Seven Basic Possibilities for What Happened at the South Tower

Six possibilities are described here.

The seventh possibility is that there was a more-or-less conventional plane that was able to shoot something ahead of itself to create the opening in the tower wall. One possibility is that there were pre-planted explosives in the building along the wall, and there was a trigger (electrical charge?) that sensed the plane (this could explain the entry flash seen in some videos) and helped blow the wall inwards as the plane went in-- even along the wings. Another possibility is the plane shot a beam or missile to create the entry hole (again explaining the entry flash). Either way, this conventional plane would have had to be destroyed completely by the floor slabs and columns once inside. This entry mechanism would have to be combined with video fakery to account for the discrepencies between the videos.

I tend not to like this explanation, though I can't really rule it out either. The biggest problem with the pre-planted explosives-trigger idea is that it seems like a cumbersome way of setting up the attack, and there is the technical problem of getting the right explosives along the inner walls without people not involved in the plot knowing about it. The problem with the plane shooting something ahead of itself (beam or missile) is that it is hard to see how this accounts for the wing holes or what happened to the tail. Another problem is this theory doesn't really explain why so many of the videos of the 2nd plane are fake-looking with a cartoonish plane.

The bottom line is that this explanation doesn't seem more likely than my top two possibilities (non-conventional plane + video fakery or pure video fakery).