alternating current (AC). Electricity in which the flow of electrons periodically reverses its direction and continually changes its amplitude.
alternation (alternating current electricity). One half of a cycle of alternating current or voltage. There are two alternations in one cycle: the positive alternation and the negative alternation. See illustration for sine wave.
alternator (electrical machine). A rotating electric generator that produces alternating current. While all
rotating generators produce AC, a DC generator uses some type of rectifier to change the AC into DC before it leaves the generator.
Alternators used on light airplanes are three-phase AC generators with built-in three-phase, solid-state rectifiers to change the AC into DC before it leaves the alternator.
alternator control unit. The solid-state control for an alternator that contains the voltage and current sensors and controls the alternator field current to maintain the correct voltage and current within the alternator rating.
altimeter. An aneroid barometer whose dial is calibrated in feet or meters above a reference pressure level.
An altimeter measures the difference between the pressure of the air surrounding it and a reference pressure which is set on the barometric pressure dial on the face of the instrument. This pressure difference is expressed in feet or meters.
altimeter setting. Station pressure (the barometric pressure at the location the reading is taken) which has been corrected for the height of the station above sea level.
When the altimeter setting is entered into the barometric pressure window of an altimeter, the instrument will indicate its height above mean, or average, sea level. This is called indicated altitude.
altimeter setting indicator. A precision aneroid barometer at a weather observation station, calibrated to indicate directly the altimeter setting.
altitude. The height of an object expressed in units of distance above a reference plane, usually above mean sea level (MSL) or above ground (AGL).
See corrected altitude, density altitude, indicated altitude, pressure altitude, radar altitude, and true altitude.
altitude (geometric figure). The greatest perpendicular distance from any side of the figure to its opposite side.
altitude engine. An aircraft reciprocating engine equipped with a supercharger that allows it to maintain its rated sea-level horsepower to a specified higher altitude.
14 CFR Part 1: “A reciprocating aircraft engine having a rated takeoff power that is producible from sea level to an established higher altitude.”
altitude readout (radar display). The altitude of an aircraft transmitted via the Mode C transponder feature that is visually displayed in 100-foot increments on a radarscope having altitude readout capability.
altitude reservation (air traffic control). Airspace utilization under prescribed conditions normally employed for the mass movement of aircraft or other special user requirements which cannot otherwise be accomplished.
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