TM 1-1510-218-10 2-42 (2)   During   ground   test   of   the   engine   fire detection  system,  an  erroneous  indication  of  system fault  may  be  encountered  if  an  engine  cowling  is  not closed  properly,  or  if  the  aircraft  is  headed  toward  a strong   external   light   source.      In  this  circumstance, close the cowling and/or change the aircraft heading to enable a valid system check. (3)   One rotary switch, placarded FIRE PROTECTION    TEST,   on   the   copilot's   subpanel   is provided   to   test   the   engine   fire   detection   system, Figures 2-6  and  2-8.   Before checkout, battery power must be on and the  FIRE DETR circuit breaker must be  closed.    Switch position   DETR  1  checks  the  area forward   of   the   air   intake   of   each   nacelle,   including circuits  to  the  cockpit  alarm  and  indication  devices.   Switch position   DETR  2  checks  the  circuits  for  the upper accessory compartment of each nacelle.  Switch position   DETR   3  checks  the  circuits  for  the  lower accessory    compartment    of    each    nacelle.        Each numbered    switch    position    will    initiate    the    cockpit indications previously described. 2-31.  ENGINE FIRE DETECTION SYSTEMS T2 . a.   A pneumatic fire detection system is installed to provide an immediate warning in the event of a fire or  over  temperature  in  the  engine  compartment.  The main element of the system is a temperature sensing cable    routed    continuously    throughout    the    engine compartment,  terminating  in  a  responder  unit.    The responder  unit  is  mounted  in  the  accessory  area  on the  upper  left-hand  engine  truss  just  forward  of  the engine firewall.  The responder unit contains two sets of   contacts,   a   set   of   integrity   switch   contacts   for continuity  test  functions  of  the  fire  detection  circuitry and a set of alarm switch contacts that completes the circuit  to  activate  the  fire  warning  system  when  the detector  cable  senses  an  over  temperature  condition in critical areas around the engine. b.  The  sensor  cable  consists  of  an  outer  tube filled with an inert gas and an inner hydride core that is filled with an active gas.  The gas within the tube forms a  pressure  barrier  that  keeps  the  responder  integrity switch   contacts   closed   for   fire   alarm   continuity   test functions.      As  the  temperature  around  the  sensing cable  increases,  the  gases  within  the  tube  begin  to expand.      When   the   pressure   from   the   expanding gases   reaches   a   preset   point,   the   contacts   of   the responder alarm switch close, activating the respective fire warning system. c.   The    fire    warning    portion    of    the    system consists    of    two    annunciators,    placarded   #1    FIRE PULL     and     #2    FIRE     PULL,    located    below    the glareshield,   two   MASTER   WARNING  annunciators located on opposite sides of the glareshield, and two responder units with pneumatic sensors in the engine compartments.  Refer to Figure 2-17. d.  An integrity switch that monitors the system is held   in   the   closed   position.      If  the  detector  should develop  a  leak,  the  loss  of  gas  pressure  would  allow the   integrity   switch   to   open   and   signal   a   lack   of detector  integrity.    The  system  then  will  not  operate during the system test function. e.   Testing  the  systems  integrity,  availability  of power,  and  the  alarm  annunciators  (#1 and   #2 FIRE PULL    and    MASTER    WARNING)   is   accomplished through   two   switches   located   on   the   copilot's   left subpanel.   The switches are placarded  ENGINE FIRE PROTECTION   TEST  LEFT   DET  /  EXT   and   RIGHT DET   /   EXT.    When  either  the   LEFT  or  the   RIGHT switch is placed in the   DET position, electrical current flows  through  a  5-ampere  circuit  breaker,  placarded FIRE  DETR,  located  on  the  overhead  circuit  breaker panel   and   the   engine   fire   detector   circuitry   to   the integrity  switch  contacts  in  the  respective  responder unit   causing   the   respective   alarm   annunciators   to illuminate.  If the circuit breaker opens, the system will not  operate  during  a  test,  or  activate  the  annunciator lights,     if     the     detector     cable     should     sense     an overtemperature condition.   The system may be tested either pre/post flight, or in flight as desired. 2-32.  ENGINE FIRE EXTINGUISHER SYSTEM. a.   The    fire    extinguisher    system    utilizes    an explosive squib and valve which, when opened, allows the  distribution  of  the  pressurized  extinguishing  agent through     a     plumbing     network     of     spray     nozzles strategically located in the fire zones of the engines.