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. 

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. 

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. 

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. 

Asymmetrical loading.  The loading of a propeller disk that causes one side to produce more thrust than the other side. 

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. 

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. 

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. 

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. 

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. 

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. 

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. 

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. 

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. 

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. 

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. 

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. 

