TM 1-1510-225-10 2-82 NORMAL   position,   heat   is   provided   for   the   major portion of each windshield.  In the HIGH  position, heat is provided at a higher watt density to a smaller portion of   the   windshield.      The   lever   lock   switch   feature prevents  inadvertent  switching  to  the HIGH   position during system shutdown. 2-60.  PRESSURIZATION SYSTEM. a.   Description.    A  mixture  of  engine  bleed  air and ambient air is available for cabin pressurization at a  rate  of  approximately  10  to  17  pounds  per  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  ±  0  input  of ambient     air     flow     by     allowing     ambient     air     flow introduction through the left flow control unit first, then 4  seconds  later  allowing  ambient  air flow through the right flow control unit. b.  Pressure  Differential.    The  pressure  vessel is  designed  for  a  normal  working  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  pressure  altitude  of  10,000  feet  at  an  aircraft altitude of 34,000 feet. c.   Pressurization Controller. The pressurization    controller,    located    on    the    pedestal extension,  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  rat e  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 center  subpanel.    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   center subpanel.  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  switch,  placarded    CABIN  PRESS  DUMP  / PRESS /  TEST, located on the pedestal extension, is in the PRESS mode.  The safety valve, adjacent to the outflow  valve,  provides  pressure  relief  in  the  event of an outflow valve failure.  The safety 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  switch.    It  is  wired through the right landing gear safety switch.   If either of  these  switches  is  open,  or  if  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.    The   BLEED AIR   VALVES    switches    on    the    copilot's    subpanel control   an   integral   electric   solenoid   firewall   shutoff valve.    A  solenoid,  operated  by  the  right  landing  gear safety  switch,  controls  the  introduction  of  ambient  air to  the  cabin  upon  takeoff.    Both  the  ambient  airflow control  valve  and  the  bleed  airflow  control  valve  are motor driven. The unit receives bleed air from the engine into an ejector that draws ambient air into the venturi of the