[Callidon]

Basically, the speed at which the aircraft were travelling at the time, coupled with the lightwieght materials and inherant flexibility of the wing design means that as the wings meet the outer structure of the tower, they kinda fold back, therefore not penetrating in the horizontal manner i think you imagine.

As for the tail sction the same is true there. The "fin" itself folds down when it impacts. It helps if you can imagine the mechanics of an arrow with a feather flight. the main shaft, or fuselage, of the arrow is a semi-rigid construct, it can crumple as they did on 9/11, but essentially they pierce the body(tower) they impact. Then imagine the flights as the wings, they are lighweight in nature and will fold back to the body of the arrow allowing them to pass thorugh the body(tower) without having to create their own channel with which to pass through.

This means that you won't see the wings/tail section make extra holes or damage as they are essentially folding back to pass through the hole made by the main fuselage section.

Hope this clears up you question, if not, i'll be glad to help