Automated Surface Observing System (ASOS).  Weather  reporting system which provides surface observations every  minute via digitized voice broadcasts and printed reports.  Automated Weather Observing System (AWOS).  Automated weather reporting system consisting of various  sensors, a processor, a computer-generated voice subsystem,  and a transmitter to broadcast weather data.  Automatic dependent surveillance—broadcast (ADS–B).  A function on an aircraft or vehicle that periodically  broadcasts its state vector (i.e., horizontal and vertical  position, horizontal and vertical velocity) and other  information.  Automatic direction finder (ADF).  Electronic navigation  equipment that operates in the low- and medium-frequency  bands. Used in conjunction with the ground-based  nondirectional beacon (NDB), the instrument displays the  number of degrees clockwise from the nose of the aircraft  to the station being received.  Automatic terminal information service (ATIS).  The  continuous broadcast of recorded non-control information in  selected terminal areas. Its purpose is to improve controller  effectiveness and relieve frequency congestion by automating  repetitive transmission of essential but routine information.  Autopilot.  An automatic flight control system which keeps  an aircraft in level flight or on a set course. Automatic pilots  can be directed by the pilot, or they may be coupled to a radio  navigation signal.  Aviation medical examiner (AME).  A physician with  training in aviation medicine designated by the Civil  Aerospace Medical Institute (CAMI).  Aviation Routine Weather Report (METAR).  Observation  of current surface weather reported in a standard international  format.  AWOS.  See Automated Weather Observing System.  Axes of an aircraft.  Three imaginary lines that pass through  an aircraft’s center of gravity. The axes can be considered  as imaginary axles around which the aircraft rotates. The  three axes pass through the center of gravity at 90° angles to  each other. The axis from nose to tail is the longitudinal axis  (pitch), the axis that passes from wingtip to wingtip is the  lateral axis (roll), and the axis that passes vertically through  the center of gravity is the vertical axis (yaw).  Bypass ratio.  The ratio of the mass airflow in pounds per  second through the fan section of a turbofan engine to the  mass airflow that passes through the gas generator portion  of the engine.  Block altitude.  A block of altitudes assigned by ATC to  allow altitude deviations; for example, “Maintain block  altitude 9 to 11 thousand.”  Biplanes.  Airplanes with two sets of wings.  Bernoulli’s Principle.  A principle that explains how the  pressure of a moving fluid varies with its speed of motion.  An increase in the speed of movement causes a decrease in  the fluid’s pressure.  BC.  See back course.  Basic empty weight (GAMA).  Basic empty weight  includes the standard empty weight plus optional and special  equipment that has been installed.  Barometric scale.  A scale on the dial of an altimeter to which  the pilot sets the barometric pressure level from which the  altitude shown by the pointers is measured.  Baro-aiding.  A method of augmenting the GPS integrity  solution by using a nonsatellite input source. To ensure that  baro-aiding is available, the current altimeter setting must  be entered as described in the operating manual.  Balance tab.  An auxiliary control mounted on a primary  control surface, which automatically moves in the direction  opposite the primary control to provide an aerodynamic  assist in the movement of the control.  Back course (BC).  The reciprocal of the localizer course  for an ILS. When flying a back-course approach, an aircraft  approaches the instrument runway from the end at which the  localizer antennas are installed.  Azimuth card.  A card that may be set, gyroscopically  controlled, or driven by a remote compass.  Axial flow compressor.  A type of compressor used in a  turbine engine in which the airflow through the compressor  is essentially linear. An axial-flow compressor is made up of  several stages of alternate rotors and stators. The compressor  ratio is determined by the decrease in area of the succeeding  stages.  B  G-5 