TM 1-1510-218-10 5-15 5-33.  ICING LIMITATIONS (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  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    aircraft    surfaces.        If    any    of    these conditions are observed, the icing environment should be exited as soon as practicable. a.   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 ½–inch accumulation). b.  A 30% increase in torque per engine required to  maintain  an  desired  airspeed  in  level  flight  (not  to exceed 85% torque) when operating at recommended holding speed. c.   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 is determined by comparing pre–icing condition entry speed to the indicated speed after a surface and antenna deice cycle is completed. d.  Any variations from normal indicated airspeed between the pilot’s and copilot’s airspeed indicators. 5-34.  ICING LIMITATIONS (SEVERE). Severe icing may result from environmental  conditions  outside  of  those for which the aircraft is certified.  Flight in freezing   rain,   freezing   drizzle,   or   mixed icing  conditions  (supercooled  liquid  water and ice crystals) may result in a buildup on protective surfaces exceeding the capability  of  the  ice  protection  system,  or may   result   in   ice   forming   aft   of   these protected surfaces.  This ice may not shed using   ice   protection   systems,   and   may seriously   degrade   the   performance   and controllability of the aircraft. a.   During   flight,   severe    icing   conditions   that exceed those for which the aircraft is certified shall be determined  by  the  following  visual  cues.    If  one  or more  of  these  visual  cues  exist,  immediately  request priority  handling  from  air  traffic  control  to  facilitate  a route or an altitude change to exit the icing conditions: (1)  Unusually  extensive  ice  accreted  on  the airframe in areas not normally observed to collect ice. (2)  Accumulation   of   ice   on   the   upper   (or lower,  as  appropriate)  surface  of  the  wing  aft  of  the protected area. (3)  Accumulation    of    ice    on    the    propeller spinner farther aft than normally observed. b.  Since the autopilot may mask tactile cues that indicate  adverse  changes  in  handling  characteristics, use of the autopilot is prohibited when any of the visual cues  specified  above  exist,  or  when  unusual  lateral trim    requirements    or    autopilot    trim    warnings    are encountered while the aircraft is in icing conditions. 5-35.  OXYGEN REQUIREMENTS. a.   One oxygen mask must be provided for each passenger  and  crewmember.    A  minimum  10–minute supply of supplement oxygen shall be available during flight  at  or  above  an  altitude  of  25,000  feet  based  on the highest total aircraft oxygen flow rates. b.  In  addition,  sufficient  oxygen  will  be  carried for each flight, assuming a decompression will occur at the  altitude  or  point  of  flight  that  is  most  critical  from the    standpoint    of    oxygen    need,    and    that    after decompression the aircraft will descend, in accordance with the emergency procedures, to a flight altitude that will allow successful termination of the flight. WARNING WARNING