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platinum fine-wire spark plug (reciprocating engine component). An aircraft spark plug whose
electrodes are made of small-diameter platinum wire. Platinum spark plugs are especially suited for use in
reciprocating engines whose fuel contains a large amount of tetraethyl lead. Tetraethyl lead produces lead
oxides inside the firing-end cavity of a spark plug, and enough of this oxide can build up to foul the spark
plug, preventing its firing.
Fine-wire platinum electrodes allow the firing-end cavity to be open enough that the gases
containing the lead are blown out, preventing the formation of the lead buildup. Platinum can operate at
such high temperatures that any lead deposits which form on the electrodes will immediately burn off.
play. A term commonly used to mean relative movement between parts. There is said to be play in the
controls of an aircraft if the control surfaces can be moved without the cockpit controls moving.
P-lead (aircraft ignition system component). The commonly used term for the wire that connects the
primary circuit of an aircraft magneto to the ignition switch. A magneto is turned off by grounding its
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 445
P-lead.
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 with a gas turbine engine to stabilize the pressure of the air
before it enters a double-entry centrifugal compressor.
Plexiglas. The registered trade name for a transparent acrylic plastic material used for aircraft windshields,
canopies, and side windows.
pliers. A form of gripping hand tool having two pivoted handles and two notched, or serrated, jaws.
Common slip-joint pliers are used to hold small objects and to cut and bend wire.
plow wind (meteorology). A straight-line, spreading downdraft in front of a thunderstorm. A plow wind is
also called a first gust.
plumb. A condition of being straight up and down, or vertical.
plumb bob. A pointed weight with a string attached to a hole in line with its point. When the string is
fastened to an object, the point of the plumb bob is directly below the point of attachment, and a freely
hanging plumb bob always points directly toward the center of the earth. Anything that aligns with the
string attached to the plumb bob is said to be plumb.
plumbing. A term referring to all of the tubing and hoses used to connect the hydraulic and pneumatic
system components in an aircraft; also the tubing and hoses that carry fuel and lubricating oil.
plumb line. The string to which a plumb bob is attached.
plutonium. A naturally radioactive, silvery, metallic chemical element used as a nuclear fuel. Plutonium’s
symbol is Pu, its atomic number is 94, and the mass number of its most stable isotope is 244.
ply rating (tire rating). A rating used for aircraft and automobile tires to indicate their relative strength.
The ply rating does not indicate the actual number of plies of fabric in the tire; rather, it indicates the
number of plies of cotton fabric needed to produce the same strength as the actual plies.
plywood. A form of manufactured wood product made of thin layers (plies) of wood veneer glued together.
The grain of each alternate ply of veneer runs at 90° or 45° to the plies next to it. Plywood is strong,
lightweight, rigid, and relatively inexpensive.
PMA (Parts Manufacturer Approval). An approval, granted by the FAA under 14 CFR Part 21, that
allows a person to produce a modification or replacement part for sale for installation on a type-certificated
product.
PMB (plastic media blasting). A method of removing paint from an aircraft surface by dry-blasting it with
tiny plastic beads.
pneumatic altimeter. A form of aneroid barometer whose dial is calibrated in feet or meters, rather than
units of pressure. A pneumatic altimeter has a barometric scale that can be adjusted to any desired reference
pressure, and the pointers of the instrument measure the difference between the existing pressure and this
reference pressure in terms of feet or meters.
When the local altimeter setting is set on the barometric scale, the altimeter shows indicated
altitude. When standard sea level pressure of 29.92 inches of mercury or 1013.2 millibars is set on the
barometric scale, the altimeter shows pressure altitude.
Another type of altimeter is an absolute (radar or radio) altimeter. It measures the actual distance
between an aircraft and the terrain below it by measuring the time needed for an electrical signal to pass
from the aircraft to the ground and back to the aircraft. See radio altimeter.
pneumatic drill motor (air drill). A small, hand-held air turbine fitted with a chuck to hold a twist drill.
　
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