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reciprocating engine. Its purpose is to extract some of the energy from the exhaust gas that would otherwise
be lost. The output shaft of the turbine is coupled through a hydraulic clutch to the engine crankshaft.
power section (reciprocating engine component). The section of the crankcase of a radial engine to
which the cylinders are attached.
powers of ten (mathematics). A handy mathematical tool in which very large and very small numbers are
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 453
changed to numbers between 1 and 10 with a superscript (exponent) to show the number of places to move
the decimal point. Powers of ten is another name for scientific notation. Numbers greater than 1 have a
positive exponent, and numbers smaller than 1 have a negative exponent.
100 = 1
101 = 10
102 = 100
103 = 1,000
104 = 10,000
105 = 100,000
106 = 1,000,000
power stroke (reciprocating engine operation). The stroke in the operating cycle of a reciprocating
engine in which both of the valves are closed, the fuel-air mixture has been ignited and burned, and the
piston is being forced downward by the expanding gases. Work is done in a reciprocating engine only
during the power stroke.
power supply (electrical component). A circuit or device that changes alternating current at the
power-line voltage into direct current of the required voltage. Many power supplies include a voltage
regulator circuit that holds the output voltage constant as the load current changes.
power transformer (electrical component). The transformer in an electrical power supply that changes
the line voltage into the voltages needed for the circuits using the power supply.
PPI (plan position indicator). A radar indicator in which the returns from the targets appear as bright
spots at the locations they would have on a circular or fan-shaped map of the area being scanned. The range
of the target is indicated by the distance of the spot from the center of the scope, and the bearing of the
target is indicated by the radial angle between the spot and a vertical reference line.
Some radar antenna rotate, and their PPI scopes are circular. Other antenna oscillate, and their PPI
scopes are fan shaped.
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 454
practice instrument approach (air traffic control). An instrument approach procedure conducted by a
VFR or an IFR aircraft for the purpose of pilot training or proficiency demonstrations.
praseodymium. A soft, silvery, malleable, ductile, rare-earth, metallic chemical element. Praseodymium’s
symbol is Pr, its atomic number is 59, and its atomic weight is 140.91. Praseodymium is used in the
manufacture of colored glass and as an alloy with other metals.
Pratt truss. A type of truss structure in which the longitudinal members are separated by vertical members
that carry only compression loads, and diagonal members that carry only tensile loads.
Most fabric-covered airplane wings are built with a Pratt truss. The wing spars are the main
structural members, the compression struts or compression ribs carry only compressive loads, and the drag
and anti-drag wires carry only tensile loads.
preamplifier (electronic circuit). An electronic circuit that amplifies an extremely weak input signal to a
value strong enough to be used in other amplifiers. Preamplifiers used in some sound systems have
provisions for mixing inputs from more than one microphone and for equalizing the sounds by amplifying
some frequencies more than others.
prearranged coordination (air traffic control). A standardized procedure which permits an air traffic
controller to enter the airspace assigned to another air traffic controller without verbal coordination. The
procedures are defined in a facility directive which ensures standard separation between aircraft.
precession (gyroscopic characteristic). One of the two basic characteristics of a gyroscope. The other
basic characteristic is rigidity in space. Precession causes a force applied to a spinning gyroscope to be felt,
not at the point the force is applied, but at a point 90° in the direction of rotation from that point.
precious metal. Metals that are valuable because they are scarce. Gold, silver, and platinum are examples
of precious metals.
precipitation (meteorology). Any or all forms of water particles, whether liquid or solid, that fall from the
atmosphere and reach the surface. Precipitation is a major class of hydrometeor and is distinguished from
cloud and virga in that it reaches the surface.
precipitation heat treatment. A method of increasing the strength of heat-treated aluminum alloy. After
the aluminum alloy has been solution-heat-treated by heating and quenching, it is returned to the oven and
　
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