TM 55-1510-220-10 shield.      The   lever   lock   feature   prevents   inadvertent switching to the HI position during system shutdown. 2-60.  PRESSURIZATION SYSTEM. a. Description.      A   mixture   of   bleed   air   from   the engines, and ambient air, is available for pressurization to the cabin at a rate of approximately 10 to 17 pounds per minute.  The flow control unit of each engine controls the 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, ten seconds later, air flow  through  the  right  flow  control  unit.    The  bleed  air switches,  located  on  the  overhead  control  panel  (fig.    2- 12) operate an integral electric solenoid which controls the bleed air to the firewall shutoff valves. b. Pressure   Differential.      The   pressure   vessel   is designed for a normal working pressure differential of 6.0 PSI, which will provide a cabin pressure altitude of 3870 feet  at  an  aircraft  altitude  of  20,000  feet,  and  a  nominal cabin altitude of 9840 feet at an aircraft altitude of 31,000 feet. c. Cabin  Altitude  and  Rate-of-Climb  Controller.    A control   panel   is   installed   on   the   copilot's   side   of   the subpanel (fig.  2-6) for operation of the system.  A knob, placarded   INC   RATE   controls   the   rate   of   change   of pressurization. A control, placarded CABIN CONTROLLER  is  used  to  set  the  desired  cabin  altitude. For proper cabin pressurization, the CABIN CONTROLLER   should   be   set   500   feet   above   cruise altitude.  For landing select 500 feet above field pressure altitude.        The    selected    altitude    is    displayed    on    a mechanically  coupled  dial  above  the  control,  placarded CABIN   ALT-FT.      Mechanically   coupled   to   the   cabin altitude  dial,  placarded  ACFTX1000.    This  dial  indicates the   maximum   altitude   the   aircraft   may   be   flown   at   to maintain the desired cabin altitude without exceeding the design  pressure  differential.    A  switch,  placarded  CABIN PRESS    DUMP-PRESS-TEST,    is    provided    to    control pressurization.  The switch is spring loaded to the PRESS position.    In  the  DUMP  position,  the  safety  valve  will  be opened and the cabin will be depressurized to the aircraft altitude.        In    the    PRESS    position,    cabin    altitude    is controlled  by  the  CABIN  CONTROLLER  control.    In  the TEST position, the landing gear safety switch is bypassed to   enable   testing   of   the   pressurization   system   on   the ground.    Operating  instructions  are  contained  in  Chapter 8. d. Cabin    Rate-of-Climb    Indicator.        An    indicator, placarded CABIN CLIMB, is installed on the copilot's side of  the  instrument  panel  (fig.    2-30).    The  cabin  rate-of- climb   controller   is   calibrated   in   thousands-of-feet   per- minute change in cabin altitude. e. Cabin  Altitude  Indicator.    An  indicator,  placarded CABIN ALT, is installed in the instrument panel (fig.  2-30) 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 on the inner dial.  Maximum differential is 6.1 PSI. 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 operation requiring oxygen.  This light is protected by a 5- ampere breaker, placarded PRESS CONTR. 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 upon 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 solenoid valve operated by   the   cabin   pressure   dump   switch   adjacent   to   the controller.  It is also wired through the right main 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 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. 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