Let me see if I understand this situation well enough. . .

Had the plane crashed in a horizontal position, the fall of the World Trade Center buildings would have been much delayed. As wise and strategic-minded as he is, Bin Laden planned that the plane be tilted as it hit the building, so as to damage more floors. The wings which carried the plane's fuel and the engines would hit the other floors and explode in flames, and as you quoted Jason, this will then cause the iron foundations to melt and weaken the structure of the building.

Edit:

Question: Did the people in the other floors hear or feel the explosion loud enough for them to assume that an accident has occurred and the premises must be evacuated immediately? It seems that since a lot of people died, a lot of people could've not been aware that there was an explosion. Is there any truth to this?

It is understandable that the people on the higher floors couldn't evacuate because of the damaged floors below them, but how about the people on the lower floors? Where they able to evacuate quick enough? Why or why not?

Thanks a lot! :)