TM 1-1510-223-10 minute.    The  flow  control  unit  of  each  engine  controls bleed    air    from    the    engine    to    make    it    usable    for pressurization, by mixing ambient air with the bleed air, depending upon aircraft altitude and ambient temperature.  On takeoff, excessive pressure bumps are prevented    by    landing    gear    safety    switch    actuated solenoids  incorporated  in  the  flow  control  units.    These solenoids,   through   a   time   delay,   stage   the   input   of ambient air flow by allowing ambient air flow introduction through the left flow control unit first, then 4 seconds later air flow through the right flow control unit b. Pressure  Differential.    The   pressure   vessel   is designed  for  a  normal  workidng  pressure  differential  of 6.5  PSI,  which  will  provide  a  cabin  pressure  altitude  of 8,000  feet  at  an  aircraft  altitude  of  29,700  feet,  and  a cabin  altitude  of  10,000  feet  at  an  aircraft  altitude  of 34,000 feet c. Pressurization   Controller.      The   pressurization controller,  located  on  the  copilot's  subpanel  (fig.    2-6), provides  a  display  of  the  selected  altitude,  an  altitude selector,  and  a  rate  control  selector.    The  cabin  and aircraft  altitude  display  is  a  mechanically  coupled  dial. The  outer  scale,  (CABIN  ALT)  of  the  display,  indicates the selected cabin altitude, the inner scale (ACFT ALT) indicates    the    corresponding    altitude    at    which    the maximum    differential    pressure    would    occur.        The indicated value on each scale is read as placarded, 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   on   the   copilot's subpanel (fig.  2-6).  It is calibrated in thousands of feet per minute change in cabin altitude. e. Cabin Altitude Indicator.  An indicator, placarded CABIN ALT, is located on the copilot's subpanel (fig. 2- 6).      The   longer   needle   indicates   aircraft   altitude   in thousands   of   feet   on   the   outside   dial.      The   shorter needle indicates pressure differential in PSI 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,  located  on  the  copilot's subpanel  (fig.    2-6),  is  in  the  pressurize  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.  It is 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    pressure differential  of  6.5  -.1.0  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 BT LEED AIR valve switches on the overhead control panel (fig.    2-13).    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. (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 2-52