TM  55-1510-221-10 ment  panel  (fig.  2-29)  above  the  cabin  rate-of-climb integral  electric  solenoid  which  controls  the  bleed  air indicator.   The   longer   needle   indicates   aircraft   alti- to  the  firewall  shutoff  valves.  A  normally  open  sole- tude   in   thousands-of-feet   on   the   outside   dial.   The noid  operated  by  the  landing  gear  safety  switch  con- shorter   needle   indicates   pressure   differential   in   PSI trols  the  introduction  of  ambient  air  flow  to  the on  the  inner  dial.  Maximum  differential  is  6.1  PSI. cabin  on  takeoff. f.   Outflow   Valve.   A     pneumatically     operated outflow  valve,  located  on  the  aft  pressure  bulkhead, maintains   the   selected   cabin   altitude   and   rate-of- climb  commanded  by  the  cabin  rate-of-climb  and altitude  controller  on  the  copilot’s  instrument  panel. 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   opera- tion  requiring  oxygen.  This  light  is  protected  by  a 5-ampere  breaker,  placarded  PRESS  CONTR. (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  automati- cally.  When  the  aircraft  is  on  the  ground,  circuitry from  the  landing  gear  safety  switch  prevents  ambi- ent  air  from  entering  the  flow  control  unit  to  pro- vide   maximum   heating. 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  instrument  panel  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  sole- noid   valve   operated   by   the   cabin   pressure   dump switch  adjacent  to  the  controller.  It  is  also  wired through  a  landing  gear  safety  switch.  If  either  of these  switches  is  open,  or  the  vacuum  source  or  elec- trical   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. (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. 2-60. OXYGEN SYSTEM. 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  side- wall  upholstery  access  panel  in  the  baggage  section of  the  aft  compartment. i. Flow  Control  Unit.  A flow control unit for- ward  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.   2-12)   operates   an a.  Description.  The  oxygen  system  (fig.  2-19) is  provided  primarily  as  an  emergency  system,  how- ever,  the  system  may  be  used  to  provide  supplemen- tal   (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  bulk- head.   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   PRES- SURE,   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  pan- els.   Both   cylinders   are   interconnected,   so   refilling can  be  accomplished  through  a  single  tiller  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  flow  planning  rates  vs  alti- tude.  Table  2-5  shows  oxygen  duration  capacities  of the  system. 2-46