TM 1-1510-225-10   5-17 5-35.  ICING LIMITATIONS (S EVERE). Severe    icing    may    result    from    environ- mental    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. NOTE All icing detection lights must be operative prior to flight into icing conditions at night.   This supersedes any relief provided by the Master Minimum Equipment List or equivalent. 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; or 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. 5-36.  CROSSWIND LIMITATIONS. Landing with wind conditions in excess of the    demonstrated    crosswind    component may result in damage to the aircraft.  This should only be attempted during emergency situations.  The maximum demonstrated crosswind component is 25 knots at 90. 5-37.  OXYGEN REQUIREMENTS. a.   Oxygen  requirements  will  be  in  accordance with AR 95-1. b.  Oxygen     system     data/duration     tables     are found in Chapter 2. 5-38.  CABIN PRESSURE LIMITS. Maximum cabin differential pressure is 6.6 psi. 5-39.  CRACKED WINDSHIELD. NOTE Heating   elements   may   be   inoperative   in area of crack. a.   External    Crack    In-flight.        If    an    external windshield   crack   is   noted,   no   action   is   required   in flight. b.  Internal  Crack  In-flight.    If  it  is  determined that  an  internal  crack  has  occurred  in  flight,  perform the Internal Crack In-flight procedure in Chapter 9. c.   External  Crack  on  Ground.    If the cracking of  only  the  windshield's  outer  ply  is  observed  on  the ground,     unpressurized     flight     may     be     conducted provided the following procedures are observed: 1.   Cracking    must    not    significantly    impair visibility. 2.   Cracking   must   not   interfere   with   use   of windshield wipers. 3.   Heating  elements  must  be  operative  for flight into icing. 4.   A  temporary  placard  must  be  fabricated and  placed  in  clear  view  of  the  pilot  until the  windshield  is  replaced.    The  placard should read as follows: WARNING WARNING