Corrected entry: When John Connor flies the helicopter in the first attack sequence, he does not put a seatbelt on. But when it crashes, he releases the seatbelt to then fall onto the roof of the helicopter (it has crashed upside down).
bazzaboeing
6th Jun 2009
Correction: Pure speculation. The camera shot is framed not to show what Connor is doing the moment he gets in the seat, and it's a matter of at least 8-10 seconds before the helicopter starts lifting off, plenty of time for a clearly experienced pilot to quickly put his seatbelt on.
GalahadFairlight