When any large amount of damage is done to helicopter (Usually from a missle of any sort), the Main Rotor locks up, making it impossible to auto rotate. This prohibits the player any way to save his vehicle if he's shot once by anything really.

While the engine should die if you get hit by a rocket, and possibly the tail rotor if hit right, the main rotor blades should not lock up under most circumstances. During engine fail, the rotor blades should still be turning, which generates lift so we can attempt a safe landing (An autorotation).

It would seem to me that as soon as your main rotor takes significant damage, the blades lock up immediately. Evidence of this can be seen by starting up a helicopter while on the ground, and shooting the engine block. The blades will slow down to a stop (Like normal). However if you target the main rotor, you'll see it stop completely.