One other thing that might explain why that much of the building was not damaged.
If you've ever touched an airplane wings, its not veryt sturdy at all. Like rapier said it is made out of aluminum. When that airplane hit the pentagon, more than likely doing 400mph+, the intesity of the heat from the explosion would be more than enough to disintegrate the wings. After all, that where all the fuel is held is in the wings. Sure the wings probably hit a little bit, but also remember that it was clipping poles on its way in. So when it did hit the building the wings probably folded backwards.
