Pitch distribution . The gradual change in pitch angle of a propeller blade from the root to the tip.

Plane of rotation . The plane in which a propeller blade rotates. The plane of rotation is perpendicular to the propeller shaft.

Planetary gears . A type of large-ratio reduction gearing. A series of small planetary gears are mounted on a spider attached to the output shaft. The planetary gears rotate between a fixed sun gear and a driven ring gear.

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 positivedisplacement 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.

Pre-ignition . Ignition of the air/fuel mixture inside the cylinder of an engine before the time for normal ignition. Pre-ignition 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 continuousflow 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.