TM 1-1520240-BD SECTION III.  COMBAT THREATS 1-10.  THREAT   DESCRIPTION .      The   primary   threats confronting   Army   helicopters   in   combat   include   the armor-piercing (AP) and armor-piercing-incendiary (API) projectiles, the high-explosive-incendiary (HEI) projectiles, and several types of ground-to-air and air-to- air missiles.  Nuclear,  biological,  and  chemical  (NBC) warfare also poses a threat to the successful accomplishment  of  the  aviation  mission.    In  addition  to the threats helicopters may encounter in flight, they will be exposed to damage by bombs, mortars, and artillery while on the ground. 1-11.  ARMOR-PIERCING AND ARMOR-PIERCING INCENDIARY  PROJECTILES.    AP  and  API  projectiles will   cause   most   of   the   damage.      These   projectiles consist  of  a  hard  tough  core,  designed  for  maximum penetration   The   API   projectile   also   has   a   thermally active  filler.    The  active  filler  is  located  in  front  of  the passive  core.    Upon  impact,  the  core  penetrates  the outside  of  the  target.    This  gives  the  projectile  a  fire- starting capability if flammable materials are present. a. Armor-Piercing Projectile.  The damage caused by  an  armor-piercing  projectile  depends  on  its  mass, velocity,  and  angle  at  impact.    The  primary  damage  is caused by the penetrator. b. Armor-Piercing-lncendiary Projectile. API projectiles can cause major damage to an aircraft if the fuel cells are penetrated.  Bulging or rupturing of the fuel cell  walls  and  surrounding  structures  can  be  caused  by hydraulic  ram  effects.    The  incendiary  mechanism  can result  in  fire  and  explosions.    Intense  heat  from  fires may   reduce   the   temper   and   strength   of   surrounding metals. 1-12.  HIGH-EXPLOSIVE-INCENDIARY PROJECTILES.        HEI    projectiles    will    create    much greater repair problems.  The HEI projectile consists of a fuse mechanism, explosive charge, tracer element, and an outer casing (Figure 1-7). a. Fuses.  Two types of fuses are used:  time- delay  and  proximity.    The  time-delay  fuse  is  activated when the projectile strikes a surface, delaying detonation  of  the  charge  for  varying  lengths  of  time. The  proximity  or  point  detonation  fuses  are  activated and detonation occurs upon or just prior to contact with the   target.      Detonation   causes   the   shell   casing   to rupture.      The   projectile   breaks   into   fragments   and accelerates them to high velocities.  The velocity of the projectile added to the velocity of the fragments caused by the explosive charge focus the fragments into a cone (Figure  1-8).    In  addition  to  fragments,  the  explosive charge produces a shock wave which travels above the speed    of    sound    initially    ahead    of    the    accelerating fragments.    Structures  close  to  the  point  of  detonation are stressed by the shock wave and overpraises before the    fragments    impact.        The    damage    mechanisms associated with  the  point detonation  or proximity-fused HEI  are  basically  the  same  as  the  time-delayed  fused HEI.  The exception is that the blast and fragmentation effects occur at the target surface and continue into the interior  structure.    Overpressure  effects  are  generally less  severe  because  the  explosion  does  not  take  place inside the fuselage. Figure 1-7.  Typical HEI Projectile 1-11