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 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.
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