TM 1-1510-225-10 3C-58 3C-37.  GROUND COLLISION AVOIDANCE SYSTEM. The ground collision avoidance 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. The ground collision avoidance system will provide  no  warning  for  stabilized  descent in  the  landing  configuration  into  terrain, unless the aircraft is following an operating electronic glideslope or a correct minimum descent  altitude  has  been  set  on  the  radio altimeter (activating mode 6). a.    Description.    The  GCAS  is  provided  to  aid the   flight   crew   in   terrain   avoidance.      The   GCAS controls and indicators are indicated in Figure 3C-31. 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 2,450 feet 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   which   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     alert/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 GCAS computer located in the nose avionics compartment,  pilots  and  copilot’s  glareshield  warning lights,  flight  compartment  speakers/headsets,  a  flap override switch, a  GPWS INOP annunciator light, and a  system  circuit  breaker.    The  system requires inputs from  the  aircraft's  radio  altimeter,  air  data  computer, ILS     glideslope     receiver,     pilot/static     system,     flap position,    and    gear    position.        The    GCAS    system receives  115  Vac  power  through  a  1-ampere  circuit breaker,   placarded   GPWS   POWER,   located  on  the copilots’ 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 GS.  The  PULL UP indicator lights will flash when the threshold  activation  limits  of  modes  1,  2,  3, or 4 have been exceeded.  The  BELOW GS indicator lights  will  illuminate  continuously  when  the  threshold activation   limits   of   mode   5   have   been   exceeded.   Pressing the  BELOW GS 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   Indications.      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  annunciation  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). WARNING