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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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.  G-4