Glossary Abradable strip.  A strip of material in the compressor  housing of some axial-flow gas turbine engines. The tip of  the compressor blade touches the abradable strip, and wears,  or abrades a groove in it. This groove ensures the minimum  tip clearance. Accumulator.  A hydraulic component that stores a  non-compressible fluid, such as oil, under pressure. An  accumulator has two compartments separated by a flexible  or movable partition with one compartment containing  compressed air. When oil is pumped into the other  compartment, the partition moves over, further compressing  the air which holds pressure on the oil. Abradable tip (compressor blade tip).  The tip of some  axial-flow compressor blades constructed so that it will  abrade, or wear away, upon contact with the compressor  housing, which ensures the minimum tip clearance between  the blade and the housing.  AD (ashless dispersant) oil.  A mineral-based lubricating oil  used in reciprocating engines. This oil does not contain any  metallic ash-forming additives, but has additives that disperse  the contaminants and hold them in suspension until they can  be removed by filters. Absolute pressure.  Pressure referenced from zero pressure  or a vacuum. AC.  Alternating current. Electrical current in which  the electrons continually change their rate of flow and  periodically reverse their direction. ADC.  Air data computer. An electronic computer in an  aircraft that senses pitot pressure, static pressure, and total air  temperature. It produces an indication of altitude, indicated  airspeed, true airspeed, and Mach number. The output  of ADC is usable by any of the engine or flight control  computers. ACC.  Active clearance control. A system for controlling the  clearance between tips of the compressor and turbine blades  and the case of high-performance turbofan engines. When  the engine is operating at maximum power, the blade tip  clearance should be minimum, and the ACC system sprays  cool fan discharge air over the outside of the engine case. This  causes the case to shrink enough to decrease the tip clearance.  For flight conditions that do not require such close clearance,  the cooling air is turned off, and the case expands to its normal  dimensions. The control of the ACC system is done by the  FADEC, or full-authority digital electronic control. ADI (antidetonation injection) system.  A system used with  some large reciprocating engines in which a mixture of water  and alcohol is sprayed into the engine with the fuel when  operating at extremely high power. The fuel-air mixture is  leaned to allow the engine to develop its maximum power,  and the ADI fluid absorbs excessive heat when it vaporizes. Adiabatic change.  A physical change that takes place  within a material in which heat energy is neither added to the  material, nor taken away. If a container of gas is compressed,  with no heat energy added to or taken from it, the gas will  become hotter; its temperature will rise. Acceleration.  The amount the velocity of an object is  increased by a force during each second it is acted upon by  that force. Acceleration is usually measured and expressed  in terms of feet per second, per second (fps²). Accessory end.  The end of a reciprocating engine on which  many of the accessories are mounted. Also, called the anti- propeller end. Aeromatic propeller.  A patented variable-pitch propeller  that has counterweights around the blade shanks and the  blades angled back from the hub to increase the effects of  aerodynamic and centrifugal twisting forces. This propeller  automatically maintains a relatively constant rpm for any  throttle setting. G-1