prevailing wind (meteorology). The direction from which the wind blows most frequently.
preventive maintenance. Maintenance procedures followed to keep a piece of equipment working properly. Changing the oil, cleaning the spark plugs, cleaning the fuel and oil strainers, and checking or replacing hose connections are examples of reciprocating engine preventive maintenance. 14 CFR Part 1: “Simple or minor preservation operations and the replacement of small standard parts not involving complex assembly operations.”
prick punch. A sharp-pointed punch used to mark a location when laying out metal parts. A prick punch makes only a small mark on the surface of the metal, just large enough to accurately position a center punch.
primary air (gas turbine engine). The air that flows through the combustors of a gas turbine engine and is used in the combustion process.
primary cell (electrochemical cell). An electrochemical cell that changes chemical energy into electrical energy. A primary cell cannot be recharged, and it must be replaced when it becomes discharged. A carbon-zinc battery, such as used in a flashlight, is an example of a primary cell.
primary circuit (magneto ignition system). The circuit in a magneto ignition system in which the primary current flows. Primary current is produced when lines of flux from the rotating magnet cut across the turns of wire in the primary winding of the magneto coil. At the instant the primary current is greatest, a set of breaker points opens the circuit and the flow stops immediately. The magnetic field caused by the primary current collapses, cuts across the many turns of wire in the secondary winding of the coil, which is wound around the primary winding, and induces a high voltage in it.
The primary circuit includes the primary winding of the coil, the breaker points, a condenser, and an ignition switch. The breaker points, condenser, and switch are in parallel with each other and in series with the coil.
primary flight controls (aircraft controls). The flight controls that cause an aircraft to rotate about its three axes. In an airplane, the primary flight controls are the ailerons, elevators, and rudder.
primary fuel (gas turbine engine). Fuel that flows from the nozzles in a gas turbine engine under conditions of low airflow.
primary radar. Radar in which the beam from the transmitter bounces off a target and is returned to the same antenna and directed to the receiver.
primary structure (aircraft structure). The portion of the structure of an aircraft that would seriously disable the aircraft if it were to fail. The primary structure includes such items as the wings, controls, and engine mounts.
primary winding (ignition system component). The winding in a magneto or ignition coil between the source of voltage and the breaker points. The primary winding is normally made of comparatively large diameter wire and has a small number of turns, typically about 200.
primary winding (transformer winding). The winding of a transformer connected across the power line. The primary winding is the input winding.
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