TM 1-1510-224-10 the    outflow    and    safety    valves    to    prevent    outside atmospheric   pressure   from   exceeding   cabin   pressure during rapid descent. h. Drain.  A drain in the outflow valve static control line   is   provided   for   removal   of   accumulated   moisture. The drain is located behind the lower sidewall upholstery access    panel    in    the    baggage    section    of    the    aft compartment. i. Flow  Control  Unit.    A  flow  control  unit,  located forward  of  the  firewall  in  each  engine  nacelle,  controls bleed  air  flow  and  the  mixing  of  ambient  air  to  make  up the  total  air  flow  to  the  cabin  for  pressurization,  heating, and   ventilation.      An   integral   electric   solenoid   firewall shutoff   valve   is   controlled   by   the   ENVIRO   &   PNEU BLEED   AIR   valve   switches   on   the   overhead   control panel   (fig.   2-15).      A   solenoid,   operated   by   the   right landing  gear  safety  switch,  controls  the  introduction  of ambient air to the cabin upon takeoff.  Both the ambient air flow control valve and the bleed air flow control valve are motor driven. The unit receives  bleed  air  from  the  engine  into  an ejector  which  draws  ambient  air  into  the  venturi  of  the nozzle.    The  mixed  air  is  then  forced  into  the  bleed  air line routed to the cabin. Bleed air flow is controlled automatically.  When the aircraft  is  on  the  ground,  circuitry  from  the  landing  gear safety switch prevents ambient air from entering the flow control  unit  to  provide  maximum  heating.    The  bleed  air firewall shutoff valve in the control unit is a spring-loaded bellows-operated  valve  that  is  held  in  the  open  position by bleed air pressure.  When the electric solenoid is shut off, or when bleed air diminishes on engine shutdown (in both  cases  the  pressure  to  the  firewall  shutoff  valve  is cut off), the firewall valve closes. 2-61.  OXYGEN SYSTEM. a. Description.    The  oxygen  system  (fig.    2-26)  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  unpressurized  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,    allowing    refilling    to    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). b. Regulator control panels. WARNING The   rapid   loss   of   oxygen   pressure when the aircraft is above 12,500 feet is  a  valid  cause  for  alarm.    Should this     condition     arise,     descend     as rapidly    as    possible    to    an   altitude which   does   not   require   the   use   of oxygen. CAUTION When   the   oxygen   system   is   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. 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. 2-61