P-lead.
Primary lead. The wire that connects
the primary winding of a magneto to the ignition switch. The magneto is turned
off by grounding its P-lead.
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. The unit of pressure
produced when one newton of force acts uniformly over an area of one square
meter. One pascal is equal to 14.503 ∙ 10-5 (0.00014503) psi. The kilopascal
(kPa) is easier to manipulate. 1 kPa = 1,000 Pa = 0.14503 psi.
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.
PCB. Plenum chamber burning.
A method of thrust augmentation used on engines with vectored nozzles. Fuel
injected into the fan-discharge air is burned to increase thrust.
Peak voltage. The
voltage of AC electricity that is measured from zero voltage to the peak of
either alternation.
Penetrant dwell time. The length of time a part is left in the penetrant when preparing
it for inspection by the fluorescent or dye penetrant method. The hotter the
part and the longer the penetrant dwell time, the smaller the fault that will
be detected.
Performance number. The rating of antidetonation characteristics of a reciprocating
engine fuel that is better than the high rating reference fuel, iso-octane.
Performance numbers are greater than 100.
Permanent magnet. A piece of hardened steel that has been exposed to a strong
magnetizing force which has aligned the spin axes of the electrons surrounding
its atoms. The high retentivity of the material causes the electrons to retain
their magnetic orientation.
Permanent-mold casting. A casting made in a reusable metal mold. The walls of
permanent-mold castings can be made thinner than similar walls made by sand
casting.
Permeability. A
measure of the ease with which lines of magnetic flux can pass through a
material.
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.
Phase sequence, or phase rotation. The sequence with which the output phases of a
three-phase generator are connected to the load. Reversing the phase sequence
of a generator from A-B-C to A-C-B prevents the generator from being
synchronized with the others on the bus.
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.
Pi (π) filter. An electronic filter used to prevent radio frequency energy
produced in the ignition exciter from feeding back into the aircraft electrical
system. The filter is made of an inductor with a capacitor on its input and
output. The name is derived from the resemblance of the three components on a
schematic diagram to the Greek letter pi (π).
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 pin. See
wrist pin.
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 angle. The
angle between the chord line of a propeller blade and the plane of rotation.
See blade angle.
Pitch distribution. The gradual change in pitch angle of a propeller blade from the
root to the tip.
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.
Plane of rotation. The plane in which a propeller blade rotates. The plane of
rotation is perpendicular to the propeller shaft.
Planer. A woodworking power tool
used to smooth the surfaces of a piece of wood.
Planetary gears. A type of large-ratio reduction gearing. A series of small
planetary gears are mounted on a spider attached to the output shaft. The
planetary gears rotate between a fixed sun gear and a driven ring gear.
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
chamber. An enclosed chamber in which air can
be held at a pressure slightly higher than that of the surrounding air. Plenum
chambers are used to stabilize the pressure of the air before it enters a
double entry centrifugal compressor.
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.
POH. Pilot’s Operating
Handbook. A document published by the airframe manufacturer and approved by the
FAA that lists the operating conditions for a particular model of aircraft.
Engine operating parameters are included in the POH.
Pole shoe. Inward
extensions from the field frame of a generator around which the field coils are
wound.
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.
Poppet valve. A
T-shaped valve with a circular head. Poppet valves are used to cover the intake
and exhaust openings in the cylinder head of a reciprocating engine. The valves
are held closed by one or more coil springs and are opened by a cam lobe or a
rocker arm pushing on the end of the valve stem.
Porcelain. A
hard, white, translucent ceramic material that was used as the insulator in
some of the early aircraft spark plugs.
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.
Positive-displacement pump. A fluid pump that moves a specific volume of fluid each time it
rotates. Spur-gear pumps, gerotor pumps, and vane pumps are all
positive-displacement pumps.
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 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.
Power. The time rate of doing
work. Power is force multiplied by distance (work), divided by time.
Power-assurance check. A test run made of a gas turbine engine to determine how its
performance compares with its precious performance as new or freshly
overhauled.
Powerplant. The
complete installation of an aircraft engine, propeller, and all accessories
needed for its proper function.
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.
Preignition. Ignition
of the fuel-air mixture inside the cylinder of an engine before the time for
normal ignition. Preignition is often caused by incandescent objects inside the
cylinder.
Prepreg. Preimpregnated
fabric. A type of composite material in which the reinforcing fibers are
encapsulated in an uncured resin. Prepreg materials are cut to size and shape
and laid up with the correct ply orientation, and the entire component is cured
with heat and pressure.
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 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 carburetor. A carburetor installed on some aircraft reciprocating engines that
uses the pressure difference between air inside the venture and ram air
entering the carburetor to produce a fuel-metering force. Pressure carburetors
have generally been replaced with continuous-flow fuel injection systems.
Pressure cooling. A method of air cooling a reciprocating engine in which the
cylinders are enclosed in tight-fitting shrouds. The cowling is divided into
two compartments by baffles and seals, with half of each cylinder in each
compartment. Ram air is directed into one compartment, and the pressure in the
other is decreased by air flowing over a flared exit or adjustable cowl flaps.
The pressure difference across the cylinders causes cooling air to be drawn
through the fins to remove the unwanted heat.
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.
Pressure. A
measure of force applied uniformly over a given unit of surface area.
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-injection carburetor. A multibarrel pressure carburetor used on large radial and
V-engines. Fuel is metered on the basis of air mass flowing into the engine and
is sprayed under pressure into the eye, or center, of the internal supercharger
impeller.
Prevailing torque. The torque required to turn a threaded fastener before it contacts
the surface it is intended to hold.
Primary winding. The winding in a magneto or ignition coil that is between the
source of voltage and the breaker points. The primary winding is normally made
of comparatively large diameter wire, and has a small number of turns,
typically about 200.
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.
Profile tip (compressor blade tip). The tip of an axial-flow compressor bladed whose
thickness is reduced to give it a higher resonant frequency so it will not be
subject to the vibrations that would affect a blade with a squared tip. The
profile tip also provides a more aerodynamically efficient shape for the high
velocity air that is moved by the blade. Profile tips often touch the housing
and make a squealing noise as the engine is shut down. For this reason profile
tips are often called squealer tips.
Profilometer. A precision measuring instrument used to measure the depth of the
hone marks in the surface of a cylinder wall.
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.
Prony brake. An
instrument used to measure the amount of horsepower an engine is delivering to
its output shaft. The engine is operated at a specific rpm, and a brake is
applied to its output shaft. The amount of torque applied to the brake is
measured, and this, with the rpm, is converted into brake horsepower.
Propeller end. The end of a reciprocating engine to which the propeller is
attached.
Propeller. A
device for propelling an aircraft that has blades on an engine-driven shaft and
that, when rotated, produces by its action on the air, a thrust approximately
perpendicular to its plane of rotation. It includes control components normally
supplied by its manufacturer, but does not include main and auxiliary rotors or
rotating airfoils of engines.
PropFan engine. The registered trade name by Hamilton Standard of an
ultra-high-bypass turbine engine. See UHB engine.
Propulsive efficiency. A measure of the effectiveness with which an aircraft engine
converts the fuel it burns into useful thrust. It is the ratio of the thrust
horsepower produced by a propeller to the torque horsepower of the shaft
turning the propeller. The nearer the speed of the aircraft is to the speed of
the exhaust jet or propeller wake, the less kinetic energy is lost in the jet
or wake, and the higher the propulsive efficiency.
PRT. Power recovery turbine.
A turbine driven by exhaust gases from several cylinders of a reciprocating
engine. Energy extracted from exhaust gases by the turbine is coupled, through
a fluid clutch, to the engine crankshaft.
Pulsating DC. Direct
current whose voltage periodically changes, but whose electrons flow in the
same direction all of the time.
Pulse-jet engine. A type of air-breathing reaction engine used during World War II
to power jet-propelled missiles. Fuel is sprayed into the combustion chamber
and ignited. As the heated air expands, it closes the one-way shutter valve in
the front of the engine and exits the engine through the nozzle at the rear. As
soon as the pressure inside the combustion chamber decreases, air enters
through the shutter valve and more fuel is ignited. The thrust is produced in a
series of pulses.
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.
Push fit. A
fit between pieces in a mechanical assembly that is close enough to require the
parts to be pushed together. A push fit is looser than a press fit, but closer
than a free fit.
Pusher engine. An engine installed with the propeller facing the rear of the
aircraft. Thrust produced by the propeller mounted on a pusher engine pushes
rather than pulls the aircraft.
Pusher powerplant. A powerplant whose propeller is mounted at the rear of the
airplane and pushes, rather than pulls, the airplane through the air.
Pusher propeller. A propeller installed on an aircraft engine so that it faces the
rear of the aircraft. Thrust from the propeller pushes rather than pulls the
aircraft.
PV diagram. A diagram showing the relationship between the volume of a
cylinder and the pressure during a cycle of engine operation.
PVC (Polyvinylchloride). A thermoplastic resin used to make transparent tubing for
insulating electrical wires.