TM 1-1510-218-10 8A-26 8A-59.  ICE AND RAIN (TYPICAL). While   in   icing   conditions,   if   there   is   an unexplained 30% increase of torque needed to maintain airspeed in level flight, a  cumulative  total  of  2  or  more  inches  of ice accumulation on the wing, an unexplained  decrease  of  15   KIAS,   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.    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 2 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%    increase    in    torque    per    engine required   to   maintain   desired   airspeed   in level   flight,   not   to   exceed   85%   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- deice cycle is completed. 4.   Any     variations     from     normal     indicated airspeed  between  the  pilot’s  and  copilot’s airspeed indicators. b.    Typical   Icing.      Icing    occurs   because    of super-cooled water vapor such as fog, clouds, or rain. The  most  severe  icing  occurs  on  aircraft  surfaces  in visible moisture or precipitation with a true outside air temperature between -5 °C and +1 °C; however, under some  circumstances,  dangerous  icing  conditions  may be encountered with temperatures below –10 °C.  The surface  of  the  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   that   could produce    frost    or    icing    conditions,    the    pilot's    and copilot's  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. c.    Rain.  Rain presents no  particular  problems other  than  slippery  runways,  restricted   visibility,  and occasional incorrect airspeed indications. d.    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. e.    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. f. 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. g.    Cruise Flight.  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.  Do not operate   deicer   boots   continuously.      Allow   at   least 1/2-inch    of    ice    on    the    wing    deicer    boots    before activating the deicer boots.  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. h.    Landing.    Extreme  care  must  be  exercised when  landing  on  ice  or  slippery  runways.    Excessive use    of    either    brakes    or    power    may    result    in    an WARNING