TM 1-1510-218-10 2-74 (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 gauge, placarded     PNEUMATIC PRESSURE,     located     on     the     copilot's     subpanel, Figures 2-6 and 2-8. 2-61.  PROPELLER     ELECTROTHERMAL     DEICER SYSTEM  C D T1 . a.   Description.  Electrothermal deicer boots are cemented   to   each   propeller   blade   to   prevent   ice formation  or  to  remove  ice  from  the  propellers.   Each thermal boot consists of one outboard and one inboard heating  element,  and  receives  electrical  power  from the   deicer   timer.      This   timer   sends   current   to   all propeller thermal boots and prevents the deicers 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, Figures 2-6 and 2-8.   Two 20-ampere circuit breakers,    placarded    PROP    ANTI-ICE    LEFT    and RIGHT, on the overhead circuit breaker panel, protect the propeller electrothermal deicer system. b.  Normal Operation.    A  control  switch  on  the overhead    control    panel,    placarded    PROP    OFF    / AUTO, is provided to activate the automatic system.  A deice   ammeter   on   the   right   subpanel   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 20-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 each approximately  30  seconds  indicates  switching  to  the next group of heating elements by the timer. c.   Alternate    Operation.        The    manual    prop deice 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   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 prop deice is operating.   The prop deice ammeter will not indicate any load in the manual mode of operation. 2-62.  PROPELLER ELECTRIC DEICE SYSTEM T2 . a.   Description.      The  propeller  electric  deicer system  consists  of  electrically  heated  single  element deice boots, slip ring and brush block assemblies, prop ammeter, a timer for automatic operation, three power distribution   panel   circuit   breakers,   two   prop   deice control circuit breakers, and two system switches.  The system    utilizes    a    metal    foil    type,    single    heating element   energized   by   dc   voltage.      Heat   from   the elements dislodges the ice formed on the blades and the ice is thrown off by centrifugal force. CAUTION Propeller   deice   should   not   be   operated when the propellers are not turning.  Static operation   may   damage   the   brushes   and slip ring. b.  Automatic Operation.   The automatic deice system  is  controlled  by  a  two  position  toggle  switch, placarded PROP AUTO / ON, located on the overhead control  panel.    When the switch   is  placed  in  the   ON position,    the    propeller    ammeter,    located    on    the copilot's inboard subpanel, indicates the current (14 to 18-amperes  load)  passing  through  the  system,  per propeller cycle. Direct current (dc) flows from the automatic timer to  the  brush  block  assembly  mounted  on  the  front  of the  engine  case.    The  brush  assembly  conducts  the current   to   the   slip   rings   installed   on   the   propeller spinner bulkhead.   The slip rings distribute the current to  all  heating  elements  on  one  propeller.    The timer then  diverts the current to all heating elements on the other propeller for the same length of time.   The timer switches  every  90  seconds,  resulting  in  a  complete cycle   of   approximately   3   minutes.      The  cycles  will continue  as  long  as  the  AUTO  switch  is  in  the    ON position.   The  AUTO deice switch is protected by a 5- ampere  circuit  breaker,  placarded   PROP  ANTI-ICE  /  CONTR,    located    on    the    overhead    circuit    breaker