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intended to hold.
prevailing visibility (meteorology). The greatest horizontal visibility which is equaled or exceeded
throughout half of the horizon circle. The half of the horizon circle need not be a continuous half.
prevailing westerlies (meteorology). The dominant west-to-east motion of the atmosphere, centered over
the middle latitudes of both hemispheres.
prevailing wind (meteorology). The direction from which the wind blows most frequently.
preventive maintenance. Maintenance procedures followed to keep a piece of equipment working
properly. Changing the oil, cleaning the spark plugs, cleaning the fuel and oil strainers, and checking or
replacing hose connections are examples of reciprocating engine preventive maintenance. 14 CFR Part 1:
“Simple or minor preservation operations and the replacement of small standard parts not involving
complex assembly operations.”
prick punch. A sharp-pointed punch used to mark a location when laying out metal parts. A prick punch
makes only a small mark on the surface of the metal, just large enough to accurately position a center
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 459
punch.
primary air (gas turbine engine). The air that flows through the combustors of a gas turbine engine and is
used in the combustion process.
primary cell (electrochemical cell). An electrochemical cell that changes chemical energy into electrical
energy. A primary cell cannot be recharged, and it must be replaced when it becomes discharged. A
carbon-zinc battery, such as used in a flashlight, is an example of a primary cell.
primary circuit (magneto ignition system). The circuit in a magneto ignition system in which the primary
current flows. Primary current is produced when lines of flux from the rotating magnet cut across the turns
of wire in the primary winding of the magneto coil. At the instant the primary current is greatest, a set of
breaker points opens the circuit and the flow stops immediately. The magnetic field caused by the primary
current collapses, cuts across the many turns of wire in the secondary winding of the coil, which is wound
around the primary winding, and induces a high voltage in it.
The primary circuit includes the primary winding of the coil, the breaker points, a condenser, and
an ignition switch. The breaker points, condenser, and switch are in parallel with each other and in series
with the coil.
primary flight controls (aircraft controls). The flight controls that cause an aircraft to rotate about its
three axes. In an airplane, the primary flight controls are the ailerons, elevators, and rudder.
primary fuel (gas turbine engine). Fuel that flows from the nozzles in a gas turbine engine under
conditions of low airflow.
primary radar. Radar in which the beam from the transmitter bounces off a target and is returned to the
same antenna and directed to the receiver.
primary structure (aircraft structure). The portion of the structure of an aircraft that would seriously
disable the aircraft if it were to fail. The primary structure includes such items as the wings, controls, and
engine mounts.
primary winding (ignition system component). The winding in a magneto or ignition coil 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.
primary winding (transformer winding). The winding of a transformer connected across the power line.
The primary winding is the input winding.
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.
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 460
primer (reciprocating engine component). A small, hand-operated pump, used to spray gasoline into the
induction system for starting the engine.
primer-surfacer (finishing system component). A type of primer used in a finishing system to provide a
good bond between the surface and the topcoat material. A primer-surfacer has enough body to be sanded
smooth to cover minor roughness of the surface being finished.
printed circuit (PC). An electrical circuit in which the conductors joining the components are made of
conductive strips bonded onto the surface of an insulating board. A thin sheet of copper or other conducting
material is bonded to a sheet of fiberglass-reinforced resin, and the desired pattern of conductors is drawn
on the copper. All of the unwanted copper is chemically etched away, and only the desired conductors are
left.
The component leads are assembled through holes in the PC board and soldered to the copper
　
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