TM 1-1520-240-BD 1-15.  STRUCTURE DAMAGE MODES . a. Projectile   Damage.      The      principal   airframe damage  modes  are  defined  in  Table  1-1.    The  most frequent damage is caused by ballistic projectiles.  The projectiles include solid penetrators such as: •    AP and API rounds. •    Various-size fragments from the HEI threat. •    Larger  metal  fragments  from  bombs,  missiles, and artillery. These  projectiles  travel  at  high  velocity  and  may  have great    mass.        The    kinetic    energy    allows    them    to penetrate deeply into airframe structures, causing much damage.    Damage  caused  by  these  projectiles  will  be complete  penetration  in  the  form  of  holes  and  section losses.  Ricochets cause spalls and gouges.  The stress of  the  impact  may  cause  cracks.    Solid  projectiles  and fragments     may     also     be     imbedded     in     structures. Petalling   is   a   form   of   damage   caused   by   projectiles when they penetrate thin skins, causing the metal to tear and deform. b. Blast and Overpressure Damage. HEI explosive  threats  pose  hazards  in  addition  to  projectile damage.    The  explosive  blast  may  prestress  structures causing them to buckle, cripple, and misalign. Separation  of  joints  and  loss  of  mechanical  fasteners may  also  appear.    When  an  explosion  occurs  within enclosed    sections    of    the    airframe,    it    causes    an overpressure    which    may    overstress    structures    and produce structural deformation. c. Fire  Damage.    API  and  HEI  incendiary  threats have a fire-starting capability if flammable materials are present.    Intense  and  prolonged  heat  may  weaken  and damage structural materials.  High temperatures reduce the   hardness   of   metals,   reducing   their   strength   and stiffness.    Metal  may  melt  under  extreme  heat.    Heat damaged    metals    may    yield    and    crack    under    the continued stress of flying. d. Secondary Damage.  All of the damage modes described above are the direct result of combat.  When damage  to  the  aircraft  causes  one  or  more  structural parts to become unserviceable, the remaining parts may be overstressed and damaged as the aircraft continues its flight.  This secondary damage may be in the form of cracks,  crippling,  or  buckling  and  loss  or  damage  to mechanical fasteners.  Secondary damage may happen away  from  the  site  of  the  original  battle  damage.    This will depend on how the stress loads are redistributed in the structure when parts are removed or are unserviceable. SECTION IV.  BATTLE-DAMAGE ASSESSMENT 1-16.  DAMAGE   ASSESSMENT   PROCEDURES.      In peacetime,   flight   safety   requires   restoring   damaged structure to its original condition.  Consideration is given to strength, corrosion protection, and cosmetic appearance.        Repairs    are    devised    by    the    aircraft engineering  authority  where  expert  advice  is  available and   times   is   not   a   critical   factor.      During   combat, damage will be quite different, as will the repairs.  Time will be of the essence, and the engineering authority and advice   will   not   be   available.      Sufficient   strength   to maintain operational flying is the primary concern of the assessment   and   repair.      In   some   aircraft,   extensive damage  may  require  little  work;  in  others  the  smallest crack could be catastrophic.  When a damaged aircraft is flown, it can be assumed that some structural strength is   still   present.      However,   this   does   not   necessarily mean that there is sufficient strength remaining to carry out  the  next  sorties  as  additional  weight  of  fuel  and armament must be considered.  An assessor must bring together  the  facts  concerning  the  damage,  the  role  the aircraft  has  to  fulfill,  and  the  repairs  necessary  for  the aircraft to carry out its next sortie.  Damage assessment markings are shown in Figure 1-9. a. Use  of  Logic  Trees.    Simplified  logic  trees  are provided   in   each   chapter   of   this   manual   to   aid   the assessor. b. Figure    1-10    is    an    example    of    an    overall systematic  check  logic  diagram  that  the  assessor  may use during an aircraft battle damage inspection. 1-13