Thank you very much for that!
I understand it was a case of compressibility effects such that rendered control surfaces ineffective?
I wonder if in that situation (a steeper and steeper dive with nose heavy air
plane) you would be able to roll the wings and achieve inverted flight so that the nose heavy tendency would then tend to reduce the pitch. Not much time to do that from 15,000 at Mach .80, anyway...
Many phenomena have been discovered after a few good pilots died, specially in the old times. Maybe there are still a few traps set by nature awaiting us. Not many, I deem. At least not as many as the traps set by computers and software, of which we still have many to find... By fallin in them.