In the early 1950's, the British De Havilland twin boom Vampire jet fighter was used in the Royal Australian Air Force. The original Rolls Royce Goblin engine was replaced by the more powerful RR Nene engine. This required the installation of two additional air intakes to the wing root intakes. The additional intakes looked like the carburettor intakes seen on piston engines fitted to typical aircraft such as the DC3.
These additional intakes were installed on top of the fuselage directly behind the cockpit. At the time the Vampires were not fitted with ejection seats.
Shortly after these modified intakes were installed three fatal loss of control accidents occurred during high speed dives. The unfortunate pilots were able to broadcast over their radios what was happening to their aircraft.
These were traced to shock waves forming over the top of the curved shape of the intakes at around Mach 0.76. In turn, this resulted in almost complete loss of elevator effectiveness. The resulting Mach Tuck proved deadly as the aircraft would be still vertical at 10,000 ft with no hope of pulling out before ground contact and no hope of baling out without an ejection seat.
In those days, the dual version of the Vampire was not ready for service and thus we flew the single seat Vampires with the only instruction being a ground briefing and someone leaning over into the cockpit to show you how to start the engine. You got a wave of the hand and told go and fly the thing. Most of us had only 240 hours at the time, and coming straight off Mustangs to the single seat Vampire.
The second trip on the Vampire was done at high altitude in formation with a fighter combat instructor who was experienced on the Vampire and who, in theory, could recognise the onset of compressibility before an inexperienced pilot could. Remember, there was no ejection seat.
The FCI would start the demonstration by flying as No 2 to the "student" Vampire pilot and directing him to enter a full power dive to build up speed. When the FCI could feel his own aircraft starting to tuck under, he would call over the radio for No 1 to close the throttle and extend the dive brakes. The aircraft would slow up and the exercise was complete.
At the time, after I did the exercise with my FCI, I personally wondered why all the fuss about this theory of Mach Tuck. Because I never felt anything untoward during the dive and recovery. Some FCI's were understandably twitchy about delaying recovery too late. To avoid the onset of serious compressibility that may not have been instantly recognised by the student in the other Vampire, these FCI would call off the exercise prematurely while in the initial part of the dive and before onset of compressibility started.
Later, the problem was resolved by removing the air intakes (known as Elephant Ears) from the top of the fuselage and installing them under the fuselage. Now in a high speed dive, the aircraft would pitch up without pilot input approaching Mach 0.77 and Mach Tuck was no more a problem. Ejection seats were installed and that eased anxiety about having to manually bale out if a problem occurred.