TM 1-1510-218-10 8-31 temperature  of  freezing  or  below  for  it  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 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 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. 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 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. NOTE When   operating   on   wet   or   icy   runways, refer   to   Stopping   Distance   Factors   Chart shown in Chapter 7. 8-66.  ICING (SEVERE). a.     The   following   weather   conditions   may   be conducive to severe in-flight icing. 1.   Visible  rain  at  temperatures  below  0  °C ambient air temperature. 2.   Droplets  that  splash  or  splatter  on  impact at  temperatures  below  0  °C  ambient  air temperature. b.    The following procedure for exiting a severe icing environment is applicable to all flight phases from takeoff to landing. 1.   Monitor    the    ambient    air    temperature. While severe icing may form at temperatures as cold as –18 °C, increased vigilance is warranted at temperatures  around  freezing  with  visible moisture present. 2.   Upon  observing  the  visual  cues  specified in the limitations section for the identification   of   severe   icing   conditions indicated   in   Chapter   5,   accomplish   the following: a.   Immediately   request   priority   handling from   ATC   to   facilitate   a   route   or   an altitude change to exit the severe icing conditions  in  order  to  avoid  extended exposure    to    flight    conditions    more severe     than     those     for     which     the airplane has been certified. b.   Avoid abrupt and excessive maneuvering     that     may exacerbate control difficulties. c.   Do not engage the autopilot. d.   If   the   autopilot   is   engaged,   hold   the control wheel firmly and disengage the autopilot.