TM 1-1520-240-BD (3)  Inspecting for Structural Changes. (a)         Structural    Changes.        Inspect    for structural  changes  when  the  aircraft  has  taken  an  HEI hit   or   the   aircraft   has   flown   with   primary   structure damaged    or    missing.        Inspect    both    the    damaged structure    and    the    surrounding    undamaged    area    for evidence of buckling, crippling, and misalignment.  This type    of    damage    is    usually    revealed    by    kinks    and wrinkling  and  "oil  canning"  of  skin  panels.    Sheared, pulled-through, and missing fasteners are also indications of structural changes. (b)    Buckling,  Crippling,  and  Misalignment. Structural changes in the form of buckling, crippling, and misalignment can happen as a result of blast pressures. These  could  be  associated  with  a  HEI  attack  or  as  a result  of  the  overstress  placed  on  a  member.    Such members    may    have    to    carry    the    load    of    another member which has been broken or crippled by projectile damage.    This  type  of  damage  can  be  critical  to  the structural   performance   of   a   member   and   may   also cause  interference  of  a  member  and  may  also  cause interference with mechanical moving components which may   bind   or   jam.      This   is   especially   true   when   the member  has  to  support  compression  loads.    A  stringer that  has  been  damaged  can  continue  to  support  some tension; for example, but may be completely ineffective in compression. (c)       Twisting   or   Bowing.      Examine   the component    for    alignment    and    signs    of    twisting    or bowing.  Use a straight edge to inspect these conditions. (4)      Inspecting   for   Embedded   Projectiles   and Fragments. (a) Embedded  Projectile.    The  effect  of an    embedded    projectile    or    fragment    in    a    tension member can be as that of a hole or crack of the same size.    The  embedded  object  creates  an  interruption  in the  structural  section.    Since  it  is  difficult  to  detect,  it must  be  assumed  that  the  projectile  has  nearly  gone through the structure. (b) Embedded Projectile Inspection. Inspect  for  embedded  projectiles  and  fragments  when either of the following conditions occur: 1      The    aircraft    has    suffered    HEI damage. 2    Inspection  of  the  aircraft  indicates that  a  solid  projectile  has  not  exited  the  aircraft,  has broken apart, or has created shrapnel by striking internal components. (c)       Projectile   Path   Determination.      The determination  of  projectile  paths  will  aid  in  identifying internal structures that may contain embedded projectiles  or  fragments.    Inspect  all  structures  in  the region using a bright light and magnifying glass.  Clearly mark  embedded  objects  and  record  them  on  DA  Form 2404, Figure 2-4. (5)  Inspecting for Fire Damage. (a)     Armor   Piercing   Incendiary   (API)   and HFI Fire Damage.  The API and HEI threats include the possibility  of  fire  damage.    These  threats  have  a  fire- starting capability if flammable materials are present. (b)      Fire   Effects.      Some   fires   may   not adversely    effect    metal    airframe    structures.        When aluminum   is   exposed   to   temperatures   above   300°F (149°C) for a prolonged period, the temper and strength of the material will be reduced. (c)  Initial Fire Damage Evidence.  The first signs  of  possible  fire  damage  will  be  a  discoloration  of the   structure.      Any   discoloration   indicates   that   the member    has    been    exposed    to    high    temperature. Conduct  a  hardness  test  to  determine  if  the  temper  of the    material    has    changed.        Such    tests    should    be conducted  in  accordance  with  standard  practices.    If  a hardness test shows heat damage, record the information   on   DA   Form   2408   and   dearly   mark   the member. (6)   Detecting Substructural Damage in Adjoining Areas. (a)        Secondary    Damage.          When     the airframe    has    been    subjected    to    severe    overstress, members    undamaged    by    projectiles    may    bend    or buckle.    This  might  be  caused  by  explosive  blast  or maneuvering   loads   imposed   on   damaged   structures. Sometimes this secondary damage will occur in a region away  from  the  primary  area  of  damage.    The  airframe near   the   projectile   damage   should   be   inspected   for evidence of secondary damage. (b)   Secondary Damage Indicators.  Inspect the   skin   for   creases,   wrinkles,   and   dents.      Inspect fasteners  for  chipped  or  flaked  paint,  looseness,  and serviceability.  These conditions are signs of damage to structure.    Open  or  remove  access  panels  and  doors, determine whether the frame is warped, and inspect the interior   members   for   cracks   and   structural   changes. Clearly mark and record all damage. 2-7