Originally Posted by
MechEngr
STS tries to ensure that the trim load is zero. This is why the Lion Air crew reported of MCAS "STS is running backwards" because it was adding to the trim load and not making it go away. The fact that an unexpected 10, 20, 30, 40, 50 ,60 pounds of trim load was on the wheel is enough to tell there is a trim problem and using the wheel trim switch countered the trim load occurred to the first Lion Air crew and the captain of the second Lion Air crew, who apparently thought using it was obvious enough he didn't mention it to the First Officer.
How would the crew know there was an electromechanical failure? Do they rip the wiring apart looking for the short circuit before turning off the trim switches? How long is "continuous?" STS doesn't run at top speed for 30 solid seconds, which is more than enough to put 100 pounds on the wheel. Trim will stop at the upper or lower limits of travel, so by definition it cannot be "continuous." I had a recent electrical issue in my house - power would cut out and come back on - from a loose wire at the distribution transformer waving in the breeze and sometimes making a short circuit to ground. If a similar situation happened, intermittent, but interfering trim problem by wiring defect, say by chafing, or a loose bit of solder in a trim switch, would that also be a hands-up, cannot be solved situation?
I am trying to point out that the initial symptoms of the MCAS failure were, for all intents and purposes, so similar to normal operation of the trim system that it wouldn’t immediately trigger a SOP disconnect of power to the stabiliser. It ran for a bit, then stopped, then did some more, and was able to be countered by use of the manual trim switches. None of that screams “runaway” until you look at it post-hoc with system knowledge that was not disseminated to line crews at the time.
A (plausible) electromechanical failure would be when nothing you can do with the normal flight deck controls can stop the trim running in a particular direction, so swift intervention is necessary before it goes to the stops. On my current type (777) you get a warning as soon as the monitoring picks this up. If you disconnect the trim every time it moves automatically, you’d do it shortly after takeoff on every flight. There is no indication to the pilots as to whether it’s MCAS, STS or even the other pilot doing the trimming, apart from the speed, and that doesn’t really help much; an intermittent fault would, again, look like normal operation until it really showed its hand.
In a critical, high workload phase of flight, near the ground, experiencing something novel that doesn’t easily categorise and requires cognition and an accurate mental systems model (not present, through no fault of the pilots) to diagnose would confuse even experienced operators. That’s why we use rule-based behaviour for Time Critical Events, such as RTO, GPWS, Windshear and Trim Runaway, but these are triggered by
specific criteria which are learnt and practiced by rote because there is not time for pontification. Sadly, I think the accident crews never really got beyond the startle/react phase as there were too many audible, tactile and mental distractions to allow much in the way of a diagnostic loop to develop.