Pitch pocket (wood defect). Pockets of pitch that appear in the growth rings of a piece of wood. 

Pitot pressure.  Ram air pressure used to measure airspeed. The pitot tube faces directly into the air flowing around the aircraft. It stops the air and measures its pressure. 

Plain-weave fabric. Fabric in which each warp thread passes over one fill thread and under the next. Plain-weave fabric typically has the same strength in both warp and fill directions. 

Plan position indicator (PPI).  A type of radar scope that shows both the direction and distance of the target from the radar antenna. Some radar antenna rotate and their PPI scopes are circular. Other antenna oscillate and their PPI scopes are fan shaped. 

Planer.  A woodworking power tool used to smooth the surfaces of a piece of wood. 

Plasticizer. A constituent in dope or lacquer that gives its film flexibility and resilience. 

Plastic media blasting (PMB).  A method of removing paint from an aircraft surface by dry-blasting it with tiny plastic beads. 

Plastics. The generic name for any of the organic materials produced by polymerization. Plastics can be shaped by molding or drawing. 

Plenum. An enclosed chamber in which air can be held at a pressure higher than that of the surrounding air. 

Ply rating.  The rating of an aircraft tire that indicates its relative strength. The ply rating does not indicate the actual number of plies of fabric in the tire; it indicates the number of piles of cotton fabric needed to produce the same strength as the actual piles. 

Plywood.  A wood product made by gluing several pieces of thin wood veneer together. The grain of the wood in each layer runs at 90° or 45° to the grain of the layer next to it. 

Pneumatics. The system of fluid power which transmits force by the use of a compressible fluid. 

PNP transistor. A bipolar transistor made of a thin base of N-type silicon or germanium sandwiched between a collector and an emitter, both of which are made of P-type material. 

Polyester fibers. A synthetic fiber made by the polymerization process in which tiny molecules are united to form a long chain of molecules. Polyester fibers are woven into fabrics that are known by their trade names of Dacron, Fortrel, and Kodel. Polyester film and sheet are known as Mylar and Celenar. 

Polyester resin.  A thermosetting resin used as a matrix for much of the fiberglass used in composite construction. 

Polyurethane enamel. A hard, chemically resistant finish used on aircraft. Polyurethane enamel is resistant to damage from all types of hydraulic fluid. 

Polyvinyl chloride. A thermoplastic resin used in the manufacture of transparent tubing for electrical insulation and fluid lines which are subject to low pressures. 

Position error.  The error in pitot-static instruments caused by the static ports not sensing true static air pressure. Position error changes with airspeed and is usually greatest at low airspeeds. 

Potential energy.  Energy possessed in an object because of its position, chemical composition, shape, or configuration. 

Potentiometer.  A variable resistor having connections to both ends of the resistance element and to the wiper that moves across the resistance. 

Pot life.  The length of time a resin will remain workable after the catalyst has been added. If a catalyzed material is not used within its usable pot life, it must be discarded and a new batch mixed up. 

Power. The time rate of doing work. Power is force multiplied by distance (work), divided by time. 

Power brakes.  Aircraft brakes that use the main hydraulic system to supply fluid for the brake actuation. Aircraft that require a large amount of fluid for their brake actuation normally use power brakes, and the volume of fluid sent to the brakes is increased by the use of deboosters. 

Power control valve.  A hand-operated hydraulic pump unloading valve. When the valve is open, fluid flows from the pump to the reservoir with little opposition. To actuate a unit, turn the selector valve, and manually close the power control valve. Pressurized fluid flows to the unit, and when it is completely actuated, the power control valve automatically opens. 

Precession.  The characteristic of a gyroscope that causes a force to be felt, not at the point of application, but at a point 90° in the direction of rotation from that point. 

