TM 55-1510-222-10 boot  inflation,  select  the  brake  deice  system  off.    Both sources  of  pneumatic  bleed  air  must  be  in  operation during brake deice system use.  Select the brake deice system   off   during   single   engine   operation.      Circuit protection  is  provided  by  the  5-ampere  circuit  breaker, placarded    BRAKE    DEICE,    in    the    overhead    circuit breaker panel (fig. 2-6). 2-58.  FUEL SYSTEM ANTI-ICING. a.    Description.      An   oil-to-fuel   heat   exchanger, located    in    each    engine    accessory    case,    operates continuously     and     automatically     to     heat     the     fuel sufficiently to prevent freezing of any water in the fuel. No controls are involved.  Three external fuel vents are provided on each wing.  One is recessed to prevent ice formation, while the second is flush mounted so that no ice  can  collect  upon  it,  the  third  is  electrically  heated, the  heating  is  controlled  by  the  two  toggle  switches  on the overhead control panel placarded FUEL VENT ON, LEFT and RIGHT (fig. 2-13).  They are protected by the two  5-ampere  circuit  breakers,  placarded  FUEL  VENT HEAT   RIGHT   LEFT,   located   in   the   overhead   circuit breaker panel (fig. 2-6). CAUTION To prevent overheat damage to electrically 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  TAKEOFF procedures. 2-59.      WINDSHIELD   ELECTROTHERMAL   ANTI-ICE SYSTEM. a.  Description.  Both pilot and copilot windshields are   provided   with   an   electrothermal   anti-ice   system. Each windshield is part of an independent electrothermal     anti-ice     system.          Each     system     is comprised of the windshield assembly with heating wires sandwiched between glass panels, a temperature sensor attached  to  the  glass,  an  electrothermal  controller,  two relays,  a  control  switch,  and  two  circuit  breakers.    The two  switches,  placarded  WSHLD  ANTI  ICE  NORMAL OFF  HI  PILOT  and  COPILOT,  respectively,  located  in the  overhead  control  panel  (fig.  2-13),  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  is  not  accessible  to  the flight  crew.  The  50-ampere  circuit  breakers  are  located in the power distribution panel under the floor ahead of the main spar. 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 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  portion  of  the  windshield.    The  lever  lock switch  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 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  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  altitude  of  10,000  feet  at  an  aircraft  altitude  of 34,000 feet. c.    Pressurization  Controller.    The  pressurization controller,  located  in  the  copilot's  subpanel,  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 2-49