Autosyn system.  The registered trade name of a remote  indicating instrument system. An Autosyn system uses an  electromagnet rotor, excited with 400-hertz AC, and a three- phase distributed-pole stator. Anti-icing.  Prevention of the formation of ice on a surface. anti-propeller end. The end of a reciprocating engine that does  not attach to the propeller. Also called the accessory end. APC.  Absolute pressure controller. Axial bearing load.  The load on a bearing parallel to the  shaft on which the bearing is mounted. Thrust produces an  axial load on a bearing. APU.  Auxiliary power unit. A small turbine- or reciprocating- engine-powered generator, hydraulic pump, and air pump.  APUs are installed in the aircraft and are used to supply  electrical power, air, and hydraulic pressure when the main  engines are not running. Axial turbine.  A turbine that is turned by a fluid flowing  through it in a direction that is approximately parallel to the  shaft on which the turbine wheel is mounted. Aramid fiber.  Fiber made from an organic compound of  carbon, hydrogen, oxygen, and nitrogen. It has high strength  and low density. It is flexible under load and is able to  withstand impact, shock, and vibration. Kevlar is a well- known aramid fiber. Axial-flow compressor.  A type of compressor used in  gas turbine engines. Air passes through the compressor  in essentially a straight line, parallel to the axis of the  compressor. The compressor is made of a number of stages  of rotating compressor blades between stages of stationary  stator vanes. Axis of rotation.  The center line about which a propeller  rotates. Aromatic compound.  A chemical compound such as  toluene, xylene, and benzene that is blended with gasoline  to improve its anti-detonation characteristics. Articulating rod.  See link rod. Aspect ratio.  The ratio of the length of an airfoil, such as a  compressor blade, to its width. Babbitt.  A soft silvery metal used for main bearing inserts  in aircraft reciprocating engines. Babbitt is made of tin with  small amounts of copper and antimony. Asymmetrical loading.  The loading of a propeller disk that  causes one side to produce more thrust than the other side. Back (propeller nomenclature).  The curved surface of a  propeller blade. The back of a propeller blade corresponds  to the upper surface of an airplane wing. ATF.  Aerodynamic twisting force. The aerodynamic force  that acts on a rotating propeller blade to increase its blade  angle. The axis of rotation of a blade is near the center of its  chord line, and the center of pressure is between the axis and  the leading edge. Aerodynamic lift acting through the center  of pressure tries to rotate the blade to a higher pitch angle. Back-suction mixture control.  A type of mixture control  used in some float carburetors that regulates the fuel-air  mixture by varying the air pressure above the fuel in the  float bowl. Baffle.  A thin sheet metal shroud or bulkhead used to direct  the flow of cooling air between and around the cylinder fins  of an air-cooled reciprocating engine. Atomize.  The process of breaking a liquid down into tiny  droplets or a fine spray. Atomized liquids vaporize easily. Augmentor tube.  A long, specially shaped stainless steel  tube mounted around the exhaust tail pipe of a reciprocating  engine. As exhaust gases flow through the augmentor tube,  they produce a low pressure in the engine compartment that  draws in cooling air through the cylinder fins. Bayonet stack.  An exhaust stack with an elongated and  flattened end. The gases leave the stack through a slot  perpendicular to its length. Bayonet stacks decrease both  exhaust back pressure and noise. BDC.  Bottom dead center. The position of a piston in a  reciprocating engine when the piston is at the bottom of  its stroke, and the wrist pin, crankpin, and center of the  crankshaft are all in line. Automatic intake valve.  An intake valve opened by low  pressure created inside the cylinder as the piston moves down.  There is no mechanical means of opening it. Automatic mixture control (AMC).  The device in a fuel  metering system, such as a carburetor or fuel injection system,  that keeps the fuel-air mixture ratio constant as the density  of air changes with altitude. Bell mouth.  The shape of the inlet of an augmentor tube  that forms a smooth converging duct. The bell mouth shape  allows the maximum amount of air to be drawn into the tube. G-3