TM 1-1510-224-10 cabin    lights    on    to    minimize    the    blinding    effects    of lightning.    Do  not  use  autopilot  altitude  hold.    Maintain constant  power  settings  and  pitch  attitude  regardless  of airspeed     or     altitude     indications.          Concentrate     on maintaining   a   level   attitude   by   reference   to   the   flight director/attitude   indicator.      Maintain   original   heading. Make no turns unless absolutely necessary. 8-51.  ICE AND RAIN (TYPICAL). WARNING While  in  icing  conditions,  if  there  is an     unexplained     30%     increase     of torque  needed  to  maintain  airspeed in  level  flight,  a  cumulative  total  of two or more inches of ice accumulation on the wing, an unexplained    decrease    of    15    knots IAS,    or    an    unexplained    deviation between pilot's and copilot's airspeed indicators,     the     icing     environment should be exited as soon as practicable.  Ice accumulation on the pitot  tube  assemblies  could  cause  a complete loss of airspeed indication. a. Typical  Icing.    The  following  conditions  indicate a    possible    accumulation    of    ice    on    the    pitot    tube assemblies  and  unprotected  aircraft  surfaces.    If  any  of the following conditions are observed, the icing environment should be exited as soon as practicable: (1) Total   ice   accumulation   of   two   inches   or more    on    the    wing    surfaces.        Determination    of    ice thickness     can     be     accomplished     by     summing     the estimated    ice    thickness    on    the    wing    prior    to    each pneumatic boot deice cycle (e.g.  four cycles of minimum recommended 1/2-inch accumulation). (2) A    30    percent    increase    in    torque    per engine   required   to   maintain   desired   airspeed   in   level flight  (not  to  exceed  85  percent  torque)  when  operating at recommended holding/loiter speed. (3) A   decrease   in   indicated   airspeed   of   15 knots  after  entering  the  icing  condition  (not  slower  than 1.4  power  off  stall  speed)  if  maintaining  original  power setting    in    level    flight.        This    can    be    determined    by comparing     pre-icing     condition     entry     speed     to     the indicated  speed  after  a  surface  and  antenna  deice  cycle is completed. (4) Any     variations     from     normal     indicated airspeed    between    the    pilot's    and    copilot's    airspeed indicators. Icing   occurs   because   of   supercooled   water   vapor such   as   fog,   clouds,   or   rain   collecting   on   the   aircraft surface.      The   most   severe   icing   occurs   on   aircraft surfaces  in  visible  moisture  or  precipitation  with  a  true outside    air    temperature    between    -5C     and     +1C; however,   under   some   circumstances,   dangerous   icing conditions may be encountered with temperatures below -10C.  The surface of aircraft must be at a temperature of   freezing   or   below   for   ice   to   stick.      If   severe   icing conditions    are    encountered,    ascend    or    descend    to altitudes  where  these  conditions  do  not  prevail.    If  flight into icing conditions is unavoidable, proper use of aircraft anti-icing     and     deicing     systems     may     minimize     the problems  encountered.    Approximately  15  minutes  prior to flight into temperature conditions which could produce frost  or  icing  conditions,  the  pilot  and  copilot  windshield anti-ice    switches    should    be    set    at    normal    or    high temperature  position  (after  preheating)  as  necessary  to eliminate  windshield  ice.    Stalling  airspeeds  should  be expected  to  increase  when  ice  has  accumulated  on  the aircraft   causing   distortion   of   the   wing   airfoil.      For   the same reason, stall warning devices are not accurate and should not be relied upon.  Keep a comfortable margin of airspeed  above  the  normal  stall  airspeed  with  ice-on  the aircraft.      Maintain   a   minimum   of   140   KNOTS   during sustained  icing  conditions  to  prevent  ice  accumulation on  unprotected  surfaces  of  the  wing.    In  the  event  of windshield   icing,   reduce   airspeed   to   226   KNOTS   or below. b. Rain.      Rain   presents   no   particular   problems other   than   slippery   runways,   restricted   visibility,   and occasional incorrect airspeed indications. c. Taxiing.    Extreme  care  must  be  exercised  when taxiing  on  ice,  or  slippery  runways.    Excessive  use  of either  brakes  or  power  may  result  in  an  uncontrollable skid. d. Takeoff.  Extreme care must be exercised during takeoff  from  ice  or  slippery  runways.    Excessive  use  of either  brakes  or  power  may  result  in  an  uncontrollable skid. e. Climb.    Keep  aircraft  attitude  as  flat  as  possible and  climb  with  higher  airspeed  than  usual,  so  that  the lower surfaces of the aircraft will not be iced by flight at a high angle of attack. f. Cruise Flight. (1) Prevention   of   ice   formation   is   far   more effective  and  satisfactory  than  attempts  to  dislodge  the ice    after    it    has    formed.        If    icing    conditions    are inadvertently encountered, turn on the anti-icing systems prior to the first sign of ice formation. (2) Do  not  operate  deicer  boots  continuously. Allow at least 0.5 inch of ice on the wing deicer boots or 0.25  inch  on  the  antenna  deicer  boots  before  activating to   remove   the   ice.      Continued   flight   in   severe   icing conditions  should  not  be  attempted.    If  ice  forms  on  the wing area aft of the deicer boots, climb or descend to an altitude where conditions are less severe. 8-31