Only One Tower Hit?

Yes, another 9/11 WI. Please bear with it, it'll be over soon.

So, say something goes wrong with the plot; Either one group of terrorists doesn't manage to get onto the plane, or the crew and passengers pull a Flight 93 and crash the plane into Hudson bay, or something.

Only one plane manages to hit the WTC towers; How does this affect things from here on out?
 
Well it'd probably be United Airlines Flight 175 that goes down (American Airlines Flight 11 was probably still expecting the Hijacking to be a ransom demand) thus saving the 900 odd people in the South Tower, and probably a few of the 1600 odd people in the North Tower (the 1366 people above the impact zone on the North Tower were mostly dead anyway though, no way to save them, except maybe firing a line across from the top of the South Tower).
 
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