TM 1-1510-218-10 2-75 panel.   The auto mode circuit is protected by a circuit breaker located on the dc power distribution panel. c.   Manual    Operation.        The    manual    deice system   is   provided   as   a   backup   to   the   automatic system.   The control switch for the manual system is located   in   the   ICE   &   RAIN  group  of  the  overhead control panel and is placarded  PROP MANUAL / ON.   This  switch  is  a   momentary  spring-loaded  type,  and must be held to the ON position until the ice has been dislodged from the propeller.   When the switch is held to the  ON  position  the  automatic  timer  is  overridden, and    power    is    supplied    to    all    elements    of    both propellers  simultaneously.    During  use  of  the  manual mode, the PROP AMMETER will not indicate any load; however,   both   aircraft   loadmeters   will   indicate   an approximate   0.5   load   increase.      The  manual  deice switch   is   protected   by   a   5-ampere  circuit  breaker, placarded  PROP  ANTI-ICE   CONTR,  located  on  the overhead   circuit   breaker   panel.      The  manual  deice circuits  are  protected  by  two  circuit  breakers,  located on the dc power distribution panel. 2-63.  PITOT AND STALL WARNING HEAT SYSTEM. CAUTION Pitot   heat   should   not   be   used   for  m ore than 15 minutes while the aircraft is on the ground.        Overheating    may    damage    the heating elements.  a.   Pitot Heat.  Heating elements are installed in the  pitot  masts  located  on  the  nose.    Each  heating element     is     controlled     by     an     individual     switch, placarded   PITOT   LEFT   ON   or   PITOT   RIGHT   ON, located on the overhead control panel, Figure 2-15.  It is not advisable to operate the pitot heat system on the ground except for testing or for short intervals of time to   remove   ice   or   snow   from   the   mast.      Circuit protection    is    provided    by    two    7.5-ampere    circuit breakers, placarded   PITOT   HEAT,  on  the  overhead circuit breaker panel. CAUTION The     heating     elements     protect     the     lift transducer   vane   and   faceplate   from   ice, however, a buildup of ice on the wing may change  or  disrupt  the  airflow  and  prevent the  system  from  accurately  indicating  an imminent stall. b.  Stall  Warning  Heat.    The  lift  transducer  is equipped     with     anti-icing     capability     on     both     the mounting  plate  and  the  vane.    The  heat  is  controlled by  a  switch  located  on  the  overhead  control  panel, placarded  STALL  WARN  /  ON.    The  level  of  heat  is minimal    for    ground    operation    but    is    automatically increased for flight operation through the landing gear safety  switch.    Circuit  protection  is  provided  by  a  15- ampere  circuit  breaker, placarded  STALL  WARN, on the overhead circuit breaker panel. 2-64.  STALL WARNING SYSTEM. The stall warning system consists of a transducer, a lift computer, a warning horn, and a test switch.    Angle  of  attack  is  sensed  by  aerodynamic pressure on the lift transducer vane located on the left wing   leading   edge.      When   a   stall   is   imminent,   the output of the transducer activates a stall warning horn.   The  system  has  preflight  test  capability  through  the use of a switch, placarded STALL WARN TEST / OFF /  LDG  GEAR  WARN  TEST,  on  the  right  subpanel, Figures 2-6 and 2-8.  Holding this switch in the  STALL WARN  TEST  position  actuates  the  warning  horn  by moving  the  transducer  vane.    The  circuit  is  protected by   a   5-ampere   circuit   breaker,   placarded   STALL WARN, on the overhead circuit breaker panel. 2-65.  BRAKE DEICE SYSTEM. a.   Description.        A    heated-air    brake    deice system  may  be  used  on  the  ground  or  in  flight  with gear retracted or extended.  When activated, hot air is diffused  by  means  of  a  manifold  assembly  over  the brake  discs  in   each   wheel.     Manual  and  automatic controls    are    provided.         There    are    two    primary occasions,  which  require  brake  deicing.    The  first  is when    an    aircraft    has    been    parked    in    a    freezing atmosphere   allowing   the   brake   systems   to   become contaminated  by  freezing  rain,  snow  or  ice,  and  the aircraft    must    be    moved    or    taxied.       The   second occasion  is  during  flight  through  icing  conditions  with wet  brake  assemblies  presumed  to  be  frozen,  which must be thawed prior to landing to avoid possible tire damage and loss of directional control.   Hot air for the brake deice system comes from the compressor stage of both engines obtained by means of a solenoid valve attached to the bleed air system which serves both the surface   deice   system   and   the   pneumatic   systems operation. b.  Operation.  A switch on the overhead control panel, placarded BRAKE DEICE, controls the solenoid valve  by  routing  power  through  a  control  module  box under  the  aisle  floorboards.    When  the  switch  is  on, power    from    a    5-ampere    circuit    breaker    on    the overhead circuit breaker panel is applied to the control module.  A 10-minute timer limits operation and avoids excessive  wheel  well  temperatures  when  the  landing gear is retracted.   The control module also contains a circuit  to  the  green   BRAKE  DEICE  ON  annunciator light,  and  has  a  resetting  circuit  interlocked  with  the