parking brake. A brake system used to keep an aircraft from rolling when it is parked. The parking brake holds pressure on the brake system when there is no operator at the controls.
partial obscuration (meteorology). A designation of sky cover when part of the sky is hidden by surface-based obscuring phenomena.
partial-panel instrument flight. Instrument flight by reference to only a limited number of flight instruments. Partial-panel flight uses the airspeed indicator, turn and slip indicator, or turn coordinator, and clock to keep the aircraft straight and level and to maneuver it. Partial-panel flight assumes the attitude gyro and horizontal situation indicator (or gyro horizon and directional gyro) to be inoperative.
partial pressure (gas pressure). The percentage of the total pressure of a mixture of gases produced by each of the individual gases in the mixture. The atmosphere surrounding the earth is a mixture of gases of which nitrogen makes up about 78% and oxygen 21%. If the pressure of the atmosphere is 29.92 inches (760 mm) of mercury, the partial pressure of the nitrogen is 78% of this, or 23.33 inches (592.8 mm) of mercury, and the partial pressure of the oxygen is 21% of the total or 6.28 inches (159.6 mm) of mercury.
parting compound (composition materials fabrication). A material spread on a mold before plastic resin is put into it. The parting compound keeps the resin from sticking to the mold. Parting compound is sometimes called a parting agent.
parting film. A layer of thin plastic material that is placed between a composite lay-up and the heating blanket. It prevents the blanket sticking to the fabric.
part number (aircraft parts identification). The number assigned by the aircraft manufacturer to identify a particular aircraft part. The part number is normally the same as the drawing number for the part.