Originally Posted by
PEI_3721
wilbert33 # 58,
Your summary represents the emerging understanding.
The important issue is why these aspects could have been overlooked, unknown, or unaccounted for.
The tail force-to-speed relationship might breakdown at larger angles; the tail could be adversely affected by wing downwash. The measured stick forces during test were probably limited by high forces, thus the extremes were extrapolated (non linear), supported by aerodynamic / structural theory (prediction). This may not have included the elevator, but presumably some aspect was considered in identifying the need for the yo-yo manoeuvre - when; 737-200 vintage extrapolated to new variants.
Longer fuselage, higher thrust engines, increased tail area ?
Similarly the additive effect of nose up recovery elevator which would be an opposing force (couple) on the tail mechanism could have been misjudged - not a normal evaluation of cross-control; possibly no data or extrapolation.
There has also been discussions that with large deviations of the combined tail angle and elevator, the elevator control suffers jack stall, thus is less effective.
The elevator feel adjustment is a well established device used with stick shake (AoA), it would be an unwanted additive to tail trim offset, but not a problem in isolation - AoA fail, no MCAS trim input.
Do we need superman ? No, we require humans in design, evaluation, test, and certification, who can foresee relevant eventualities whilst constrained by management and working practices, and subject to normal human variability due to the working environment. (Reasonable judgement)
The industry requires a robust processes of design - certification, something which will tolerate the variabilities above, so that pilots, with similar human limitations, can manage what is expected of them; manage the residual uncertainty.
yanrair, # 60,
‘So - the trick was and still is, don’t let it get out of trim.’
Aviation safety is not founded on “tricks”, particularly those which might not be described or repeated.
The are significant differences between what you ‘know’ (know what), and ‘how’ to use what you know; knowhow.
Apologies for poor use of English Wilbert- for "trick " read "correct procedure/technique". Since STAB loads increase rapidly with airspeed changes it is not recommended to change speed. Pariculary with Jammed Stabiliser where it won't budge. Stay at same speed and plane stays in trim. Until of course you have to slow down to land but that is a different trick - sorry, procedure!
Cheers and thanks for your post.
Y