Well, it's hard to say for sure. 'Aviator' could be entirely fictional, or it might be loosely based on some true events or real-life aviators, but with significant alterations for the sake of the story.
The Aviator does have a basis in truth. It tells the story of Howard Hughes, though some artistic liberties were likely taken to make it more engaging for the audience.
In fact, 'Aviator' draws heavily from true events, though some artistic liberties might have been taken for dramatic effect. But the core elements are based on reality.