Glossary Absolute accuracy.  The ability to determine present position  in space independently, and is most often used by pilots. Absolute altitude.  The actual distance between an aircraft  and the terrain over which it is flying. Absolute pressure.  Pressure measured from the reference  of zero pressure, or a vacuum. A.C.  Alternating current. Acceleration error.  A magnetic compass error apparent when  the aircraft accelerates while flying on an easterly or westerly  heading, causing the compass card to rotate toward North. Accelerometer. A part of an inertial navigation system  (INS) that accurately measures the force of acceleration in  one direction. ADF.  See automatic direction finder. ADI.  See attitude director indicator. ADM.  See aeronautical decision-making. ADS–B.  See automatic dependent surveillance–broadcast . Adverse yaw.  A flight condition at the beginning of a turn in  which the nose of the aircraft starts to move in the direction  opposite the direction the turn is being made, caused by the  induced drag produced by the downward-deflected aileron  holding back the wing as it begins to rise. Aeronautical decision-making (ADM). A systematic  approach to the mental process used by pilots to consistently  determine the best course of action in response to a given  set of circumstances. A/FD.  See Airport/Facility Directory. Agonic line.  An irregular imaginary line across the surface of  the Earth along which the magnetic and geographic poles are in  alignment, and along which there is no magnetic variation. Aircraft approach category.  A performance grouping of  aircraft based on a speed of 1.3 times the stall speed in the  landing configuration at maximum gross landing weight. Air data computer (ADC).  An aircraft computer that  receives and processes pitot pressure, static pressure, and  temperature to calculate very precise altitude, indicated  airspeed, true airspeed, and air temperature. AIRMET.  Inflight weather advisory issued as an amendment  to the area forecast, concerning weather phenomena of  operational interest to all aircraft and that is potentially  hazardous to aircraft with limited capability due to lack of  equipment, instrumentation, or pilot qualifications. Airport diagram.  The section of an instrument approach  procedure chart that shows a detailed diagram of the  airport. This diagram includes surface features and airport  configuration information. Airport/Facility Directory (A/FD). An FAA publication  containing information on all airports, communications,  and NAVAIDs. Airport surface detection equipment (ASDE). Radar  equipment specifically designed to detect all principal  features and traffic on the surface of an airport, presenting the  entire image on the control tower console; used to augment  visual observation by tower personnel of aircraft and/or  vehicular movements on runways and taxiways. Airport surveillance radar (ASR). Approach control  radar used to detect and display an aircraft’s position in the  terminal area. G-1