Flying either of the Little Bird variants (not sure about the other chopper available) into the ground results in said helicopter bouncing a few times, slowly chipping health away from all available systems. Either of the pilots may die during this bouncing, but it really becomes a problem when the crash happens at high speeds (dropping from 500+ ft) and landing, only to bounce a few times and survive. Helicopters only explode when nearly every system is on red.
In ARMA 2, helicopters would typically blow up with contact to ground. Does not happen in singleplayer in ARMA 3.