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You have to prove that it couldn't have dove down and reached that speed. Which you haven't, and can't. If a plane can dive down, and remain under perfect control and reach Mach 1, there is absolutely no reason why 175 couldn't have done it.
They climbed to 52,000 feet, where they put it (the DC-8) into a half G pushover (a dive no steeper than 175 performed, and possible not as steep), at 45,000 feet, while in perfect control, the aircraft reached Mach 1.01 for 16 seconds. They were able to recover at 35,000 feet, with no damage to the aircraft.
The dive speed (Vd) is the absolute maximum speed above which the aircraft must not fly. Typically, to achieve this speed, the aircraft must enter a dive (steep descent), as the engines cannot produce sufficient thrust to overcome aerodynamic drag in level flight. At the dive speed, excessive aircraft vibrations develop which put the aircraft structural integrity at stake.
In that video, for the Airbus380 flutter test, they descended in a steady dive from 38,000 feet aiming for a Vd of Mach .96 (it's a big plane with lots of surface area) which to certify required some major modifications.
The south tower plane, according to you, would be able to exceed Mach 1.39 to Mach 1.4 when descending from the same height of 38,000 feet.. for an EAS of 510 knots, at sea level.
Vd is 420 knots for the Boeing 767 as set by the manufacturer based on wind tunnel and flight testing.
Here are those limitations, from Boeing...
At EAS (Sea Level), over test Vd - let's take a look at the range, beyond Vd for the Boeing 767, and we'll do it in full 5 knot increments, which is fair, since we're already at and beginning to exceed the threshold limit for structural failure, Vd limit, and even the smallest increments at that point can have grave effects, as the flight testers experienced with the Airbus A320 in the video contained in that link above i.
420 (Vd limit, by stress/flutter testing)
425 (which is .99 - Mach 1.0 equivalent airspeed and pressure at higher altitude of 22,000 feet - which is about the threshold from all those examples of near or just over Mach flight, while surviving, and this is very conservative, because such dives are mostly done from much higher altitudes as per your DC-8 ref cited above in which case an EAS of 425 represents an even higher Mach # up around 35,000 - 52,000 ft, well exceeding Mach 1.0 ++)
430, 435, 440, 445, 450, 455, 460, 465, 470, 475, 480, 485, 490, 495, 500, 505, 510 knots + (including windspeed, 515)
"During the descent from 12,000 feet to 6,000 feet, the aircraft groundspeed remained between 500 - 520 knots. As the aircraft made it's descent to 1000 feet, it accelerated (there goes Zaphod58's hypothesis about self propulsion at level flight on final approach) and impacted World Trade Center tower #2 at approximately 510 knots groundspeed.
Those are fine informative post however, as I have said before, I don't know anything about building construction and demolition nor the physics of airplane accent and decent.
What I do know is the foundation of a plot that encompassed 4 years of planning and was highly detailed scheme that shrewdly took advantage of Americas freedoms and turned them against their own people. I will not be part of the current, and seemingly inexhaustible number of posters at ATP that rely solely on the google database of instant expertise. I am not including you in that because you put a great deal of effort in your work here. I simply can't act like I know things that I don't. I know the human side of the collapse and the toll it took and why thousands more died than need be.
Time is short this morning but I will get back to your post later.