The planes were flown by remote control. The exact floor they wanted to hit was of course at a fixed altitude above sea level
so flying that exact altitude would be a piece of cake.. just dial in a number on the keyboard really.
They already flew remote controlled planes half a century
before 9/11.
The nuclear tests of operation greenhouse in 1951 on Enewetak Atoll already used drone airplanes for dust sampling.
"An important experimental program at GREENHOUSE was the use of unmanned, radio-controlled
drone aircraft for cloud sampling. Eight B-17 drones were flown close to the detonation to measure
blast and thermal effects and then into the nuclear cloud to collect radioactive samples."
http://www.dtra.mil/documents/ntpr/factsheets/Greenhouse.pdf
http://www.usmilitariaforum.com/forums/index.php?/topic/26802-qb-17-b-17-drone-photos-and-film/
This is a boeing B-17 drone at Eniwetok few years earlier in 1948.
Here is the raging inferno that you guys need to 'soften' the steel needed for your disintegrate by fire theory.
Nobody has that evidence.
The entire building was rigged maybe a few core columns at the impact zone were affected.
Even if the core columns were damaged to assume that the top of the building falls straight through the unaffected and massive lower parts
of the building is absurd.
This is what you guys are confident in defending :