Airspeed indicator.  A differential pressure gauge that  measures the dynamic pressure of the air through which the  aircraft is flying. Displays the craft’s airspeed, typically in  knots, to the pilot.  Alternate static source valve.  A valve in the instrument static  air system that supplies reference air pressure to the altimeter,  airspeed indicator, and vertical speed indicator if the normal  static pickup should become clogged or iced over.  Air traffic control radar beacon system (ATCRBS).  Sometimes called secondary surveillance radar (SSR), which  utilizes a transponder in the aircraft. The ground equipment is  an interrogating unit, in which the beacon antenna is mounted  so it rotates with the surveillance antenna. The interrogating  unit transmits a coded pulse sequence that actuates the aircraft  transponder. The transponder answers the coded sequence  by transmitting a preselected coded sequence back to the  ground equipment, providing a strong return signal and  positive aircraft identification, as well as other special data.  Altimeter.  A flight instrument that indicates altitude by  sensing pressure changes.  Altimeter setting.  Station pressure (the barometric pressure  at the location the reading is taken) which has been corrected  for the height of the station above sea level.  Altitude engine.  A reciprocating aircraft engine having a  rated takeoff power that is producible from sea level to an  established higher altitude.  Ambient pressure.  The pressure in the area immediately  surrounding the aircraft.  Airway.  An airway is based on a centerline that extends from  one navigation aid or intersection to another navigation aid  (or through several navigation aids or intersections); used  to establish a known route for en route procedures between  terminal areas.  Ambient temperature.  The temperature in the area  immediately surrounding the aircraft.  Airworthiness Certificate.  A certificate issued by the FAA  to all aircraft that have been proven to meet the minimum  standards set down by the Code of Federal Regulations.  Airworthiness Directive.  A regulatory notice sent out by  the FAA to the registered owner of an aircraft informing  the owner of a condition that prevents the aircraft from  continuing to meet its conditions for airworthiness.  Airworthiness Directives (AD notes) are to be complied with  within the required time limit, and the fact of compliance,  the date of compliance, and the method of compliance are  recorded in the aircraft’s maintenance records.  Alert area.  An area in which there is a high volume of pilot  training or an unusual type of aeronautical activity.  Almanac data.  Information the global positioning system  (GPS) receiver can obtain from one satellite which describes  the approximate orbital positioning of all satellites in the  constellation. This information is necessary for the GPS  receiver to know what satellites to look for in the sky at a  given time.  ALS.  See approach lighting system.  Alternate airport.  An airport designated in an IFR flight  plan, providing a suitable destination if a landing at the  intended airport becomes inadvisable.  AME.  See aviation medical examiner.  Amendment status.  The circulation date and revision  number of an instrument approach procedure, printed above  the procedure identification.  Ammeter.  An instrument installed in series with an electrical  load used to measure the amount of current flowing through  the load.  Aneroid.  The sensitive component in an altimeter or  barometer that measures the absolute pressure of the air.  It is a sealed, flat capsule made of thin disks of corrugated  metal soldered together and evacuated by pumping all of  the air out of it.  Aneroid barometer.  An instrument that measures the  absolute pressure of the atmosphere by balancing the weight  of the air above it against the spring action of the aneroid.  Angle of attack.  The angle of attack is the angle at which  relative wind meets an airfoil. It is the angle that is formed  by the chord of the airfoil and the direction of the relative  wind or between the chord line and the flight path. The  angle of attack changes during a flight as the pilot changes  the direction of the aircraft and is related to the amount of  lift being produced.  G-3 