place in triggers condition

!canMove heli and  {alive _x} count crew heli == 0 or count crew heli == 0

It checks to see if the heli is flyable and crew are dead or not in the chopper.

A simpler way would be

!canMove heli and getposATL heli select 0 < 1

The only problem if if the heli gets hung up on a roof.

(position heli select 2) < 2

Will also work as long as your heli is never going to land.