TM 55-1510-219-10 As the aircraft climbs, the controller modulates the outflow valve   to   maintain   a   selected   cabin   rate   of   climb   and increases the cabin differential pressure until the maximum   cabin   pressure   differential   is   reached.      At   a cabin  altitude  of  12,500  feet  a  pressure  switch  mounted on  the  back  of  the  overhead  control  panel  completes  a circuit  to  illuminate  a  red  warning  annunciator  light,  ALT WARN, to warn of operation requiring oxygen.  This light is   protected   by   a   5ampere   breaker,   placarded   PRESS CONTR. g. Pressurization  Safety  Valve.    Before  takeoff,  the safety   valve   is   open   with   equal   pressure   between   the cabin  and  the  outside  air.    The  safety  valve  closes  on liftoff if the CABIN PRESS CONTR switch on the copilots subpanel   is   in   the   PRESS   mode.      The   safety   valve adjacent  to  the  outflow  valve  provides  pressure  relief  in the  event  of  failure  of  the  outflow  valve.    This  valve  is also  used  as  a  dump  valve  and  is  opened  by  vacuum which  is  controlled  by  a  solenoid  valve  operated  by  the cabin pressure dump switch adjacent to the controller.  It is also wired through the right landing gear safety switch. If either of these switches is open, or the vacuum source or  electrical  power  is  lost,  the  safety  valve  will  close  to atmosphere except at maximum differential pressure of 6. 1   PSI.      A   negative   pressure   relief   diaphragm   is   also incorporated into 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  forward  of the firewall in each 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.      The bleed air switches located on the overhead control panel (fig.    2-18)  operates  an  integral  electric  solenoid  which controls  the  bleed  air  to  the  firewall  shutoff  valves.    A normally   open   solenoid   operated   by   the   landing   gear safety switch controls the introduction of ambient air flow to the cabin on takeoff. (1) The unit receives bleed air from the engine into an ejector which draws ambient air into the nozzle of the venturi.  The mixed air is then forced into the bleed air line routed to the cabin. (2) 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. (3) 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.    2-19)  is provided primarily as an emergency system, however, the system  may  be  used  to  provide  supplemental  (first  aid) oxygen.      Two   64   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 and copilot positions are  equipped  with  diluter  demand  type  regulators,  which mix  the  proper  amount  of  oxygen  for  a  given  amount  of air at altitude.  Also a first aid oxygen mask is provided in the   cabin.      Oxygen   system   pressure   is   shown   by   two gages placarded OXYGEN SUPPLY PRESSURE, located aft   of   the   pilot's   oxygen   regulator   control   panel.      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  on  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 duration capacities of the system. b. Regulator Control Panels.  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. (1) Diluter   control   lever.      The   diluter   control lever  selects  either  normal  or  100%  oxygen,  but  acts  to select only when the emergency pressure control lever is in the NORMAL position. CAUTION When not in use, the diluter control lever   should   be   left   in   the   100% OXYGEN position to prevent regulator contamination. (2) Emergency   pressure   control   lever.      The emergency   pressure   control   lever   has   three   positions. 2-47