When the towers were first built, one of the design questions was whether or not they could survive being hit by a 747 — on accident, presumably. So, that issue was taken into consideration.
At the time of the attack, more modern aircraft were used, which had much thinner skins and were much more lightly constructed. Which means that they got much more shredded by the building construction as they passed through, and which also helped take off a lot more of the fire insulation from the trusses in the building.
The 747s simulated were also near-empty and flying slow, as the assumption was an accidental collision from an airplane lost in fog trying to land. They never simulated one nearly full of fuel running full-tilt.
The two towers were designed to sustain an accidental hit by a 707 on a descent trajectory.
They were instead purposely hit by much larger 747s and at a much higher cruising speed. This meant that the kinetic energy transfer was approximately 8x larger than expected. The 747s at the time of impact also held significantly more fuel (at least 100% more) than those utilised in the 707 attack model.
Correct. Sorry, I had originally typed 767 and then in an edit swapped as everyone else said 747 and I thought perhaps I was having some sort of Mandela Effect going on. The size and speed differential holds true - a cruising speed extended-range 767 is much faster and larger than the equivalent, landing-approach 707 the building was “rated” for.
At the time of the attack, more modern aircraft were used, which had much thinner skins and were much more lightly constructed. Which means that they got much more shredded by the building construction as they passed through, and which also helped take off a lot more of the fire insulation from the trusses in the building.