Glossary Aborted takeoff.  A takeoff that is terminated prematurely  when it is determined that some condition exists that makes  takeoff or further flight dangerous. Absolute pressure.  Pressure measured from zero pressure  or a vacuum. Absolute pressure regulator.  A valve used in a pneumatic  system at the pump inlet to regulate the compressor inlet  air pressure to prevent excessive speed variation and/or  overspeeding of the compressor. Absolute zero.  The point at which all molecular motion  ceases. Absolute zero is –460 °F and –273 °C. Accumulator.  A hydraulic component that consists of two  compartments separated by a movable component, such as  a piston, diaphragm, or bladder. One compartment is filled  with compressed air or nitrogen, and the other is filled with  hydraulic fluid and is connected into the system pressure  manifold. An accumulator allows an incompressible fluid  to be stored under pressure by the force produced by a  compressible fluid. Its primary purposes are to act as a shock  absorber in the system, and to provide a source of additional  hydraulic power when heavy demands are placed on the  system. Actuator.  A fluid power device that changes fluid pressure  into mechanical motion. ADC.  Air data computer. ADF.  Automatic direction finder. ADI.  Attitude director indicator. Advancing blade.  The blade on a helicopter rotor whose  tip is moving in the same direction the helicopter is moving. Adverse yaw.  A condition of flight at the beginning of a  turn in which the nose of an airplane momentarily yaws in  the opposite direction from the direction in which the turn  is to be made.  Aerodynamic drag.  The total resistance to the movement of  an object through the air. Aerodynamic drag is composed of  both induced drag and parasite drag. See induced drag and  parasite drag. Aerodynamic lift.  The force produced by air moving over  a specially shaped surface called an airfoil. Aerodynamic  lift acts in a direction perpendicular to the direction the air  is moving. Aeronautical Radio Incorporated (ARINC).  A corporation  whose principal stockholders are the airlines. Its function is  to operate certain communication links between airliners  in flight and the airline ground facilities. ARINC also sets  standards for communication equipment used by the airlines. Aging.  A change in the characteristics of a material with time.  Certain aluminum alloys do not have their full strength when  they are first removed from the quench bath after they have  been heat-treated, but they gain this strength after a few days  by the natural process of aging. Agonic line.  A line drawn on an aeronautical chart along  which there is no angular difference between the magnetic  and geographic north poles. Air carrier.  An organization or person involved in  the business of transporting people or cargo by air for  compensation or hire. Air-cycle cooling system.  A system for cooling the air in  the cabin of a turbojet-powered aircraft. Compressor bleed  air passes through two heat exchangers where it gives up  some of its heat; then, it drives an expansion turbine where  it loses still more of its heat energy as the turbine drives a  compressor. When the air leaves the turbine, it expands and  its pressure and temperature are both low. G-1