TM 55-1510-220-10 Section VII.  UTILITY SYSTEMS 2-51.  DEFROSTING SYSTEM. a. Description.  The defrosting system is an integral part  of  the  heating  and  ventilation  system.    The  system consists of two warm air outlets connected by ducts to the heating   system.      One   outlet   is   just   below   the   pilot's windshield and the other is below the copilot's windshield. A   push-pull   control,   placarded   DEFROST   AIR,   on   the pilot's subpanel, manually controls airflow to the windshield.    When  pulled  out,  defrosting  air  is  ducted  to the  windshield.    As  the  control  is  pushed  in,  there  is  a corresponding decrease in airflow. b. Automatic Operation. 1.  Vent blower switches - As required. 2.  Cabin temperature mode selector switch - AUTO. 3.  Cabin temperature control rheostat - As required. 4.  Cabin air, copilot air, pilot air, and defrost air controls - As required. c    Manual Operation 1.  Pilot air, copilot air - IN. 2.  Cabin air and defrost air controls - Out 3.  Cabin temperature mode selector switch - MAN HEAT. 4.  Cold air outlets - As required. 5.  Manual temperature switch - As required. d. Manual  Operation.    If  the  automatic  temperature control should fail to operate, the temperature (of defrost air    and    cabin    air)    may    be    controlled    manually    by manipulating   the   CABIN   TEMP   MODE   control   switch between the OFF and MAN HEAT positions.  This control is located on the overhead control panel (fig.  2-12). 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 deicer boots which are  pneumatically  actuated.    Engine  bleed  air,  from  the engine   compressor,   is   used   to   supply   air   pressure   to inflate  the  deicer  boots,  and  to  supply  vacuum,  through the  ejector  system,  for  boot  hold  down  during  flight.    A pressure regulator protects the system from over inflation. When  the  system  is  not  in  operation,  a  distributor  valve applies vacuum to the boots for hold down. CAUTION Operation    of    the    surface    de-ice system    in    ambient    temperatures below  -40°C  can  cause  permanent damage to the de-ice boots. b. 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  1/2 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. 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) A   three   position   switch   on   the   overhead control   panel   placarded   SURF   DEICE   MANUAL   OFF SINGLE  CYCLE  AUTO,  controls  the  deicing  operation. The switch is spring loaded to return to the OFF position from   SINGLE   CYCLE   AUTO   or   MANUAL.      When   the SINGLE CYCLE AUTO 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  boots. When these boots have inflated and deflated, the cycle is complete. (3) 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. 2-42