Straight from TouchArcade.com:
X-Plane for Mac OS X, Linux, and Windows has long been considered one of the best–if not the best–flight simulators available to consumers. Its accuracy and attention to the smallest detail of flight simulation has won it numerous accolades in the flight simulation arena.
As Laminar describes it, X-Plane for Mac, Windows, and Linux is the most flexible and realistic flight simulator on the market for desktop personal computers. Using blade element theory for the most powerful and accurate flight model available for personal computers, and earning FAA certification for logging of flight time for pilots, X-Plane is the Gold Standard for accurate flight simulation, offering realism that is unmatched by any other simulator.
And now, thanks to the power of the iPhone and iPod touch, X-Plane is available for these stunning little platforms. The ability to punch a hole in the sky with X-Plane while on the go is big news for iPhone-using flight simulator fans. X-Plane retails for $9.99 [App Store] in the iTunes App Store.