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oxygen. A colorless, odorless, tasteless, gaseous chemical element that makes up about 21% of the earth’s
atmosphere. Oxygen’s symbol is O, its atomic number is 8, and its atomic weight is 15.9994. Oxygen will
not burn, but it supports combustion and combines chemically with all other elements except the inert gases.
Oxygen is absolutely necessary to support human and animal life.
ozone. An unstable form of oxygen. A molecule of ozone contains three atoms of oxygen (O3), rather than
two, as in a molecule of oxygen gas (O2). Ozone is blue in color, and it smells much like chlorine.
Ozone, which is produced by passing an electric spark through oxygen, is a powerful oxidizing
agent and is used as a bleach and in the treatment of industrial wastes. Ozone is harmful to rubber products.
ozonosphere. The layer of the earth’s atmosphere that extends from about 20 to 30 miles above the earth.
The ozonosphere has a high concentration of ozone that absorbs ultraviolet radiation from the sun.
P
pack carburizing (metal heat treatment). A method of case hardening the surface of a steel part without
hardening the inside and making it brittle. The part to be case hardened is packed in a fire-clay container
and entirely surrounded with a special ground-up charcoal. The container is sealed and put into a furnace
and heated to a temperature greater than 910°C. At this temperature, the iron takes on a crystalline form
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 424
called gamma iron.
The heated charcoal releases carbon monoxide, and since it cannot escape, it combines with the
gamma iron and forms an extremely hard surface. The depth to which the carbon penetrates the steel is
determined by the length of time it is held at this high temperature.
packing. A seal used between surfaces in which there is relative movement. When there is no movement
between the surfaces, a gasket is used to provide the seal.
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, which can be thinned for best
application, dries by evaporation or curing, the pigment is left as a film on the surface.
paint dryer. A component, usually an oil, added to a paint to improve its drying characteristics.
paint stripper. A material used to remove paint from a surface. Paint stripper contains very strong solvents
in a wax-like vehicle which prevents the solvents from evaporating before they can soak into the paint film.
Most paint strippers soften a lacquer film, so it can be wiped off. But rather than softening a film
of enamel, the stripper penetrates it and causes it to swell. When the film swells, it wrinkles and breaks
away from the surface on which it is applied. The wrinkled enamel is then scrubbed from the surface or
washed off with steam or hot water.
palladium. A soft, ductile, steel-white, tarnish-resistant metallic chemical element. Palladium’s symbol is
Pd, its atomic number is 46, and its atomic weight is 106.4. Palladium is used in the manufacture of
electrical contacts, jewelry, and surgical instruments.
Pal nut. The registered trade name for a thin, pressed-steel nut which has a shallow cup-shaped bottom
surface. A Pal nut is screwed down over the top of a plain nut to lock the nut onto a stud. When the Pal nut
is tightened, the bent-up threads are forced down, so they hold pressure on the plain nut and prevent its
vibrating loose.
pan-pan (urgency signal). The international radio-telephony urgency signal. When repeated three times,
pan-pan indicates uncertainty or alert, followed by the nature of the urgency.
pancake landing (aircraft operation). A landing in which the aircraft is flared out (leveled off) several
feet above the surface of the ground. As the aircraft loses its forward speed and aerodynamic lift, it drops to
the ground in a flat attitude.
panhead screw (aircraft hardware). A machine screw or sheet metal screw that has a large-diameter head
that is slightly crowned, or rounded, on top.
pants (airplane wheel covers). A commonly used name for the streamlined wheel covers installed on
some fixed landing gear airplanes.
paper capacitor (electrical component). A type of capacitor made of two strips of metal foil for its plates
and strips of waxed paper for its dielectric. Leads are attached to the metal foil, and the foil and waxed
paper are formed into a tight, compact roll. The entire roll is then encapsulated in a plastic resin. A mark or
line around one end of a paper capacitor identifies the lead attached to the outside layer of foil.
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
　
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