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. Pressure reducing valve (oxygen system component). A valve used in an oxygen system to change high cylinder  pressure to low system pressure. 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. 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. 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. 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. 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.) 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. 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. 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°. 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. 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. G-27