Props and the aircraft involved.

I don't know how they did it, but they did a lot better job than many more recent films, ie, Langoliers, which I watched last week. A good example is when she's flying over the mountains we see the shadows down below of both aircraft. The interior shots brought back the good ole days of flying before the government overreacted to security issues and made flying a total pain in the AR&.

My main special effect peeve was how little wind was in the cockpit, the girls hair barely blew around, that was pretty cute, I sure know what my hair would have looked like in that situation!!!!

***So I've seen 4 movies/wk in theatre for a 1/4 century, call me crazy?**

Mostly real aircraft for exteriors - with the odd studio set with process b.g during the transfer.
A studio set for all interiors apart from the odd shot of Heston in a real cabin.

Whenever you see any serious film work in a plane on a major feature film it's almost certainly a set as real aircraft are too confined to get a lot of angles, lighting is difficult, as are camera moves and real aircraft usually cost more to hire than to either hire a standing set or to build your own, using parts from a disused aircraft.