Preflight inspection.  A required inspection to determine the condition of the aircraft for the flight to be conducted. It is conducted by the pilot-in-command. 

Precipitation heat treatment.  A method of increasing the strength of heat-treated aluminum alloy. After the aluminum alloy has been solution-heat-treated by heating and quenching, it is returned to the oven and heated to a temperature lower than that used for the initial heat treatment. It is held at this temperature for a specified period of time, and then removed from the oven and allowed to cool slowly. 

Prepreg (preimpregnated fabric). A type of composite material in which the reinforcing fibers are encapsulated in an uncured resin. Prepreg materials must be kept refrigerated to prevent them from curing before they are used. 

Press-to-test light fixture.  An indicator light fixture whose lens can be pressed in to complete a circuit that tests the filament of the light bulb. 

Pressure.  Force per unit area. Hydraulic and pneumatic pressure are normally given in units of pounds per square inch (psi). 

Pressure altitude.  The altitude in standard air at which the pressure is the same as that of the existing air. Pressure altitude is read on an altimeter when the barometric scale is set to the standard sea level pressure of 29.92 inches of mercury. 

Pressure-demand oxygen system. A type of oxygen system used by aircraft that fly at very high altitude. This system functions as a diluter-demand system until, at about 40,000 feet, the output to the mask is pressurized enough to force the needed oxygen into the lungs, rather than depending on the low pressure produced when the wearer of the mask inhales to pull in the oxygen. (See diluter-demand oxygen system.) 

Pressure fueling.  The method of fueling used by almost all transport aircraft. The fuel is put into the aircraft through a single underwing fueling port. The fuel tanks are filled to the desired quantity and in the sequence selected by the person conducting the fueling operation. Pressure fueling saves servicing time by using a single point to fuel the entire aircraft, and it reduces the chances for fuel contamination. 

Pressure manifold (hydraulic system component). The portion of a fluid power system from which the selector valves receive their pressurized fluid. 

Pressure plate (brake component).  A strong, heavy plate used in a multiple-disk brake. The pressure plate receives the force from the brake cylinders and transmits this force to the disks. 

Pressure reducing valve (oxygen system component). A valve used in an oxygen system to change high cylinder pressure to low system pressure. 

Pressure relief valve (oxygen system component).  A valve in an oxygen system that relieves the pressure if the pressure reducing valve should fail. 

Pressure vessel.  The strengthened portion of an aircraft structure that is sealed and pressurized in flight. 

Primer (finishing system component).  A component in a finishing system that provides a good bond between the surface and the material used for the topcoats. 

Profile drag.  Aerodynamic drag produced by skin friction. Profile drag is a form of parasite drag. 

Progressive inspection.  An inspection that may be used in place of an annual or 100-hour inspection. It has the same scope as an annual inspection, but it may be performed in increments so the aircraft will not have to be out of service for a lengthy period of time. 

Pump control valve. A control valve in a hydraulic system that allows the pilot to manually direct the output of the hydraulic pump back to the reservoir when no unit is being actuated. 

Pureclad.  A registered trade name for clad aluminum alloy. 

Purge (air conditioning system operation).  To remove all of the moisture and air from a cooling system by flushing the system with a dry gaseous refrigerant. 

Pusher powerplant.  A powerplant whose propeller is mounted at the rear of the airplane and pushes, rather than pulls, the airplane through the air. 

PVC (Polyvinylchloride). A thermoplastic resin used to make transparent tubing for insulating electrical wires. 

Quartersawed wood. Wood sawed from a tree in such a way that the annual rings cross the plank at an angle greater than 45°. 

Quick-disconnect fitting.  A hydraulic line fitting that seals the line when the fitting is disconnected. Quick-disconnect fittings are used on the lines connected to the engine-driven hydraulic pump. They allow the pump to be disconnected and an auxiliary hydraulic power system connected to perform checks requiring hydraulic power while the aircraft is in the hangar. 

