Auto Mach Trims have their limits.
I can only talk to the Vulcan and Victor Mk 1s although a few flights in a Vulcan Mk2 as an IRE were a bonus.
The extract from memoirs has been posted some time ago on another thread but may be of interest in the current context.
The IMN reached was a transitory reading before all attention was concentrated on recovering from a serious mach tuck
Towards the end of 1957, Flt Lt Rus Law was selected to replace me at Boscombe Down. He was then completing the ETPS course at Farnborough. I commenced handing over my responsibilities for all Vulcan flight tests to others in the squadron. Flt Lt Ray Bray was to pick up on weapon carriage and release trials. So, one day we were out over Lyme Bay in Vulcan 892 with a full load of practice bombs. One point for measurement was to be at the corner of the aircraft flight envelope with the weapon bay doors open. This was 415 Kts IAS, 0.98 IMN and 3.5 g. The cross over for IMN and IAS was at about 27,000 ft.
Ray was flying the aircraft from the right seat. We started a spiralling descent at high mach number from above 35,000 ft, aiming to reach all of the test conditions together at 27,000 ft.
The Vulcan had an increasing nose down pitching moment due to shock wave effects as speed increased above 0.88 IMN. To artificially correct this instability, Avro had inserted an auto mach trimmer in the elevator controls. The trimmer responded to mach number by extending increasing up elevator without any change to stick position or feel until it ran out of authority at 0.98 IMN. There was then little remaining elevator movement available, especially for manoeuvre.
We were thus spiralling down with almost full up-elevator. I had discussed these limits during our pre-flight briefing so we were a bit wary.
As it happened, Ray pulled a little too tightly into the spiral just short of our target conditions and speed dropped off. He relaxed stick back pressure to recover speed which increased too quickly and to overshoot just as we had reached the limit of the auto mach trimmer. The result was an abrupt increase in nose-down pitch and an attempt to control this by further back-stick. But there was no more left, the elevators were at full authority.
I grabbed the stick with both hands, stopped the roll and tried to milk some more elevator. With two of us pulling hard on the stick, we found it to be very firmly against the stops. But, the nose was pitching down at an increasing rate as mach number went up past 1.0. I let go of the stick with my right hand and pulled all engines back to idle, before pondering the effects of the speed brakes if I were to extend them now at a speed well above their maximum operating speed. There was a real risk of them failing structurally. By now we were going through the vertical with the Mach No reaching 1.04+ . I was considering pushing under and slowly rolling upright.
I took the risk and slammed down the speed brake control and felt them bight. IAS and IMN started to come back and, slowly at first, the elevators started to pitch the nose up. We came back through the vertical at about 18,000 ft and soon gained normal control. Continuing buffet reminded us that the weapon bay doors were still open. To my relief they closed as we continued to pull out of our dive, regaining level flight at 8,000 ft. Any ships below would have received a very substantial sonic bang.
There was a residual abnormal noise and my concern about the speed brakes returned. I called base and asked for assistance from any airborne aircraft in the vicinity. A Canberra was vectored towards us as I headed back towards Boscombe Down. I found that, as I slowed the aircraft, the noise became worse. At 150 Kts the noise in the cockpit was like blowing across the top of an empty bottle. Soon we had a Canberra pilot looking us over. He spotted a small access hatch open under the nose and no other problem of external significance. The hatch turned out to be the access to the oxygen filling connections.
Ground inspection revealed that the rear bulkhead of the weapons bay had been deformed. If that had let go we would have lost our tail.
This turned out to be the first time that a V bomber had gone supersonic. It was not to be a normal event. In the next year one of B Squadron's Victors ran-away nose down after losing a pitot tube and broke up over the Bristol channel.
I continued to learn the elements of survival as a test pilot. A wide general knowledge of all aircraft systems was basic, together with a solid appreciation of the strengths and weaknesses of aircraft structures. Necessary also was an ability to continuously assess all factors and limitations applying to critical areas of flight and performance, always matching all aspects to one's own personal and crew abilities.