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allow liquids or gases to seep (pass slowly) through the material. Cast iron and certain types of wood are
porous, while glass and hard steel are not. They have very little porosity.
porous chrome plating. A method of treating the walls of aircraft reciprocating engine cylinders that
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hardens them and increases their ability to hold lubricants. Worn cylinder barrels may be ground so their
bore is straight and round. Then hard chromium is electroplated on the walls to a depth that brings the
diameter of the cylinder bore back to its original dimension.
The surface of the hard chrome plating resembles a maze of spiderwebs, with thousands of tiny,
interconnected cracks. After the correct depth of chromium has been deposited, the electroplating current is
reversed, and the tiny cracks enlarge enough to hold oil.
Porous chrome plating provides a hard, wear-resistant surface over which the piston rings ride.
The oil trapped in the tiny grooves furnishes lubrication to minimize piston ring and cylinder wall wear and
helps the rings seal.
porpoising (flight condition). An unstable flight condition in which an aircraft continually oscillates along
its longitudinal axis. Porpoising is caused by the aircraft possessing neutral dynamic longitudinal stability.
port side. The left side of an aircraft or ship, as viewed when looking forward.
position error (aircraft instrument error). An error in the indication of flight instruments connected to
the static air system (altimeter, airspeed indicator, and vertical speed indicator). This error is caused by the
air at the entrance to the static system not being absolutely still. The amount of position error changes with
the angle of attack or airspeed of the aircraft and is usually greatest at low airspeeds when the angle of
attack is the highest.
position line (aircraft drawings). A line used to show the extreme position to which a part can be moved.
positive charge (electrical charge). An unbalanced electrical condition caused by a deficiency of electrons.
A positive charge can be produced by chemical cells, photocells, and thermocouples.
positive control. 14 CFR Part 1: “Control of all air traffic, within designated airspace, by air traffic
control.”
positive control area. See PCA.
positive-displacement pump (fluid pump). A type of fluid pump that moves a specific amount of fluid
each time it rotates. Since the pump is always moving fluid, some form of relief valve must be used to
relieve pressure when the system cannot use the fluid as fast as the pump moves it. Gear pumps, gerotor
pumps, and vane pumps are examples of positive-displacement pumps.
positive feedback (electronic amplifier). A condition in an electronic amplifier in which part of the
amplified signal is fed back from the output to the input. Positive feedback, also called regenerative
feedback, is used to reinforce the input signal.
positive ion. An atom that has become electrically unbalanced by losing one or more electrons. Since there
are more protons in the nucleus than there are electrons surrounding the nucleus, the atom has a positive
charge.
positive logic (digital electronics). The form of logic in which the more positive voltage represents a logic
one and the less positive voltage or zero voltage represents a logic zero.
positive number. A number greater than zero.
positive static stability (aircraft stability). The production of a force that causes an aircraft to return to a
condition of straight and level flight after it has been disturbed from this condition.
positive temperature coefficient. The change of a value, such as resistance, capacitance, or physical
length, with temperature in such a way that the value increases as the temperature increases.
positive terminal (electrical power source). The terminal of a source of electrical energy through which
electrons return to the source after they have passed through the external circuit.
positive vorticity (meteorology). Vorticity caused by cyclonic turning. Positive vorticity is associated with
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upward motion of the air.
positron. An unstable atomic particle. A positron has the same mass and spin characteristics as an electron,
but it has a positive electrical charge. (An electron has a negative electrical charge.)
potable water. Water carried in an aircraft for the purpose of drinking.
potassium. A soft, silvery-white, lightweight, alkali metal chemical element. Potassium’s symbol is K, its
atomic number is 19, and its atomic weight is 39.102. Potassium salts are important in the manufacture of
　
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