacts  an obstruction that prevents the cone from properly seating  inside the propeller hub. Bourdon tube.  The major component in a gage-pressure  measuring instrument. It is a thin-wall metal tube that has  an elliptical cross section and is formed into a curve. One  end of the tube is sealed and connected to an arm that moves  the pointer across the instrument dial, and the open end is  anchored to the instrument case. The pressure to be measured  is directed into the open end, which causes the elliptical  cross section to become more circular. As the cross section  changes, the curve straightens and moves the pointer over  the dial by an amount proportional to the amount of pressure. Bypass engine.  Another name for a turbofan engine. See  turbofan engine. Bypass ratio.  The ratio of the mass of air moved by the fan  to the mass of air moved by the core engine. Calendar month.  The measurement of time used by the FAA  for inspection and certification purposes. One calendar month  from a given date extends from that date until midnight of  the last day of that month. Cam.  An eccentric, or lobe, on a rotating shaft that changes  rotary motion into linear motion. A cam is mounted on the  magnet shaft in a magneto to push upward on the insulated  breaker point to separate, or open, the points when the magnet  is in a particular location. Brayton cycle.  The constant-pressure cycle of energy  transformation used by gas turbine engines. Fuel is sprayed  into the air passing through the engine and burned. Heat from  the burning fuel-air mixture expands the air and accelerates it  as it moves through the engine. The Brayton cycle is an open  cycle in that the intake, compression, combustion, expansion,  and exhaust events all take place at the same time, but in  different locations within the engine. Cam engine.  A reciprocating engine with axial cylinders  arranged around a central shaft. Rollers on the pistons in  the cylinders press against a sinusoidal cam mounted on the  shaft to produce rotation of the shaft. British thermal unit (Btu).  The basic unit of heat energy  in the English system. One Btu is the amount of heat energy  needed to raise the temperature of one pound of pure water  from 60 ˚F to 61˚F. Cam-ground piston.  A reciprocating engine piston that is  not round, but is ground so that its diameter parallel to the  wrist pin is slightly smaller than its diameter perpendicular  to the pin. The mass of metal used in the wrist pin boss,  the enlarged area around the wrist pin hole, expands when  heated, and when the piston is at its operating temperature,  it is perfectly round. BSFC.  Brake specific fuel consumption. A measure of the  amount of fuel used for a given amount of power produced  by a heat engine. BSFC is expressed in pounds of fuel burned  per hour for each brake horse-power the engine is producing. Buckets.  The portions of aft-fan blades that are in the exhaust  of the core engine. Buckets drive the fan from energy received  from hot gases leaving the core engine. Can-annular combustor.  A type of combustor used in some  large turbojet and turbofan engines. It consists of individual  cans into which fuel is sprayed and ignited. These cans mount  on an annular duct which collects the hot gases and directs  them uniformly into the turbine. Bungee cord.  An elastic cord made of small strips of rubber  encased in a loosely braided cloth tube that holds and protects  the rubber, yet allows it to stretch. The energy in a stretched  bungee cord may be used to crank a large aircraft engine. Burner.  See combustor. Capacitance afterfiring.  The continuation of the spark  across the gap in a shielded spark plug after the fuel-air  mixture in the cylinder is ignited. Afterfiring is caused by  the return of electrical energy stored in the capacitance of the  shielded ignition leads. Capacitance afterfiring is eliminated  by the use of a resistor in the spark plug. Burnish.  To smooth the surface of a metal part that has been  damaged by a deep scratch or gouge. Metal piled at the edge  of the damage is pushed back into the damage with a smooth,  hard steel burnishing tool. G-5