TM 1-1510-225-10 3B-202 The   GCAS   is   a   completely   automatic   system (requiring  no  input  from  the  crew)  which  continuously monitors    the    aircraft's    flight    path    at    altitudes    of between 50 and 2450 feet Above Ground Level (AGL). The system provides visual and aural advisory/warning  messages  to  the  crew  in  the  event the  aircraft  is  flown  along  a  projected  flight  path  that would result in contact with the terrain under any of six modes of operation. NOTE The   operating   temperature   range   of   the computer is  -15 °C to 71 °C. When    operating    in    temperatures    below -15 °C, the system may be unreliable. All alter/warning signals are inhibited below 50 feet AGL. System    messages    always    precede    the "Whoop, Whoop" aural tone and "Pull Up" warnings,  thus  allowing   the  crew  time  to identify    and    correct    the    specific    flight situation.        The    system    messages    and warnings    are    discussed    in    the    various mode descriptions.  In all GCAS warning modes, the warnings cease and  the  system  resets  when  the  pilot  establishes  a positive  pull up and climb.  The system consists of a ground collision avoidance system computer located in the  nose  avionics  compartment,  pilot's  and  copilot's glareshield warning lights, flight compartment speakers/headsets,  a  flap  override  switch,  a   GPWS INOP  annunciator  light,  and  a  system  circuit  breaker, placarded   GPWS   POWER,  located  on  the  copilot's sidewall circuit breaker panel. b.  Controls, Indicators, and Functions.  (1)   GCAS Glareshield Switch-Indicators.   The pilot and copilot are each provided with two GCAS system   switch-indicators   located   on   their   respective sides of the glareshield.  The left switch-indicator is red and is placarded  PULL UP in large letters and  GPWS TEST  in  smaller  white  engraved  letters  below.    The right    switch-indicator    is    yellow    and    is    placarded BELOW G/S.  The  PULL  UP indicator lights will flash when  the  threshold  activation  limits  of  modes  1,  2,  3, or 4 have been exceeded.  The BELOW G/S indicator lights  will  illuminate  continuously  when  the  threshold activation   limits   of   mode   5   have   been   exceeded.   Pressing the  BELOW G/S switch-indicator when below 1000   feet   AGL   will   deactivate   the   deviation   below glideslope   mode.      The   glideslope   mode   will   be   re- armed when the aircraft climbs back through 1000 feet AGL.    Pressing  the   PULL  UP,  GPWS  TEST  switch- indicator will activate the GCAS self-test sequence. (2)   GCAS   Aural   Warning   Indicators.      The following   is   a   list   of   aural   indications.      Due   to   the possibility  of  activating  more  than  one  condition  at  a time, the following warning priority has been established.    A  voice  annunciator  of  a  higher  priority will  immediately  override  a  lower  priority  message  in progress.    A  lower  priority  message  will  function  only upon the cessation of a higher order warning. 1.   “Whoop,  Whoop,  Pull  Up"  (modes  1 and 2). 2.   "Terrain" (mode 2). 3.   "Too Low Terrain" (mode 4). 4.   "Too Low Gear" (mode 4). 5.   "Too Low Flap" (mode 4). 6.   "Minimums" (mode 6). 7.   "Sink Rate" (mode 1). 8.   "Don't Sink" (mode 3). 9.   "Glideslope" (mode 5). (3)   GCAS  Flap  Override  Switch.  A guarded switch placarded  GPWS FLAP OVRD, located on the pedestal extension, provides a means of disabling the "Too Low Flap" aural warning to facilitate landing with the  flaps  in  the  UP  position.    The  GCAS  "Too  Low Flap," aural warning will not be activated if the flap is in the    APPROACH    or    LANDING    positions    (40°    or greater). (4)   GCAS System Inoperative Caution Annunciator    Light.        Illumination    of    the    cautionary (yellow)   annunciator   light   placarded    GPWS   INOP, located   on   the   caution/advisory   annunciator   panel, warns the flight crew that the GCAS is not functional. c.   Normal Operation. (1)   Turn-On    Procedure.        The    GCAS    is operable   when   the   following   conditions   have   been met: 1.   Battery switch – ON. 2.   Inverter switch – NO. 1 or  NO. 2. 3.   AVIONICS  MASTER  POWER  switch – ON. 4.   GPWS POWER circuit breaker – In.