It was severe uncontrollable mach tuck that caused three Australian built Vampire Mk 30's to go in like darts near the RAAF fighter base at Williamtown near Sydney in the very early Fifties. With no ejection seats yet fitted, the three pilots were unable to abandon their aircraft. The problem was shock waves forming over newly installed air intakes called "Elephant Ears situated just to the rear of the canopy on top of the fuselage. Around Mach 0.75, the shockwaves drastically affected elevator effectiveness. I understand the coming September issue of the UK magazine "Todays Pilot" has an article about these aircraft.