TM 1-1510-225-10 8A-26 If fuel tanks are completely filled during hot weather, fuel expansion may cause overflow, creating a fire hazard. j.    Before    Leaving    Aircraft.        Use    normal procedures   in   Section   II.      Take   extreme   care   to prevent  sand  or  dust  from  entering  the  fuel  and  oil system during servicing.  During  hot  weather,  release the brakes immediately after installing wheel chocks to prevent brake disc warpage. 8A-58.  TURBULENCE AND THUNDERSTORM OPERATION. CAUTION Due    to    the    comparatively    light    wing loading,  control  in  severe  turbulence  and thunderstorms is critical.  Since turbulence imposes    heavy    loads    on    the    aircraft structure,  make  all  necessary  changes  in aircraft  attitude  with  the  least  amount  of control pressures to avoid excessive loads on the aircraft structure. Thunderstorms  and  areas  of  severe  turbulence should be avoided.  If such areas are to be penetrated, it will be necessary to counter rapid changes in attitude and     accept     major     indicated     altitude     variations. Penetration  should  be  at  an  altitude  which  provides adequate  maneuvering  margins  as  a  loss  or  gain  of several  thousand  feet  of  altitude  may  be  expected. The    recommended    penetration    speed    in    severe turbulence  is  170 KIAS.    Pitch  attitude  and  constant power   settings   are   vital   to   proper   flight   technique. Establish recommended penetration speed and proper attitude prior to entering turbulent air to minimize most difficulties.        False    indications    by    the    pressure instruments   due   to   barometric   pressure   variations within   the   storm   make   the   instruments   unreliable. Maintaining  a  pre-established  attitude  will  result  in  a fairly constant airspeed.  Turn cockpit and cabin lights on to minimize the blinding effects of lightning.  Do not use  autopilot  altitude  hold.    Maintain  constant  power settings  and  pitch  attitude  regardless  of  airspeed  or altitude indications.  Concentrate on maintaining a level attitude   by   reference   to   the   flight   director/attitude indicator.    Maintain  original  heading.    Make  no  turns unless absolutely necessary. 8A-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. WARNING WARNING