TM 1-1510-218-10 8-30 CAUTION During    hot    weather,    if    fuel    tanks    are completely    filled,    fuel    expansion     may cause   overflow,   thereby   creating   a   fire hazard. j. Before   Leaving   Aircraft.      Take   extreme care to prevent sand or dust from entering the fuel and oil   system   during   servicing.      During   hot   weather, release  the  brake  immediately  after  installing  wheel chocks to prevent brake disc warpage. 8-64.  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’s 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   that   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 them 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   FD/Attitude   Indicator. Maintain  original  heading  and  don't  make  any  turns unless absolutely necessary. 8-65.  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. a.    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 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 a 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 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. b.    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 WARNING