Originally Posted by
john_tullamarine
I would be interested in your opinion on the automatic use of stabiliser trim to correct pitch feel
In respect of the Boeing, I can only speculate.
For lighties, the normal way to influence stability deficiencies is by means of elevator circuit downsprings or bob weights depending on what problems are being tweaked. For a typical SAS requirement, the use of a variable downspring for the high alpha, low speed, high thrust environment is usual. With a bit of clever engineering, this latter widget modifies the speed-force characteristic away from whatever the engine normal forces might be doing in the way of nasty things.
My guess, and I emphasise that is all it is, is that Boeing cast about for, and used, something the aircraft had (a controllable stab) to mimic the more conventional techniques. One presumes, when it worked OK (ie no failure consideration), the TPs were happy to sign off ? From my reading of the tea leaves, it was just such a tragedy that the FMECA deficiencies had such a profoundly undesired outcome with the two mishap aircraft.
Perhaps part of the problem is the absence
There's naught unusual about stick force stability problems and the relevant design engineering folks have a variety of tools at their disposal to influence whatever undesirable stuff might come out of the FT program. Very few aircraft have no stability problems to sort out during the work up program. Boeing's approach might have been a tad unusual but the gameplan, presumably, was to get over the certification bar .. somewhere along the way, though, the design review processes fell down somewhat ...
Thank you for the answer. Much appreciated.
The engineers will not have relied solely on failure mode analysis software I imagine, but will have gone through the mechanics and electronics / electrics design with a fine tooth comb too?