TM 1-1510-218-10 3A-44 1.  CODE OFF Flag 2.  MILLIBARS Display 3.  IN HG Display 4.  Needle Pointer 5.  Drum Display Figure 3A-26.  Pilot’s Encoding Altimeter (2)   MILLIBARS.    Indicates  local  barometric pressure in millibars.  Adjusted by use of set knob. (3)   IN     HG. Indicates     local     barometric pressure in inches of mercury.  Adjusted by use of set knob. (4)   Needle Indicator. Indicates aircraft altitude    in    hundreds    of    feet    with    subdivisions    at fifty-foot intervals. (5)   Drum  Display.    Indicates  aircraft  altitude in   ten   thousands,   thousands,   and   hundreds   of   feet above sea level. c.   Encoding Altimeter Operation. NOTE If the altimeter does not read within 70 feet of  field  elevation  when  the  correct  local barometric   setting   is   used,   the   altimeter needs   calibration   or   internal   failure   has occurred.    An  error  of  greater  than  70  feet also  nullifies  use  of  the  altimeter  for  IFR Flight. (1)   Turn-On  Procedure.    Encoding  altimeter will  operate  when  transponder  is  operating  with   M-C switch set to center position. 1.   Barometric   set   knob   –   Set   desired altimeter setting in IN HG window.   2.   CODE  OFF  flag  –  Check  not  visible. Needle indicator  – Check operation. d.  Encoding Altimeter Emergency Operation. 1.   Altimeter circuit breaker  – Pull, if encoder fault occurs. 3A-33.  GROUND PROXIMITY ALTITUDE ADVISORY SYSTEM. The    ground    proximity    altitude    advisory system  will   provide  little,  if  any,  warning for flight into precipitous terrain approaching  a  sheer  wall  if  there  is  little gradually rising terrain before reaching the steep terrain. a.   Description.   The Ground Proximity Altitude Advisory System (GPAAS) is provided to aid  the  flight crew in terrain avoidance.  Refer to Figure 3A-27. The  GPAAS  is  a  completely  automatic  system, requiring  no  input  from  the  crew,  which  continuously monitors    the    aircraft's    flight    path    at    altitudes    of between 100 and 2000 feet AGL. The  GPAAS  computer  processes  the  data  and, when    conditions    warrant,    selects    the    appropriate digitized    voice    advisory/warning    message    from    its memory.    This  message  is  then  announced  over  the pilot’s  and  copilot's  audio  system.    If  the  condition  is not  corrected,  the  GPAAS  will  rearm,  and  will  again announce   and   repeat   the   warning   if   the   condition recurs.      The   GPAAS   computer   remains   ready   to announce   a   different   message   during   the   intervals between repetitions.  All messages are disabled below 100 feet AGL. WARNING