TM 1-1510-225-10 2-79 4.   CABIN  AIR,  COPILOT  AIR,  PILOT  AIR, and DEFROST AIR Controls – As required. c.   Manual Operation. If the automatic temperature     control     should     fail     to     operate,     the temperature    of    defrost    air    and    cabin    air    can    be controlled   manually   by   setting   the    CABIN   TEMP MODE switch to the  MAN  HEAT position, then using the     MANUAL TEMP     switch     to     set     the     desired temperature.    This  control  is  located  on  the  copilot's subpanel.      Use   the   following   procedure   for   manual operation: 1.   PILOT and  COPILOT  AIR Controls – In. 2.   CABIN  AIR and   DEFROST  AIR  Controls – Out. 3.   CABIN    TEMP    MODE    Switch     –    MAN HEAT. 4.   Cold Air Outlets – As required. 5.   MANUAL TEMP Switch – As required. 2-52.  SURFACE DEICING SYSTEM. a.   Description.    Ice  accumulation  is  removed from   each   inboard   and   outboard   wing   leading  edge and  both  horizontal  stabilizers  by  the  flexing  of  deice boots  that  are  pneumatically  actuated.    Bleed  air  is used  to  supply  air  pressure  to  inflate  the  deice  boots and to supply vacuum through the ejector system.  A pressure    regulator    protects    the    system    from    over inflation.      When   the   system   is   not   in   operation,   a distributor valve keeps the boots held down by vacuum supplied through the ejector system. CAUTION Operation  of  the  surface  deice  system  in ambient  temperatures  below    –40   °C   can cause    permanent    damage    to    the    deice boots.  NOTE Under    conditions    where    one    bleed    air source  is  inoperative,  sufficient  bleed  air pressure  for  deice  boot  inflation  may  not be available.  Prior to deice boot inflation, check   the   regulated   bleed   air   pressure gauge    for    a    minimum    of    16    psi.        If insufficient     pressure     exists,     increasing engine N₁ and/or decreasing aircraft altitude will increase bleed air pressure. b.  Operation. (1)   Deice Boots.  Deice boots are intended to remove ice after it has formed rather than prevent ice formation.    For  the  most  effective  deicing  operation, allow  at  least  1/2  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. Never cycle the system rapidly.  This may cause   the  ice  to  accumulate  outside  the contour  of  the  inflated  boots  and  prevent ice removal. (2)   Deice   Switch.        A    two    position    deice switch,     placarded     DEICE     CYCL,     on     the     pilot's subpanel, controls the deicing operation.  The switch is spring loaded to return to the off position from SINGLE or MANUAL. (a)  SINGLE Position.  When the SINGLE position   is   selected,   the   distributor   valve   opens   to inflate   the   wing   boots.      After   an   inflation   period   of approximately 6 seconds, an electronic timer switches the    distributor    to    deflate    the    wing    boots    and    a 4-second  inflation  begins  in  the  horizontal  stabilizer, stabilon,  and  taillet  boots.    When  these  boots  have inflated and deflated, the cycle is complete. (b)  MANUAL  Position.    If  the  switch  is held  in  the   MANUAL   position,   the   boots   will  inflate simultaneously  and  remain  inflated  until  the  switch  is released.    The  switch  will  return  to  the   OFF  position when released.  After the cycle, the boots will remain in the vacuum hold down condition until again actuated by the switch. (3)   Bleed  Air.    Either  engine  is  capable  of providing sufficient bleed air for all requirements of the surface  deice  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. 2-53.  PROPELLER ELECTRIC DEICE SYSTEM. a.   Description.      The   propeller   electric   deice system  includes  electrically  heated  deice  boots,  slip rings    and    brush    block    assemblies,    a    timer    for automatic operation, ammeter, circuit breakers for left and  right  propeller  and  control  circuit  protection,  and two   switches   located   on   the   pilot's   subpanel,   for automatic or manual control of the system. WARNING