TM   55-1520-220-10 8-71. 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. The   following   conditions   indicate   a   possible accumulation  of  ice  on  the  pitot  tube  assemblies  and unprotected airplane surfaces. If any of these 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 !&inch accumulation. (2)  A  30  percent  increase  in  torque  per  engine required to maintain an 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. a.   Typical   Icing.  Typical   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 before   ice   will   stick   to   the   aircraft.   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   co-pilot   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  restricted  visibility  and  occasional  incorrect  airspeed indications. c.   Taxing.  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  he  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 he iced by flight at a high angle  of  attack. f. Cruise Flight. (1)  Prevention  of  Ice  Formation.  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)  Deicer  Boots.  Do  not  operate  deicer  boots continuously. Allow at least one-half inch of ice on the boots  before  activating  the  deicer  boots  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 au altitude where conditions are less  severe. g.  Landing.  Extreme  care  must  be  exercised  when landing on ice or slippery runways.   Excessive use of either brakes or power may result in an uncontrollable skid. Ice accumulation on the aircraft will result in higher stalling airspeeds due to the change in aerodynamic characteristics 8-32 Change 7