Remove all fuel from the chopper.Name the people that must be in the chopper guy1, guy2, etc, name the chopper "chopper" and make a trigger.Type in the condition field "Guy1 in chopper AND guy2 in chopper AND guy3 in chopper", etc.Type in activation field "chopper setfuel 1" This is only good for the start of mission, as if someone dies the chopper will never take off.If it isnt the start, i think this will work for the condition:
(Guy1 IN Chopper OR !Alive Guy1) AND (Guy2 IN chopper OR !Alive Guy2) AND (GUY3 IN Chopper OR !ALIVE GUY3), etc