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alpha cutoff frequency (transistor characteristic). The frequency at which the current gain (alpha) of a
common-base transistor stage drops off to 0.707 of its low-frequency value.
alpha hinge (helicopter rotor). The hinge at the root of a helicopter rotor blade that allows the tip of the
blade to move back and forth in its plane of rotation. The axis of the alpha hinge is perpendicular to the
plane of rotor rotation. An alpha hinge is also called a lead-lag hinge.
alpha iron. The arrangement of the molecules in iron when its temperature is below 911°C.
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alphanumeric symbols. Symbols made up of all of the letters in our alphabet, numerals, punctuation
marks, and certain other special symbols.
alpha particle. A positively charged nuclear particle having the same mass as the nucleus of a helium atom.
Alpha particles consist of two protons and two neutrons and are emitted at a high speed in certain types of
radioactivity.
alpha ray (electromagnetic radiation). A stream of alpha particles which ionize the gas through which
they pass. These particles cause a fluorescent screen to glow. An alpha ray is made up of the
lowest-frequency radio emission.
alpha wave (brain waves). Electrical signals produced in the brain that have a pulse frequency of eight to
13 hertz. Alpha waves are produced by a normal human brain when in its relaxed state.
alpha wave detector. A sensitive electronic device used to detect and display alpha waves from the human
brain.
alteration. A change to a certificated aircraft, aircraft engine, propeller, or appliance.
alternate airdrome (ICAO). An aerodrome to which an aircraft may proceed when it becomes either
impossible or inadvisable to proceed to or to land at the aerodrome of intended landing.
alternate airport. 14 CFR Part 1: “An airport at which an aircraft may land if a landing at the intended
airport becomes inadvisable.”
alternate air valve (reciprocating engine induction system). A valve in the induction system of a
reciprocating engine that directs air either from the air filter or from a point inside the engine nacelle into
the carburetor or the fuel injection system.
The alternate air valve on an engine equipped with a carburetor is called a carburetor heat valve.
alternate static air valve (aircraft instrument system). A valve in the static air system that supplies
reference air pressure to the altimeter, airspeed indicator, and vertical speed indicator.
Normally these instruments pick up their reference air pressure from a static vent on the outside of
the aircraft. If this vent ever becomes clogged with ice, the alternate static air valve can be opened, and the
instruments will be supplied with static air from some unpressurized area inside the aircraft.
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
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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
　
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