TM   55-1510-219-10 8-57.  ICE  AND  RAIN  (TYPICAL). W A R N I N G 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  recommendation  ½-inch  accumulation. (2)  A  30  percent  increase  in  torque  per  engine required  to  maintain  a  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   Ice.   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 he  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.   Taxiing.   Extreme  are  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.   Cruising   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  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 an 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 and  increased  weight  of  the  aircraft  due  to  ice  buildup. Approach   and   landing   airspeeds   must   be   increased accordingly. 8-26 Change  3