TM 55-1510-220-10 (4) Either     engine     is     capable     of     providing sufficient   bleed   air   for   all   requirements   of   the   surface deicer system.  Check valves in the bleed air and vacuum lines  prevent  backflow  through  the  system  during  single- engine  operation.    Regulated  pressure  is  indicated  on  a gage,   placarded   PNEUMATIC   PRESSURE,   located   on the copilots subpanel. 2-53.  ANTENNA DEICING SYSTEM. a. Description.  The antenna de-ice system removes ice   accumulation   from   the   dipole   antennas.      Pressure regulated bleed air from the engines supplies pressure to inflate the boots.  To assure operation of the system in the event    of    failure    of    one    engine,    a    check    valve    is incorporated  in  the  bleed  air  line  from  each  engine  to prevent  loss  of  pressure  through  the  compressor  of  the inoperative   engine.      Inflation   and   deflation   phases   are controlled by a distributor valve. b. Antenna De-ice System Switch.  The antenna de- ice   system   is   controlled   by   a   switch   placarded   ANT DEICE, SINGLE OFF MANUAL located on the overhead control panel (fig.  2-12).  The switch is spring loaded to return to the OFF position from the SINGLE or MANUAL position.  When the switch is set to the single position, the system  will  -,run  through  one,  timed  6  second  inflation- deflation cycle.  When the switch is held in the MANUAL position, the boots will inflate and remain inflated until the switch is released. c. Operation. (1) De-ice   boots   are   intended   to   remove   ice after it has formed rather than prevent its formation.  -For the most effective deicing operation, allow at least 1/8 to 1/4 inch of ice to form on the boots before attempting ice removal.  Very thin ice may crack and cling to the boots instead of shedding. NOTE Never cycle the system rapidly, this may   cause   the   ice   to   accumulate outside  the  contour  of  the  inflated boots and prevent ice removal. 2-54.        PROPELLER    ELECTROTHERMAL    ANTI-ICE SYSTEM. a. Description.      Electrothermal   anti-ice   boots   are cemented to each propeller blade to prevent ice formation or  to  remove  the  ice  from  the  propellers.    Each  thermal boot  consists  of  one  outboard  and  one  inboard  heating element,  and  receives  electrical  power  from  the  de-ice timer.    This  timer  sends  current  to  all  propeller  de-ice boots and prevents the boots from overheating by limiting the  time  each  element  is  energized.    Four  intervals  of approximately   30   seconds   each   complete   one   cycle. Current consumption is monitored by a propeller ammeter on    the    copilot's    subpanel.        Two    20-ampere    circuit breakers  placarded  PROP  ANTI-ICE  LEFT  and  RIGHT and  5-ampere  propeller  control  circuit  breaker  placarded CONTR on the overhead circuit breaker panel (fig.  2-27), protect  the  propeller  electrothermal  de-ice  system  during manual operation.  A 25 ampere circuit breaker placarded PROP AUTO, protects the system in automatic operation. b. Automatic   Operation.      A   control   switch   on   the overhead  control  panel  placarded  PROP  OFF  AUTO  is provided   to   activate   the   automatic   system.      A   deice ammeter   above   the   pedestal   registers   the   amount   of current  (14  to  18  amperes)  passing  through  the  system being  used.    During  AUTO  operation,  power  to  the  timer will   be   cut   off   if   the   current   rises   above   25   amperes. Current  flows  from  the  timer  to  the  brush  assembly  and then   to   the   slip   rings   installed   on   the   spinner   backing plate.  The slip rings carry the current to the deice boots on the propeller blades.  Heat from the boots reduces the grip  of  the  ice  which  is  then  thrown  off  by  centrifugal force,  aided  by  the  air  blast  over  the  propeller  surfaces. Power  to  the  two  heating  elements  on  each  blade,  the inner  and  outer  element,  is  cycled  by  the  timer  in  the following  sequence:  right  propeller  outer  element,  right propeller  inner  element,  left  propeller  outer  element,  left propeller  inner  element.    Loss  of  one  heating  element circuit on one side does not mean that the entire system must be turned off.  Proper operation can be checked by noting the correct level of current usage on the ammeter. An  intermittent  flicker  of  the  needle  approximately  each 30   seconds   indicates   switching   to   the   next   group   of heating elements by the timer. c. Manual  Operation.    The  manual  propeller  de-ice system is provided as a backup to the automatic system. A  control  switch  located  on  the  overhead  control  panel, placarded   PROP   INNER   OUTER,   controls   the   manual override    relays.    When    the    switch    is    in    the    OUTER position,  the  automatic  timer  is  overridden  and  power  is supplied to the outer heating elements of both propellers simultaneously.  The switch is of the momentary type and must be held in position until the ice has been dislodged from  the  propeller  surface.    After  deicing  with  the  outer elements, the Change 2      2-43