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of rotation.
pitch axis (aircraft axis). The lateral axis of an aircraft that extends from wing tip to wing tip and passes
through the center of gravity. This is the axis about which the aircraft pitches. See axes of an aircraft.
pitch distribution (propeller specification). The gradual twist in the propeller blade from shank to tip.
pitch pocket (wood defect). Cavities filled with pitch that appear in the growth rings of a piece of wood.
pitot pressure. Ram air pressure used to measure airspeed. The pitot tube faces directly into the air flowing
around the aircraft. It stops the air and allows the pressure caused by its movement to be measured.
pitch setting. 14 CFR Part 1: “The propeller blade setting as determined by the blade angle measured in a
manner, and at a radius, specified by the instruction manual for the propeller.”
pitot-static tube. A combination pickup used to sample pitot pressure (ram air pressure) and static pressure
(still air pressure). These two pressures, taken from outside a moving aircraft, are combined to measure
airspeed. Pitot pressure is picked up through an open-end tube that sticks directly into the airstream, and
static pressure is picked up through small holes or slots in the side of the pitot-static tube, or through flush
ports on the side of the aircraft.
pitot tube. An open-end tube that points directly into the air flowing over an aircraft structure. The pitot
tube samples the pressure of the ram air. This pressure is taken into the airspeed indicator where it is
compared with static air pressure to produce the airspeed indication.
pitting corrosion. A type of corrosion that forms on the surface of metal as small, localized pits filled with
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Page 442
the salts of corrosion.
PK screw. See Parker-Kalon screw.
placard. A notice placed on an aircraft in plain sight of the appropriate crew member to give information
pertinent to the operation of the aircraft. Operations limitations as well as maintenance specifics are often
displayed by placards.
plain bearing. A bearing made of a material, such as lead or babbitt, that has a low coefficient of friction.
Plain bearings can support only loads applied in a direction perpendicular to their surface, and are used as
crankshaft bearings in most reciprocating engines.
plain flap (aircraft secondary control). A type of wing flap in which a part of the wing trailing edge
inboard of the aileron is hinged to fold downward in flight to change the camber of the wing.
plain nut. A form of hexagonal nut that has no provisions for locking it onto a bolt or stud.
plain overlap seam (aircraft fabric seam). A type of machine-sewed seam used to join two pieces of
aircraft fabric. The edge of one piece of fabric overlaps the edge of the other, and one or two rows of
stitches are used to join the pieces. In a plain overlap seam, the thread passes through two layers of fabric.
plain rib (airplane wing rib). A wing rib used to give the wing its airfoil shape. Plain ribs are also called
former ribs.
plain washer (aircraft hardware). A disk of metal with a hole in its center. Plain washers are used to
provide a smooth surface on which a nut can ride, and to fill the space between a surface and the nut to
adjust for the grip length of a bolt.
plain-weave fabric. Fabric in which each warp thread passes over one fill thread and under the next.
Plain-weave fabric normally has the same strength in both warp and fill directions.
plan area. The area of a surface as viewed from the top.
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plane of rotation (helicopter rotor). The plane in which the rotor blades travel. The plane of rotation is
perpendicular to the rotor shaft, which is the axis of rotation of the rotor blades.
plane of symmetry (aircraft layout). An imaginary vertical plane that passes through the longitudinal axis
of an aircraft and divides the aircraft into two symmetrical halves.
planer. A woodworking power tool used to smooth the surfaces of a piece of wood.
planetary electrons. Electrons that circle around the nucleus of an atom in rings, or shells.
planetary gear system. A system of gears that changes the speed of rotation of a shaft, but does not change
its direction of rotation. The sun gear is mounted on the crankshaft and is surrounded by and meshes with a
set of planetary gears mounted on a carrier, or spider, that is part of the propeller shaft. The planetary gears
also mesh with and rotate inside the fixed ring gear. When the sun gear turns, the planetary gear spider
rotates inside the ring gear and turns the propeller shaft at a speed slower than that of the crankshaft.
　
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