Plenum chamber.  An enclosed chamber in which air can be  held at a pressure slightly higher than that of the surrounding  air. Plenum chambers are used to stabilize the pressure of the  air before it enters a double entry centrifugal compressor. POH.  Pilot’s Operating Handbook. A document published  by the airframe manufacturer and approved by the FAA that  lists the operating conditions for a particular model of aircraft.  Engine operating parameters are included in the POH. Pole shoe.  Inward extensions from the field frame of a  generator around which the field coils are wound. Poppet valve.  A T-shaped valve with a circular head. Poppet  valves are used to cover the intake and exhaust openings in  the cylinder head of a reciprocating engine. The valves are  held closed by one or more coil springs and are opened by a  cam lobe or a rocker arm pushing on the end of the valve stem. Porcelain.  A hard, white, translucent ceramic material that  was used as the insulator in some of the early aircraft spark  plugs. Positive-displacement pump.  A fluid pump that moves  a specific volume of fluid each time it rotates. Spur-gear  pumps, gerotor pumps, and vane pumps are all positive- displacement pumps. Power.  The time rate of doing work. Power is found by  dividing the amount of work done, measured in floor-pounds,  by the time in seconds or minutes used to do the work. Power  may be expressed in foot-pounds of work per minute or in  horsepower. One horsepower is 33,000 foot-pounds of work  done in one minute, or 550 foot pounds of work done in one  second. Power-assurance check.  A test run made of a gas turbine  engine to determine how its performance compares with its  precious performance as new or freshly overhauled. Powerplant.  The complete installation of an aircraft engine,  propeller, and all accessories needed for its proper function. Preignition.  Ignition of the fuel-air mixture inside the  cylinder of an engine before the time for normal ignition.  Preignition is often caused by incandescent objects inside  the cylinder. Prepreg.  Preimpregnated fabric. A type of composite  material in which the reinforcing fibers are encapsulated in  an uncured resin. Prepreg materials are cut to size and shape  and laid up with the correct ply orientation, and the entire  component is cured with heat and pressure. Pressure.  A measure of force applied uniformly over a given  unit of surface area. Pressure altitude.  The altitude in standard atmosphere at  which the pressure is the same as the existing pressure. Pressure carburetor.  A carburetor installed on some aircraft  reciprocating engines that uses the pressure difference  between air inside the venture and ram air entering the  carburetor to produce a fuel-metering force. Pressure  carburetors have generally been replaced with continuous- flow fuel injection systems. Pressure cooling.  A method of air cooling a reciprocating  engine in which the cylinders are enclosed in tight-fitting  shrouds. The cowling is divided into two compartments  by baffles and seals, with half of each cylinder in each  compartment. Ram air is directed into one compartment, and  the pressure in the other is decreased by air flowing over a  flared exit or adjustable cowl flaps. The pressure difference  across the cylinders causes cooling air to be drawn through  the fins to remove the unwanted heat. Pressure-injection carburetor.  A multibarrel pressure  carburetor used on large radial and V-engines. Fuel is  metered on the basis of air mass flowing into the engine  and is sprayed under pressure into the eye, or center, of the  internal supercharger impeller. Prevailing torque.  The torque required to turn a threaded  fastener before it contacts the surface it is intended to hold. Primary winding.  The winding in a magneto or ignition coil  that is between the source of voltage and the breaker points.  The primary winding is normally made of comparatively  large diameter wire, and has a small number of turns, typically  about 200. Profile tip (compressor blade tip).  The tip of an axial-flow  compressor bladed whose thickness is reduced to give it a  higher resonant frequency so it will not be subject to the  vibrations that would affect a blade with a squared tip. The  profile tip also provides a more aerodynamically efficient  shape for the high velocity air that is moved by the blade.  Profile tips often touch the housing and make a squealing  noise as the engine is shut down. For this reason profile tips  are often called squealer tips. Profilometer.  A precision measuring instrument used to  measure the depth of the hone marks in the surface of a  cylinder wall. G-21