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The housing in which the turbine is mounted has a trigger-actuated air valve that controls the speed of the
turbine by controlling the amount of air flowing through it.
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 446
Pneumatic drill motors are used in aircraft manufacture and maintenance because they are safer
than electric drill motors, and they give the operator excellent control of the drill speed. Pneumatic drill
motors do not overheat when they are stalled.
pneumatics (fluid power). The branch of fluid power systems that uses a compressible fluid to transfer
energy.
PN junction (semiconductor device). The surface in a semiconductor diode or transistor between the
P-material and the N-material. When the PN junction is forward-biased (positive voltage to the P-material),
holes in the P-material and free electrons in the N-material drift to the junction. Here they combine by the
free electrons filling the holes. A maximum number of electrons flow across a forward-biased PN junction.
When the PN junction is reverse-biased (positive voltage to the N-material), the holes and free
electrons are drawn away from the junction and very few electrons cross it.
PNP transistor (semiconductor device). A bipolar transistor made of P-type and N-type silicon or
germanium. The base, made of N-type material, is between a collector and an emitter, both made of P-type
material. A very small current flow in the emitter-base circuit controls a much larger flow of current
between the emitter and collector.
POH (Pilot’s Operating Handbook). An FAA-approved document published by the airframe
manufacturer that lists the operating conditions for a particular model of aircraft. Engine operating
parameters are included in the POH.
pointer (analog instrument). The thin strip of metal or small rod moved by an analog instrument
mechanism over a calibrated scale. The position of the pointer over the scale shows the value of the
measurement being made. Instrument pointers are sometimes called hands or needles.
point-to-point wiring (electronic component assembly). A method of assembling components in an
electronic device. The components are mounted on a chassis, and wires are routed from the p ins on one
component to the pins on another and soldered. Components built with printed circuit boards and integrated
circuit chips have, to a great extent, replaced components made with point-to-point wiring.
poise. A unit of viscosity measurement in the centimeter-gram-second system. One poise is equal to one
dyne-second per square centimeter. Centipoise is a more commonly used value, and one centipoise is equal
to 0.01 poise.
position lights. Another name for the navigation lights mounted on the wings and tail of an aircraft.
position report (air traffic control). A report over a known location as transmitted by an aircraft to ATC.
polar air (meteorology). An air mass with characteristics developed over high latitudes, especially within
the subpolar highs. Continental polar air (cP) has cold surface temperatures, low moisture content, and,
especially in its source regions, great stability in its lower layers. It is shallow in comparison with Arctic air.
Maritime polar (mP) air initially possesses similar properties to those of continental polar air, but as it
passes over warmer water, it becomes unstable, with a higher moisture content.
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 447
polar front (meteorology). A semipermanent, semicontinuous front, separating air masses of tropical and
polar origins.
polarity. The possession or manifestation of two opposing characteristics. A battery has polarity because
one terminal has an excess of electrons and the other a deficiency. A magnet has polarity because the lines
of magnetic flux leave one pole and return to the other.
polarized capacitor (electronic component). A capacitor that can be used only in a DC circuit. When
using a polarized capacitor, attention must be paid to the polarity of the voltage to which it is connected.
Most electrolytic capacitors are polarized and are marked so they can be correctly installed in a
circuit. If a polarized capacitor is incorrectly installed, enough current can flow through it to destroy it.
polarized light. Light in which all of the electromagnetic energy is vibrating in the same plane.
polarized receptacle (electrical wiring receptacle). An electrical receptacle whose sockets are arranged in
such a way that its mating plug can be inserted in only one way. Nonpolarized receptacles for house wiring
have two flat sockets of the same size, and a plug can be inserted with either pin fitting into either socket.
Polarized receptacles used in modern house wiring have three sockets. A round socket is
　
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