TM 1-1510-223-10 then diverts power to all heating elements on the other propeller  for  the  same  length  of  time.    This  cycle  will continue  as  long  as  the  switch  is  in  the  ON  position. The   system   utilizes   a   metal   foil   type   single   heating element, energized by DC voltage.  The timer switches every   90   seconds,   resulting   in   a   complete   cycle   in approximately 3 minutes. c. Manual Operation.   The manual propeller deice system   is   provided   as   a   backup   to   the   automatic system. The spring-loaded control switch located on the overhead control panel placarded PROP MANUAL ON, controls  the  manual  override  relay.    When  holding  the switch    in    the    ON    position    the    automatic    timer    is overridden,    and    power    is    supplied    to    the    heating elements of both propellers simultaneously.  This switch is  of  the  momentary  type  and  must  be  held  in  position for  approximately  90  seconds  to  dislodge  ice  from  the propeller surface.  Repeat this procedure as required to avoid significant buildup of ice, which will result in a loss of performance, vibration, and impingement of ice upon the   fuselage.      The   propeller   deice   ammeter   will   not indicate a load while the propeller deice system is being utilized  in  the  manual  mode.    I-However,  each  aircraft loadmeter will indicate an approximate 10% increase in load    while    the    manual    propeller    deice    system    is operating. 2-55. PITOT AND STALL WARNING HEAT SYSTEM. a. Pitot Heat Pitot heat should not be used for more than 15   minutes   while   the   aircraft   is   on   the ground.        Overheating    may    damage    the heating elements. Heating   elements   are   installed   in   both   pitot   masts, located on the nose.  Each heating element is controlled by   an   individual   switch   placarded   PITOT,   LEFT   and RIGHT ON, located on the overhead control panel (fig. 2-13).    Circuit  protection  is  provided  by  the  two  7.5- ampere circuit breakers, placarded PITOT HEAT, on the overhead   circuit   breaker   panel   (fig.      2-7).      The   true airspeed  temperature  probe  heat  control  circuit  is  also protected by these circuit breakers.  If either left or right pitot  heat  is  on,  the  true  airspeed  temperature  probe heat will be on. Heating  elements  protect  the  stall  warning lift     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.  2-7). 2-56. STALL WARNING SYSTEM. The stall warning system consists of a transducer, a lift computer,   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 (fig.    2-1).    When  a  stall  is  imminent,  the  output  of  the transducer  activates  a  stall  warning  hornr  The  system has  preflight  test  capability  through  the  use  of  a  switch placarded    STALL    WARN    TEST    OFF    LDG    GEAR WARN   TEST   on   the   copilot's   subpanel   (fig.      2-6). Holding this switch in the STALL WARN TEST position actuates   the   warning   horn   by   moving   the   transducer vane.    The  circuit  is  protected  by  the  5-ampere  circuit breaker,   placarded   STALL   located   on   the   overhead circuit breaker panel (fig.  2-7). 2-57. BRAKE DEICE SYSTEM. a. Description.  The 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  which  are 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  located  on  the  overhead control    panel    (fig.        2-13),    placarded    BRAKE    ON, controls  the  solenoid  valve  by  routing  power  through  a control module box under the aisleway floorboards.  The system is protected by a 5-ampere circuit breaker on the overhead   circuit   breaker   panel   (fig.      2-7),   placarded BRAKE  DEICE  A  10-minute  timer  limits  operation  and avoids   excessive   wheel   well   temperatures   when   the landing gear is retracted. 2-50