the debris from the gear doors is what damaged the tail.
Thanks tdracer, that makes good sense.
In the NTSB accident report it's also noted that the wings had a permanent "set" of 2 or 3 inches upward at the tips following the incident, still within Boeing tolerances for the type. And the APU broke loose from it's mounts. Any way you look at it, it's an amazing testament to the structural engineering of the 747 that it was able to tolerate such abuse without failing catastrophically.
It was calculated that the indicated airspeed of one of the 767's that hit
the WTC was over 460 Knots, the old girl stayed together as long as she could..
But when exceeding mach limits, there's more than just the dynamic pressure to be concerned with. Though many types may tolerate the forces applied to the airframe during operation beyond Vd, mach induced buffet/vibration associated with operation beyond Md can cause a host of other problems. Beyond a certain mach number, shock induced airframe buffet/vibration can render flight instruments unreadable, cause force reversals on flight controls and induce stress overloads in airframe structures.
The maximum demonstrated dive speed will be less than the speed at which any of these effects become excessive. All on an airplane which has not yet been subjected to the rigorous daily abuse associated with years of line flying and the minimal maintenance attention many airliners have to live with in the real world.
Just as a thought experiment, I wonder how a typical certification test pilot would react to the idea of being assigned to dive test a typical 737 with 20,000 typical airline cycles on it.