TM 1-1510-218-10 2-78 h.  Cabin Rate-Of-Climb Indicator  D2 T    .   An indicator,   placarded   CABIN   CLIMB,  is  installed  just ahead   of   the   control   quadrant.      It  is  calibrated  in thousands-of-feet per-minute change in cabin altitude. i.    Cabin Altitude Indicator C D1.  An indicator, placarded  CABIN  ALT,  is  installed  in  the  instrument panel,   Figure   2-17,   above   the   cabin   rate-of   climb indicator.   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 6.1 psi, Figure 2-17. j.    Cabin    Altitude    Indicator    D2    T    .        An indicator,    placarded   CABIN    ALT,   is   installed   just ahead   of   the   control   quadrant.      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 6.1 psi  D2  T1  and 6.5 ± psi  T2 . k.   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.        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,  placarded   ALT  WARN, to warn of operation requiring oxygen. l.    Safety   Valve.      Before   take-off,   the   safety valve  is  open  with  equal  pressure  between  the  cabin and the outside air.  The safety valve closes on 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 a 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 PRESS 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  electrical power is lost, the safety valve will close to atmosphere except at maximum pressure differential of 6.1 psi   D2   T1 and 6.5 ± psi  T2 . A screen in the safety valve should be cleaned at 1000-hour    intervals.        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. m.  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. n.  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.   This unit is fully pneumatic except for an   integral   electric   solenoid   firewall   shutoff   valve controlled by the  BLEED  AIR  VALVE switches on the overhead  control  panel,  Figure  2-15,  and  a  normally open   solenoid   operated   by   the   landing   gear   safety switch  which  controls  the  introduction  of  ambient  air flow to the cabin on take off. (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  airflow  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. 2-69.  OXYGEN SYSTEM   C . Do not smoke while oxygen is in use. a.   Description.  The oxygen system provides a sufficient  supply  of  oxygen  for  the  pilot,  copilot,  and nine cabin outlets to permit a descent from 31,000 feet down to 13,000 feet pressure altitude.  The system is a constant flow-type for the nine passenger outlets.  The pilot and copilot utilize diluter-demand 100% masks.  A 49-cubic   foot   lightweight   supply   cylinder   is   installed behind the aft pressure bulkhead.  The oxygen gauge on   the   copilot's   subpanel   gives   direct   readout   of cylinder pressure.  The pressure regulator and control valve   are   adjacent   to   the   supply   cylinder   and   are activated  by  a  remote  push-pull  knob  located   in  the cockpit, immediately aft of the overhead circuit breaker panel.      Pulling   the   SYSTEM  /  PULL  ON  /  CREW READY knob supplies oxygen to the pilot, copilot, and WARNING