This depends on what you're trying to say. "On the airplane" implies that something is literally on the outside of the airplane, such as the logo, paint, or something sitting on the wings. "In the...
Airplanes are vehicles that you can pay to get on to fly from one place to another. Some planes are used to fight wars and others are pssenger or cargo planes which carry things to sell.
George Carlin had a bit on this. "I'm getting IN the plane! In the plane! Let Evil Kneivel get ON the plane. I'll be in here with you folks in uniform. There seems to be less wind in here."
As strange as it may seem, they were. The Write brothers made hundreds of different paper airplane models to help them understand what they needed to do to make their airplane fly. They also modeled...