terox hit the nail there. You may start with a helicopter, but when it is destroyed, it won't come back w/o other scripting. I'm not good with eventhandlers, but a trigger like this would work:
condition: alive player6 and !driver chopper1
on activation: player6 join grpnull
That way your group respawn won't respawn you into that unit, but the poor player of player6 will be orphaned.
If you don't mind a pilot without a ride running around, you really don't need to do anything. Vehicles don't "respawn", new ones can be created if a vehicle is destroyed using another script