Ozone.  An unstable form of oxygen produced when an electric spark passes through the air. Ozone is harmful to rubber products. 

Packing.  A seal between two parts where there is relative motion. 

Paint.  A covering applied to an object or structure to protect it and improve its appearance. Paint consists of a pigment suspended in a vehicle such as oil or water. When the vehicle dries by evaporation or curing, the pigment is left as a film on the surface. 

Parabolic reflector. A reflector whose surface is made in the form of a parabola. 

Parallel circuit.  A method of connecting electrical components so that each component is in a path between the terminals of the source of electrical energy. 

Paralleling circuit.  A circuit in a multi-engine aircraft electrical system that controls a flow of control current which is used to keep the generators or alternators sharing the electrical load equally. The relay opens automatically to shut off the flow of paralleling current any time the output of either alternator or generator drops to zero. 

Paralleling relay.  A relay in multi-engine aircraft electrical system that controls a flow of control current which is used to keep the generators or alternators sharing the electrical load equally. The relay opens automatically to shut off the flow of paralleling current any time the output of either alternator or generator drops to zero. 

Parasite drag. A form of aerodynamic drag caused by friction between the air and the surface over which it is flowing. 

Parent metal. The metal being welded. This term is used to distinguish between the metal being welded and the welding rod. 

Partial pressure.  The percentage of the total pressure of a mixture of gases produced by each of the individual gases in the mixture. 

Parting film.  A layer of thin plastic material placed between a composite lay-up and the heating blanket. It prevents the blanket from sticking to the fabric. 

Pascal’s Law.  A basic law of fluid power which states that the pressure in an enclosed container is transmitted equally and undiminished to all points of the container, and the force acts at right angles to the enclosing walls. 

Performance number.  The anti-detonation rating of a fuel that has a higher critical pressure and temperature than iso- octane (a rating of 100). Iso-octane that has been treated with varying amounts of tetraethyl lead is used as the reference fuel. 

Petrolatum-zinc dust compound.  A special abrasive compound used inside an aluminum wire terminal being swaged onto a piece of aluminum electrical wire. When the terminal is compressed, the zinc dust abrades the oxides from the wire, and the petrolatum prevents oxygen reaching the wire so no more oxides can form. 

Petroleum fractions.  The various components of a hydrocarbon fuel that are separated by boiling them off at different temperatures in the process of fractional distillation. 

Phased array antenna. A complex antenna which consists of a number of elements. A beam of energy is formed by the superimposition of the signals radiating from the elements. The direction of the beam can be changed by varying the relative phase of the signals applied to each of the elements. 

Phenolic plastic. A plastic material made of a thermosetting phenol-formaldehyde resin, reinforced with cloth or paper. Phenolic plastic materials are used for electrical insulators and for chemical-resistant table tops. 

Pilot hole.  A small hole punched or drilled in a piece of sheet metal to locate a rivet hole. 

Pin knot cluster.  A group of knots, all having a diameter of less than approximately 1 ⁄ 16 inch. 

Pinked-edge tape.  Cloth tape whose edges have small V-shaped notches cut along their length. The pinked edges prevent the tape from raveling. 

Pinking shears.  Shears used to cut aircraft fabric with a series of small notches along the cut edge. 

Pinion.  A small gear that meshes with a larger gear, a sector of a gear, or a toothed rack. 

Piston. A sliding plug in an actuating cylinder used to convert pressure into force and then into work. 

Pitch (aircraft maneuver).  Rotation of an aircraft about its lateral axis. 

Pitch (rivet).  The distance between the centers of adjacent rivets installed in the small row. 

