While I was on the airplane or
While I was in the airplane?
Which preposition is correct? 
Both are correct: you would use “on” when you were not particularly thinking of what the airplane is like inside, “in” when you were thinking of something to which the interior of the airplane is relevant, such as “the person next to me in the airplane kept squirming in his seat”. The distinction is quite vague; the prepositions can be used almost interchangeably.