TM 1-1510-223-10 on  engine  shutdown  (in  both  cases  the  pressure  to  the fire-   wall   shutoff   valve   is   cut   off),   the   firewall   valve closes. 2-61. OXYGEN SYSTEM. a. Description.   The  oxygen  system  (fig.    2-24)  is provided  primarily  as  an  emergency  system,  however, the  system  may  also  be  used  to  provide  supplemental (first  aid)  oxygen.    Two  70  cubic-foot  capacity  oxygen supply     cylinders,     charged     with     aviator's     breathing oxygen,  are  installed  in  the  unpressunrized  portion  of the aircraft behind the aft pressure bulkhead.  The pilot's and copilot's positions are equipped with diluter demand type   regulators,   which   automatically   mix   the   proper amount of oxygen for a given amount of air at altitude. A  first  aid  oxygen  mask  is  also  provided  in  the  cabin. Oxygen    system    pressure    is    shown    by    two    gages placarded  OXYGEN  SUPPLY  PRESSURE,  located  on the pilot's and copilot's oxygen regulator control panels. Two   pressure   reducers,   located   in   the   unpressurized portion of the aircraft behind the aft bulkhead, lower the pressure in the system to 400 PSI, and route oxygen to the    regulator    control    panels.        Both    cylinders    are interconnected, so refilling can be accomplished through a  single  filler  valve  located  in  the  aft  right  side  of  the fuselage   exterior.      A   pressure   gage   is   mounted   in conjunction with the filler valve, and each cylinder has a pressure  gage.  Table  2-4  shows  oxygen  flow  planning rates  vs.    altitude.  Table  2-5  shows  oxygen  duration capacities  of  the  system  in  liters  per  minute  (LPM)  per mask at normal temperature and pressure, dry (NTPD). (1)   Regulator control panels. A  valid  cause  for  alarm  would  be  the  rapid loss of oxygen pressure when the aircraft is above  12,500  feet.    Should  this  condition arise,  descend  as  rapidly  as  possible  to  an altitude  which  does  not  require  the  use  of oxygen. When  not  in  use,  the  diluter  control  lever should    be    left    in    the    100%    OXYGEN position to prevent regulator contamination. Each regulator control panel contains a blinker-type flow indicator,  a  500  PSI  pressure  gage,  a  red  emergency pressure     control     lever     placarded     EMERGENCY     - NORMAL  -  TEST  MASK,  a  white  diluter  control  lever placarded 100% OXYGEN - NORMAL OXYGEN, and a green  supply  control  lever  placarded  ON  -  OFF.    The diluter   control   lever   selects   either   normal   or   100% oxygen when the emergency pressure control lever is in the NORMAL position (2)   The  emergency  pressure  control  lever  has three positions. Two positions control oxygen consumption  for  the  individual  using  oxygen,  and  the remaining   position   serves   for   testing   hose   and   mask integrity.      In   the   EMERGENCY   position,   the   control lever  causes  100%  oxygen  to  be  delivered  at  a  safe, positive  pressure.    In  the  NORMAL  position,  the  lever allows  delivery  of  normal  or  100%  oxygen,  depending upon  the  selection  of  the  diluter  control  lever.    In  the TEST MASK position, 100% oxygen at positive pressure is delivered to check hose and mask integrity. NOTE Check to ensure that the OXYGEN SUPPLY PRESSURE gage registers adequate pressure before each flight When oxygen is in   use,   a   check   of   the   supply   pressure should be made at intervals during flight, to note the quantity available and to approximate    the    supply    duration.        The outside    temperature    is    reduced    as    an aircraft ascends to higher altitudes.  Oxygen cylinders thus cooled by temperature change will show a pressure drop.  This type of   drop   in   pressure   will   rise   again   upon return to a lower or warmer altitude. (3)   The 500 PSI oxygen pressure gage provided   on   the   oxygen   control   panels   should   never indicate over 400 PSL If the pressure exceeds 400 PSL a   malfunction   of   the   pressure   reducer   is   indicated. Whenever  oxygen  is  inhaled,  a  blinker-vane  slides  into view   within   the   flow   indicator   window,   showing   that oxygen is being released  When oxygen is exhaled, the blinker vane vanishes from view. b. Oxygen masks. Pure  oxygen  will  support  combustion.    Do not smoke while oxygen is in use. Oxygen masks for the pilot and copilot are provided.  To connect  a  mask  into  the  oxygen  system,  the  individual connects  the  line  attached  to  the  mask  to  the  flexible hose  which  is  attached  to  the  cockpit  sidewall.    The microphone   in   the   oxygen   mask   is   connected   to   the audio system through the pilot's or copilots headset/oxygen     mask     microphone     selector     switch, located   on   the   instrument   panel   (fig.   2-15).      To   test mask and hose integrity, the individual 2-53