If you are "in" a helicopter, you are the pilot or a passenger. If you are "on" it, then you are on its roof. It is the same with "get in" and "get on". You "get out of" a helicopter, like you "get out of" a car. You "get off" a form of public transport, like plane, train, bus and also bike. You "get out of" something which encloses or surrounds you.

My rule of thumb is: Can you stand up and walk? If yes, ON. If no, IN. I can walk on a bus, train, plane, or boat, so I get ON and OFF them. I cannot walk in a car, taxi, van, or helicopter, so I get IN and OUT of them. I also get ON a bicycle, motorcycle, or horse because I am ON top of them, not inside them.