TM 55-1510-222-10 indicated  value  on  each  scale  is  read  per  placard  ALT FT X  1000.  The rate control selector, placarded RATE INC; regulates the rate at which cabin pressure ascends or  descends  to  the  selected  altitude.    The  rate  change selected may be from 200 to 2000 feet per minute. d.    Cabin  Rate-of-Climb  Indicator.    An  indicator, placarded   CABIN   CLIMB,   is   located   in   the   copilot's instrument  panel.    It  is  calibrated  in  thousands  of  feet per minute change in cabin altitude. e.        Cabin    Altitude    Indicator.        An    indicator, placarded    CABIN    ALT,    is    located    in    the    copilot's instrument  panel.    The  longer  needle  indicates  aircraft altitude  in  thousands  of  feet  on  the  outside  dial.    The shorter   needle   indicates   pressure   differential   in   PSI (pounds  per  square  inch)  on  the  inner  dial.    Maximum differential is 6.5 .10 PSI. f.      Outflow   Valve.      A   pneumatically   operated outflow   valve,   located   in   the   aft   pressure   bulkhead, maintains  the  selected  cabin  altitude  and  rate-of  climb commanded   by   the   cabin   rate-of-climb   and   altitude controller.        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 ALT WARN    warning    annunciator,    to    warn    of    operation requiring oxygen. g.  Safety Valve.  Before takeoff, the safety valve is  open  with  equal  pressure  between  the  cabin  and  the outside  air.    The  safety  valve  closes  upon  lift  off  if  the CABIN  PRESS  DUMP  switch  is  in  the  PRESS  mode. The    safety    valve,    adjacent    to    the    outflow    valve, provides pressure relief in the event of an outflow valve failure.  This valve is also used as a dump valve.  The safety  valve  is  opened  by  vacuum,  which  is  controlled by  a  solenoid  valve  operated  by  the  CABIN  PRESS DUMP  switch,  located  adjacent  to  the  controller.    It  is wired through the 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 pressure differential of 6.5 .10 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, 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.  2-13).    A  solenoid,  operated  by  the  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. (1) 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. (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. 2-22) 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 and copilot 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 in the pilot and copilot  oxygen  regulator  control  panels.    Two  pressure reducers, 2-50