I was looking through my viewfinder many years ago at FCO, wanting to take shots of landing in Italy, when a large man dressing in Army fatigues, carrying a machine gun, with a large dog at his side, appeared and waved his finger in the universal "don't do that" signal.

I chose not to have a conversation on my rights as a photographer.

You can read about US-based photogrpahy rights at http://www.krages.com/phoright.htm

Shooting an airplane/airport from a public road would seem to be no problem, but I'm not sure whether the inside of an airport would fall under a "public place" or a location requiring building owner permission. I know PIT has a sign stating "no photography beyond this point".