From The Hardy Boys Wiki:
Attending an airshow to help out their friend Jamal Hawkins and his dad's air taxi service should be a nice break from the normal grind for Frank and Joe Hardy. But when one of the Hawkinses' planes is mysteriously stolen -- starting a whole wave of crime at the event -- the vacation ends, and the hunt for clues begins!
The brothers' investigation takes them from the airport runway to the nearby mountains, and even into the clouds. Who, or what, is behind the trouble? Could it be one of the exhibitors, a jealous rival, or a former hotshot pilot? The crooks' plans are really taking off -- can the Hardys ground these guys for good?