There are two types of collision in physics which are named as elastic collision and inelastic collision.
Elastic collision is that type of collision in which both kinetic energy and momentum is conserved i.e total kinetic energy and momentum before collision is equal to the total kinetic energy and momentum after collision.As kinetic energy is conserved here,so less or ideally no damage occurs to the colliding bodies.
Inelastic collision is that type of collision in which only momentum is conserved but not kinetic energy .After the collision the kinetic energy is lost in forms heat,sound ,light etc.
Normally any deformation and destruction is due to the inelastic collision which results in the loss of kinetic energy.
Hence the helicopters were destroyed and deformed due to inelastic collision.
