TM 1-1510-225-10 2-80 b.  Automatic    Operation.        The    two-position switch    located    on    the    pilot's    subpanel,    placarded PROP   AUTO   /   OFF,    is    provided    to    activate    the automatic  system.    When  the  switch  is  placed  to  the AUTO  position,  the  timer  diverts  power  through  the brush  block  and  slip  ring  to  all  heating  elements  on one   propeller.      Subsequently,  the  timer  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   AUTO   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   pilot's   subpanel,   placarded   PROP   MANUAL  / OFF,  controls  the  manual  override  relay.    When  the switch is held in the  MANUAL  position,  the  automatic timer   is   overridden,   and   power   is   supplied   to   the heating   elements   of   both   propellers   simultaneously.   The 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.    However,  each  aircraft  loadmeter  will  indicate an approximate 10% increase in load while the manual propeller deice system is operating. 2-54.  PITOT 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.  Heating    elements    are    installed    in    both    pitot masts,  located  on  the  nose.   Each  heating  element  is controlled by an individual 7 ½  -ampere circuit breaker switch, placarded   PITOT,  LEFT and  RIGHT, located on the pilot's subpanel. 2-55.  STALL WARNING HEAT SYSTEM. CAUTION Heating elements  protect the stall warning 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. The   lift   transducer   is   equipped   with   anti-icing capability  on  both  the  mounting  plate  and  the  vane.   Stall warning vane heat is controlled by, and the circuit is   protected   by,   a   15-ampere  circuit  breaker  switch located   on   the   pilot's   subpanel,   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. 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.      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, located on the copilot's 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, located on the right sidewall circuit breaker panel. 2-57.  BRAKE DEICE SYSTEM. a.   Description.    The  brake  deice  system  may be used in flight with gear retracted or extended, or on the    ground.        When    the    brake    deice    system    is 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 that 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, when brake assemblies are presumed to be frozen 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.      Hot   air   is   obtained   by   means   of   a solenoid  valve  attached  to  the  bleed  air  system  which