Aviation Maintenance Terms beginning with P
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.
pieces of thin wood together
The grain of all pieces runs in the same direction.
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.
Pinion
A small gear that meshes with a larger gear, a sector of a gear, or a toothed rack.
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.
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.
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.
Plastic media blasting (PMB)
A method of removing paint from an aircraft surface by dry-blasting it with tiny plastic beads.
Plasticizer
A constituent in dope or lacquer that gives its film flexibility and resilience.
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.
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.
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.
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.
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.
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.
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.
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 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-disc brake. The pressure plate receives the force from the brake cylinders and transmits this force to the discs.
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.
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.
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.
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