pawl (ratchet device). A driving or holding link used in a ratchet mechanism. The pawl allows a ratchet wheel to turn in one direction, but keeps it from turning in the opposite direction.
payload. The amount of load carried by a vehicle over and above the load necessary for the operation of the vehicle. Payload is actually the amount of revenue-producing load an aircraft can carry.
P-band radar. Radar that operates in the frequency range between 225 and 390 megahertz. At this frequency, the wavelengths of the pulses of energy are between 133.3 and 76.9 centimeters.
PCB (plenum chamber burning). A method of thrust augmentation used on engines with vectored nozzles. Fuel injected into the fan discharge air is burned to increase thrust.
P-channel field effect transistor. A field effect transistor having a source, drain, and channel of P-material formed on a substrate of N-material. See field effect transistor.
peak current (alternating current value). The greatest amount of current flowing in one alternation of alternating current. Peak current can be measured with an oscilloscope by measuring the distance between the zero-current level and the point of the greatest deviation (the greatest distance from the zero-current line).
peak inverse voltage (PIV). The highest peak value of reverse voltage a rectifying diode can withstand before it breaks down and conducts. A semiconductor diode does not normally conduct when it is reverse-biased (the anode negative and the cathode positive). But if too high a reverse voltage is placed across the diode, it will break down and conduct.
peak-to-peak voltage (alternating current value). Voltage measured with a cathode-ray oscilloscope between the peak negative value and the peak positive value.
peak voltage (alternating current value). The greatest momentary voltage in an alternating current circuit. In sine wave AC, the peak voltage is 1.414 times the effective, or rms, value of the voltage. See effective value of AC.
peak voltage. The AC voltage measured from zero voltage to the peak of either alternation.
pedestal grinder (machine tool). A type of electric motor-driven grinding wheel mounted on a pedestal, rather than on a bench.
peel ply (composite materials fabrication). A layer of porous fabric used in the lay-up of a composite material that vents the excess resin and prevents the bagging material from sticking to the part. The peel ply is removed after the resin has cured.
peen. To flatten or round-over the end of a rivet or shaft by a series of light hammer blows around its edge.
pencil compass (drawing instrument). A drawing instrument made in the shape of a pair of dividers. One leg has a sharp point that forms the center of a circle or arc, and the other leg holds a pencil or a piece of pencil lead that marks the circumference of the circle or the arc.
pendulum. A weight suspended on a rod or cable from a fixed point, free to swing back and forth. The period of a pendulum, the time required for one complete swing, is determined by its length. A pendulum is used as the regulating device in a clock because the period of a fixed-length pendulum is always the same.
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