Autosyn system. The registered trade name of a remote indicating instrument system. The rotors in an Autosyn system are two-pole electromagnets, and the stators are delta-connected, three-phase, distributed-pole windings in the stator housings.
The rotors in the transmitter and indicator are connected in parallel and are excited with 26-volt, 400-hertz AC. The rotor in the indicator follows the movement of the rotor in the transmitter.
autotransformer. A single-winding electrical transformer that uses a carbon brush riding on a bare portion of the winding to select the number of turns used as the secondary.
Autotransformers produce a secondary voltage that can be varied from almost zero to a value higher than the primary voltage. Autotransformers do not isolate the secondary voltage from the primary voltage.

auxiliary. Anything supplemental to, or additional to, a main device or value.
auxiliary flight control. Lift-modifying devices on an aircraft. Flaps, slats, and slots are examples of auxiliary flight controls.
auxiliary fuel pump. An electrically operated fuel pump used to supply fuel to the engine for starting or in the case of failure of the engine-driven pump.
auxiliary hydraulic pump. A pump used as an alternate source of hydraulic pressure for an emergency or to produce hydraulic pressure when the aircraft engines are not operating.

auxiliary power unit. See APU.
auxiliary rotor (helicopter component). A small rotor on the tail of a single-main-rotor helicopter which produces a force to counteract the torque of the main rotor.
The pitch of the blades of an auxiliary rotor may be controlled by the pilot to rotate the helicopter about its vertical axis.
14 CFR Part 1: “A rotor that serves either to counteract the effect of the main rotor torque on a rotorcraft or to maneuver the rotorcraft about one or more of its three principal axes.”
auxiliary view (aircraft drawing). A view used on some aircraft drawings made at an angle to one of the three views of the main drawing. Auxiliary views show details that cannot be shown in the regular views used in the drawing.
avalanche diode (electrical component). Another name for a zener diode.
average value of sine wave alternating current. 0.637 time the peak value of the sine wave alternating current.
aviation. The branch of science, business, or technology that deals with any part of the operation of machines that fly through the air.