TM 1-1510-224-10 sensor attached to the glass, an electrothermal controller,  two  relays,  a  control  switch,  and  two  circuit breakers.        Two    switches,    placarded WINDSHIELD, PILOT,  NORMAL  -  OFF  -  HIGH,   and   WINDSHIELD, COPILOT,   NORMAL   -   OFF   -   HIGH,    located    on    the overhead    control    panel    (fig.        2-15),    control    system operation.        Each    switch    controls    one    electrothermal windshield   system.      The   circuits   of   each   system   are protected    by    a    5-ampere    circuit    breaker    and    a    50- ampere  circuit  breaker,  which  are  not  accessible  to  the flight crew. CAUTION To  help  prevent  windshield  cracking, windshield  heat  should  be  placed  in the  NORMAL  position  for  at  least  15 minutes    prior    to    using    the    HIGH position. b. Normal    Operation.        Two    levels    of    heat    are provided  through  the  three  position  switches,  placarded NORMAL in the aft position,  OFF in the center position, and   HIGH   after   lifting   the   switch   over   a   detent   and moving   it   to   the   forward   position.      In   the    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  the  input  of  ambient  air  flow  by  allowing  ambient air flow introduction through the left flow control unit first, then four 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 copilot's subpanel (fig.  2-8), 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.    2-8).    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.  2-8).  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.      2-8),   is   in   the   pressurize   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 DUMP 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 2-60