mientras están en el avión y al subir o bajar por las escaleras

14:48 Apr 20, 2012 

This is what makes it lovely (especially for us non-natives): The exceptions. I figured it, but was not fully sure and in any case, this is one you have to memorize, period.

But I had in mind the stereotype that "on" meant (here) all over the surface and, while I certainly know that this is not the case (and especially that no linguist would let that one slip, let alone a native one), I felt I had to check...

So thanks again to you (and the colleagues down the page). I know better now than I did before.

14:10 Apr 20, 2012 

The collocation that comes naturally to native speakers is "on" the bus/plane/train/ship/ferry... it's something we don't think about. You could also use "in" or "inside" to be more specific; for example, the movie "Snakes on a Plane" takes place inside a plane. I don't see it as a question of being right or wrong, simply doing what comes naturally. When in Rome...