Why would they want to do that when theres a hundred tones of airplane and fuel traveling at about 500 mph behind it. The other problem i have is that they would have had to fire the missile something like half of a second before it hit the tower, any earlier and the plane would have been a long way off and you would see the missile. It would be quite something to be able to judge that when you were about to hit the tower and your traveling at 500 mph.