When the President is evacuated from the White House he presumably goes to Andrews Air Force Base to board Air Force One. First off, in the movie you can see the capitol dome from Andrews. In real life you can't. Secondly, when the alien ship blows up the White House and the wall of fire spreads outward, it takes a mere seconds for it to reach Andrews and is chasing AF1 down the runway. In real life, I doubt the fire wall would be that quick to reach Andrews from Washington.