Automatic flight control system (AFCS). The full system of automatic flight control that includes the autopilot, flight director, horizontal situation indicator, air data sensors, and other avionics inputs. 

Automatic pilot (autopilot). An automatic flight control device that controls an aircraft about one or more of its three axes. The primary purpose of an autopilot is to relieve the pilot of the control of the aircraft during long periods of flight. 

Autosyn system. A synchro system used in remote indicating instruments. 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 transmitters and indicators are connected in parallel and are excited with 26-volt, 400-Hz AC. The rotor in the indicator follows the movement of the rotor in the transmitter. 

Auxiliary power unit (APU).  A small turbine or reciprocating engine that drives a generator, hydraulic pump, and air pump. The APU is installed in the aircraft and is used to supply electrical power, compressed air, and hydraulic pressure when the main engines are not running. 

Aviation snips.  Compound-action hand shears used for cutting sheet metal. Aviation snips come in sets of three. One pair cuts to the left, one pair cuts to the right, and the third pair of snips cuts straight. 

Aviator’s oxygen.  Oxygen that has had almost all of the water and water vapor removed from it. 

Avionics.  The branch of technology that deals with the design, production, installation, use, and servicing of electronic equipment mounted in aircraft. 

Azimuth.  A horizontal angular distance, measured clockwise from a fixed reference direction to an object. 

Back course.  The reciprocal of the localizer course for an ILS (Instrument Landing System). When flying a back-course approach, the aircraft approaches the instrument runway from the end on which the localizer antennas are installed. 

Backhand welding.  Welding in which the torch is pointed away from the direction the weld is progressing. 

Backplate (brake component).  A floating plate on which the wheel cylinder and the brake shoes attach on an energizing- type brake. 

Backup ring.  A flat leather or Teflon ring installed in the groove in which an O-ring or T-seal is placed. The backup ring is on the side of the seal away from the pressure, and it prevents the pressure extruding the seal between the piston and the cylinder wall. 

Balance cable.  A cable in the aileron system of an airplane that connects to one side of each aileron. When the control wheel is rotated, a cable from the cockpit pulls one aileron down and relaxes the cable going to the other aileron. The balance cable pulls the other aileron up. 

Balance panel.  A flat panel hinged to the leading edge of some ailerons that produces a force which assists the pilot in holding the ailerons deflected. The balance panel divides a chamber ahead of the aileron in such a way that when the aileron is deflected downward, for example, air flowing over its top surface produces a low pressure that acts on the balance panel and causes it to apply an upward force to the aileron leading edge. 

Balance tab.  An adjustable tab mounted on the trailing edge of a control surface to produce a force that aids the pilot in moving the surface. The tab is automatically actuated in such a way it moves in the direction opposite to the direction the control surface on which it is mounted moves. 

Balanced actuator.  A linear hydraulic or pneumatic actuator that has the same area on each side of the piston. 

Banana oil.  Nitrocellulose dissolved in amyl acetate, so named because it smells like bananas. 

Bank (verb).  The act of rotating an aircraft about its longitudinal axis. 

Barometric scale.  A small window in the dial of a sensitive altimeter in which the pilot sets the barometric pressure level from which the altitude shown on the altimeter is measured. This window is sometimes called the “Kollsman” window. base. The electrode of a bipolar transistor between the emitter and the collector. Varying a small flow of electrons moving into or out of the base controls a much larger flow of electron between the emitter and the collector. 

Base.  The electrode of a bipolar transistor between the emitter and the collector. Varying a small flow of electrons moving into or out of the base controls a much larger flow of electrons between the emitter and the collector. 

Bead (tire component).  The high-strength carbon-steel wire bundles that give an aircraft tire its strength and stiffness where it mounts on the wheel. 

