One question if anyone could answer, is there a flight regime where the electric trim motors/their gear train would not have had the power to further manipulate the stab? Seeing how great the forces were, and even though the electric trim is electrically driven and aided I imagine by a reduction gear set, it would not seem to me to be multiple magnitudes more powerful than the handle+wheel-cable/pulley driven manual system (which itself is a lever and force reduction aided system (handle+wheel plus cable movement per turn vs pulley size on the jack screw drive side) with certain mechanical advantage).Could it be postulated that at a certain speed and AOA, the electric trim may have stalled out itself, unable to bring the stab nose up even if commanded, seeing that the manual system was practically seized?
"Handling the Big Jets", D. P. Davies.
A stalled stabilise drive can occur on some types where it is possible. with a very high elevator hinge moment. to apply a load on the tailplane so high that the drive mechanism is completely defeated and fails to produce any movement. It is unlikely that this will ever occur with the aeroplane in trim since a pilot is most unlikely ever to require a manoeuvre involving such large elevator angles and high stick forces. In an upset of some kind, however, where the stabiliser has achieved a gross out-of-trim condition, this position can arise. In turbulence, for example, a pilot might have run the stabiliser rather a long way away from the trimmed condition, a large and rapid change in speed could produce a very high stick force or the autopilot height lock in a long draught could have run the stabiliser a long way. All these could result in a grossly out-of-trim stabiliser setting with the immediate need of a very high stick force to keep control of the flight path.
While it is obviously disturbing to find that the trim will not run when signalled to relieve a high stick force, very recent tests by the author have shown that recovery from this condition is comparatively simple. Just sitting there and pulling a very high load, while it is the instinctive reaction in order to produce the required flight path, only compounds the difficulty. The stabiliser will not run until the hinge moment is relieved. So, keeping the trim button engaged, slowly ease ofl‘ the stick force. The aeroplane will not react very strongly because you are not doing much trade with all that force anyway. As the force falls through the critical value (actually, about 120 lb. pull on the type tested — although you won't know this, of course) the stabiliser will run and the aeroplane will come back under control.
Some aeroplanes with powered stabilisers and manual elevators have been cleared against a full aircraft nose down runaway stabiliser condition from a simulated jet upset manoeuvre, but only after the maximum nose down stabiliser range has been restricted. Tests in a particular ease showed that. provided the proper drill was followed, the aeroplane could be recovered although not, of course, within its normal speed limitations. The dive having been entered and speed brakes pulled, both pilots had to hold maximum up-elevator forces. The aeroplane stabilised at 0.93 true Mach number in the dive and held this condition for a comparatively long period of time. As the EAS increased at constant Mach number with decreasing altitude the stability of the aeroplane returned very slowly, the elevator began to take effect and very slowly the Mach number started to fall. Once this occurred it could be seen that the recovery would be made. As the Mach number decreased the full elevator effect returned. pitch attitude decreased and the speed began to fall. The recovery rapidly improved in quality thereafter, although very high stick forces had to be maintained until level flight was established at a much lower altitude.
Power was not reduced from the cruise setting during the whole of this manoeuvre because containment of the initial dive angle was so marginal that nothing, but nothing, was allowed to add any nose down pitching moment —which is just what reducing power would have done. It should be pointed out that this manoeuvre was flown on a type where it had been proved that the stabiliser drive could not be stalled in the presence of very high stick forces.
How this rather hairy manoeuvre will be regarded by an airline pilot is not known, but it must be of some comfort to know that there is a drill which will provide a recovery, although at the expense of at large height loss. Remember that this is for a stabiliser stuck at full nose down. a most unlikely event. For the more likely [but still improbable, of course] case of a stalled drive just remember the drill described earlier: keep the trim switch selected and ease off the high load on the stick. When the stabiliser starts running again ease off the stick force progressively and stop trimming when the stick force is down to a low value.
Again. in order to be fair to the designers it must be pointed out that originally there were no requirements relating to stabiliser drive stalling, although, in some cases, an arbitrary figure of about 100 lb. stick force was used. In the light of more recent experience, requirements in this area are being applied. This is always difficult to handle retrospectively: the easiest way out has been simply to limit the maximum amount of aircraft nose down stabiliser range under power.
Since the certification dates back to when Jesus was playing full back for Jerusalem it's anyone's guess.