TM 55-1510-220-10 switch is to be held in the INNER position to perform the same  function  for  the  inner  elements  of  both  propellers. The loadmeters will indicate approximately a 5% increase of    load    per    meter    when    manual    propeller    de-ice    is operating.  The propeller de-ice ammeter will not indicate any load in the manual mode of operation. 2-55.  PITOT AND STALL WARNING HEAT SYSTEM. CAUTION Pitot  heat  should  not  be  used  for more   than   15   minutes   while   the aircraft is on the ground. Overheating may damage the heating elements. a.  Pitot Heat.  Heating elements are installed in both pitot masts,  located  on  the  nose.    Each  heating  element  is controlled  by  an  individual  switch  placarded  PITOT  ON LEFT  or  RIGHT,  located  on  the  overhead  control  panel (fig.    2-12).    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 1/2 ampere circuit breakers, placarded PITOT HEAT, on the overhead circuit breaker panel (fig.  2-27).  The INS true air speed probe (air  data  probe)  heat  control  circuit  is  also  protected  by this circuit breaker.  If either left or right pitot heat is on, INS true airspeed probe heat will be on. CAUTION The   heating   elements   protect   the stall   warning   lift   transducer   vane and face plate 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.  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 (fig. 2-27). 2-56.  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.      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. 2-57.  BRAKE DEICE SYSTEM. a. Description.      A   heated-air   brake   deice   system may be used in flight with gear retracted or extended, or on  the  ground.    When  activated,  hot  air  is  diffused  by means  of  a  manifold  assembly  over  the  brake  discs  on 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  de-ice 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  de-ice 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  aisleway  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 DE-ICE ON annunciator light, and has a resetting circuit interlocked with the gear unlock switch. When  the  system  is  activated,  the  BRAKE  DE-ICE  ON light should be monitored and the control switch selected OFF  after  the  light  extinguishes  otherwise,  on  the  next gear Change 2   2-44