TM 55-1510-220-10 accumulated  from  the  spray  of  slush  or  water.    Monitor BRAKE  DE-ICE  annunciator  for  automatic  termination  of system operation and then turn the switch OFF. (2) Flight controls.  During flight, trim tabs and controls  should  also  be  exercised  periodically  to  prevent freezing. (3)    Anti-icing equipment.   Insure that anti-icing systems   are   activated-before   entering   icing   conditions. Do  not  activate  the  surface  de-ice  system  until  ice  has accumulated  one-half  to  one  inch.    The  propeller  de-ice system  operates  effectively  as  an  anti-ice  system  and  it may   be   operated   continuously   in   flight.      If   propeller imbalance  due  to  ice  does  occur,  it  may  be  relieved  by increasing RPM briefly, then returning to desired setting. NOTE Do  not  operate  deicer  boots  continuously. Continuous operation tends to balloon the ice over the boots.  Allow at least 1/ 2 inch of  ice  to  accumulate  on  the  surface  boots and 1/8 to 1/4 inch of ice to accumulate on the antenna boots, then activate the deicer boots   to   remove   the   ice.      Repeat   this procedure as required. (4)    Ice  vanes.    Ice  vanes  must  be  extended when operating in visible moisture or when freedom from visible  moisture  cannot  be  assured,  at  5°C  FAT  or  less. Ice vanes are designed as an anti-ice system, not a de-ice system.    After  the  engine  air  inlet  screens  are  blocked, lowering  the  ice  vanes  will  not  rectify  the  condition.    Ice vanes  should  be  retracted  at  15°C  FAT  and  above  to assure adequate engine oil cooling. (5)    Stall  speeds.    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.      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. h. Descent.    Use  normal  procedures  in  Section  II. Brake    icing    should    be    considered    if    moisture    was encountered during previous ground operations or inflight in icing conditions with gear extended. i. Landing.    Landing  on  an  icy  runway  should  be attempted only when absolutely necessary and should not be   attempted   unless   the   wind   is   within   10   degrees   of runway  heading.    Application  of  brakes  without  skidding the   tires   on   ice   is   very   difficult,   due   to   the   sensitive brakes.  In order not to impair pilot visibility, reverse thrust should  be  used  with  caution  when  landing  on  a  runway covered with snow or standing water.  Use the procedures in Section II for normal landing. j. Engine   Shutdown.      Use   normal   procedures   in Section II. k. Before Leaving the Aircraft.  When the aircraft is parked   outside   on   ice   or   in   a   fluctuating   freeze-thaw temperature condition the following procedures should be followed in addition to the normal procedures in Section II. After  wheel  chocks  are  in  place,  release  the  brakes  to prevent freezing. Fill fuel tanks to minimize condensation,  remove  any  accumulation  of  dirt  and  ice from  the  landing  gear  shock  struts,  and  install  protective covers  to  guard  against  possible  collection  of  snow  and ice. 8-69.      DESERT   OPERATION   AND   HOT   WEATHER OPERATION. Dust,    sand,    and    high    temperatures    encountered during desert operation can sharply reduce the operational  life  of  the  aircraft  and  its  equipment.    The abrasive  qualities  of  dust  and  sand  upon  turbine  blades and moving parts of the aircraft and the destructive effect of   heat   upon   the   aircraft   instruments   will   necessitate hours  of  maintenance  if  basic  preventive  measures  are not followed.  In flight, the hazards of dust and sand will be difficult to escape, since dust clouds over a desert may be   found   at   altitudes   up   to   10,000   feet.      During   hot weather  operations,  the  principle  difficulties  encountered are  high  turbine  gas  temperatures  (TGT)  during  engine starting,  overheating  of  brakes,  and  longer  takeoff  and landing rolls due to the higher density altitudes.  In areas where high humidity is encountered, electrical equipment (such as communication equipment and instruments) will be subject to malfunction by corrosion, fungi and moisture absorption by nonmetallic materials. a. Preparation For Flight.  Check the position of the aircraft  in  relation  to  other  aircraft.    Propeller  sand  blast can   damage   closely   parked   aircraft.      Check   that   the landing   gear   shock   struts   are   free   of   dust   and   sand. Check instrument panel and general interior for dust and sand    accumulation.        Open    main    entrance    door    and cockpit vent storm windows to ventilate the aircraft. 8-30