breakers   located   on   the   overhead   circuit   breaker panel   placarded   FUEL   CONTR   HEAT,   LEFT   or RIGHT  (fig.  2-26). To   prevent   overheat   damage   to   electri- cally   heated   anti-ice   jackets,   the   FUEL VENT  heat  switches  should  not  be  turned ON  unless  cooling  air  will  soon  pass  over the  jackets. b.  Normal  Operation.  For   normal   operation, switches  for  the  FUEL  VENTS  anti-ice  circuits  are turned  ON  as  required  during  the  BEFORE  TAKE- OFF  procedures  (Chapter  8). 2-58.   WINDSHIELD   ELECTROTHERMAL   ANTI- ICE  SYSTEM. a.  Description.  Both  pilot  and  copilot  wind- shields  are  provided  with  an  electrothermal  anti-ice system.   Each   windshield   is   part   of   an   independent electrothermal  anti-ice  system.  Each  system  is  com- prised  of  the  windshield  assembly  with  heating  wires sandwiched  between  glass  panels,  a  temperature  sen- sor  attached  to  the  glass,  an  electrothermal  control- ler,  two  relays,  a  control  switch,  and  two  circuit breakers.   Two   switches,   placarded   WSHLD   ANTI- ICE   NORMAL   -   OFF   -   HI   -   PILOT,   COPILOT, located  on  the  overhead  control  panel  (fig.  2-l  2) 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.  The  50-ampere  cir- cuit   breakers   are   located   in   the   power   distribution panel  under  the  floor  ahead  of  the  main  spar.  The 5-ampere  circuit  breakers  are  located  on  panels  for- ward  of  the  instrument  panel. b. Normal   Operation.   Two  levels  of  heat  are provided   through   the   three   position   switches   plac- arded  NORMAL  in  the  aft  position,  OFF  in  the  cen- ter  position,  and  HI  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  HI  position,  heat is  provided  at  a  higher  watt  density  to  a  smaller  por- tion  of  the  windshield.  The  lever  lock  feature  pre- vents  inadvertent  switching  to  the  HI  position  dur- ing  system  shutdown. 2-59.   PRESSURIZATION   SYSTEM. a. Description.   A  mixture  of  bleed  air  from the  engines,  and  ambient  air,  is  available  for  pres- TM  55-1510-221-10 surization  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  alti- tude  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  alti- tude  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  Control- ler. 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 ACFTX  1000.  This  dial  indicates  the  maximum  alti- tude   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  depressu- rized  to  the  aircraft  altitude.  In  the  PRESS  position, cabin  altitude  is  controlled  by  the  CABIN  CON- TROLLER  control.  In  the  TEST  position,  the  land- ing 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   indica- tor,   placarded   CABIN   CLIMB,   is   installed   on   the copilot’s  side  of  the  instrument  panel  (fig.  2-29).  The cabin   rate-of-climb   controller   is   calibrated   in   thou- sands-of-feet  per-minute  change  in  cabin  altitude. e.  Cabin  Altitude  Indicator.  An  indicator, placarded   CABIN   ALT,   is   installed   in   the   instru- 2-45