Originally Posted by
FullWings
What struck me at the time was that there wasn’t an immediate trigger for action in the way the fault presented itself. The 737 trim is active all the time during flight; in fact it is unusual for the trim wheels to *not* be in motion for any length of time. STS, MCAS, config changes, CofG changes, etc.
The Boeing checklist trigger for trim runaway at the time was “continuous uncommanded trim motion”, which guards against an electromechanical runaway, but that wasn’t what happened - it was a software failure that only moved the trim under certain circumstances. How could you tell the difference between, say, STS doing its job and and an MCAS failure? The answer is, in the short term you couldn't, and abnormal operation appeared the same as normal operation unless you had a long diagnosis period, by which time it was too late.
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?