TM 1-1510-225-10 8-27 indicator.    Maintain  original  heading.    Make  no  turns unless absolutely necessary. 8-59.  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. The   following   conditions   indicate   a   possible accumulation  of  ice  on  the  pilot  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  times  the  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. a.    Typical   Icing.      Icing   occurs   because   of supercooled 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  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.  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. WARNING