Originally Posted by
Fly Aiprt
One must remember that the 737 was a "certified" aircraft, and so was supposed to be trimmable at any speed within the flight envelope.
unfortunately, that is not quite correct. There is an assumption that the crews can cope with an out of trim case at all times, e.g., there is always sufficient authority of the elevator to cope with an out of trim stabiliser, but reality is that is not what the rules actually require. The protection of the certification standard gives a partial coverage for out of trim conditions, of a 3 second error only in the trim at all speeds within the normal envelope effectively. That is sorely tested by the MCAS system, and even without it, there could easily be conditions where you can alter the thrust enough that an additional 3 second of error in trim can place you outside of normal protections under the certification standard. [think of the A310's, the A320 @ Perpignan etc, and other wildly out of trim from thrust/trim mismatches].
The B737 is not alone in being able to be put into a severe out of trim case, but the fact that a condition can arise where the trim cannot function at all as a result of the error is a serious lack of certification shortfall in the protections provided by certification. The only part of the regs that the MCAS appears to be actually non compliant with is related to stability augmentation systems. The trim case it may well have been compliant, the rules themselves were deficient in this case, and appear to be in any case where a condition may arise where the stabiliser can only be returned to function by undergoing special procedures that were not generally taught or discussed, could require a breach of the single crew on the controls policy, and would exceed the control input force requirement of certification. That latter item is a possible breach.
The OEM and the industry have had an expensive lesson on the real world behaviour of the worlds favourite jet transport, and as always the industry will learn from the wreckage and reinforce rules as a result. This particular deficiency is going to be more than a minor matter, personally, I would think that there is some serious issues raised in how to ensure trim is always able to be controlled. watch out for some T tabs on elevator TE in the near future etc... Perhaps nothing gets fixed.