TM 55-1510-219-10 (2) During  certain  ambient  conditions,  use  of the   brake   deice   system   may   reduce   available   engine power,   and   during   flight   will   result   in   a   TGT   rise   of approximately   20°C.      Appropriate   performance   charts should  be  consulted  before  brake  deice  system  use.    If specified   power   cannot   be   obtained   without   exceeding limits,  the  brake  deice  system  must  be  selected  off  until after takeoff is completed.  TGT limitations must also be observed   when   setting   climb   and   cruise   power.      The brake  deice  system  is  not  to  be  operated  above  15°C ambient   temperature   except   to   test   the   system.      The system  is  not  to  be  operated  for  longer  than  10  minutes (one   deicer   cycle)   with   the   landing   gear   retracted.      If operation does not automatically terminate after approximately  10  minutes  following  gear  retraction,  the system must be manually selected off.  During periods of simultaneous  brake  deice  and  surface  deice  operation, maintain   85%   Nl   or   higher.      If   inadequate   pneumatic pressure   is   developed   for   proper   surface   deicer   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    a    5-ampere    circuit    breaker,    placarded BRAKE  DEICE,  on  the  overhead  circuit  breaker  panel (fig.  2-26). 2-58.  FUEL SYSTEM ANTI-ICING. a. Description.        An    oil-to-fuel    heat    exchanger, located     on     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.    Two  external  fuel  vents  are  provided  on each wing.  One is recessed to prevent ice formation; the other is electrically heated and is controlled by two toggle switches  on  the  overhead  control  panel  placarded  FUEL VENT   ON,   LEFT   or   RIGHT   (fig.      2-18).      They   are protected   by   two   5-ampere   circuit   breakers,   placarded FUEL   VENT   HEAT,   RIGHT   or   LEFT,   located   on   the overhead  circuit  breaker  panel  (fig.    2-26).    Each  fuel control unit's pneumatic line is protected against ice by an electrically   heated   jacket   protected   by   a   7.      5ampere circuit  breaker  located  on  the  overhead  circuit  breaker panel  placarded  FUEL  CONTR  HEAT,  LEFT  or  RIGHT (fig.  2-26). 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 (Chapter 8). 2-59.        WINDSHIELD    ELECTROTHERMAL    ANTIICE 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.    Two  switches,  placarded  WSHLD ANTIICE  NORMAL  -OFF  HI  PILOT,  COPILOT,  located on the overhead control panel (fig.  2-18) 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  circuit  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 forward of the instrument panel. 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 feature prevents  inadvertent  switching  to  the  HI  position  during system shutdown. 2-60.  PRESSURIZATION SYSTEM. a. 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. b. 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.        Approximately    85%    NI    is    required    when operating with one engine.  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 2-45