As Delta717 said, it was not the initial impact that caused the collapse. That was the intention to use all large, West Coast-bound flights. They chose planes that would depart airports near New York and Washington and would be loaded with fuel. They knew what they were doing.
By the way, just to show you how strong the building were, the impact of wind alone on a high wind day is much stronger than a 767 slicing through it.
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