Answer:
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 airplane" usually means that something is inside, such as cargo and people. However, sometimes people do say "He is on the airplane" to mean that he is a passenger or crew member -- this is not technically correct, but it has come to be common usage.