Originally Posted by
737 Driver
I assume that you are aware that there are other potential sources of uncommanded and undesired stabilizer trim movement besides MCAS? We do not have separate disconnects for them either. The runaway stab trim procedure is completely and absolutely agnostic as to the source of the uncommanded movement, nor should the flight crew waste precious time trying to diagnosis the source.
As to the mantra, you may have noted that one of the steps was “Trim the Aircraft.” At all times, the MCAS input could be stopped and reversed by the use of Main Electric Trim. The flying pilot was fully capable of countering every MCAS input until such time that the crew got around to executing the runaway stab trim procedure.
BTW, it is not procedural to fiddle with the cutout switches once they have been used.
I'm aware there are other sources, like the A/P, but turning off the A/P is already possible, so I didn't mention it. I already mentioned STS, and suggested that there should be a way to disable both STS and MCAS, since they are still functional even when the A/P is off. And you should be able to do that without affecting anything else.
Of course you could also have a stabilizer runaway involving the manual electric trim, that would require disabling it with the cutout switches to stop it. But from what I read such a runaway is less likely compared to an MCAS runaway. I didn't hear anyone here saying they experienced such a stabilizer runaway on the 737.
As to not wasting time troubleshooting, actually the existing trim runaway memory items involve troubleshooting. First they tell you to turn off the A/P and A/T. If that doesn't work, use the cutout switches. If that doesn't work, hold and grasp the trim wheels. If they didn't want you to troubleshoot, they would just tell you to do all 3 steps immediately, without waiting after each step to see if it fixes the issue.
As to fiddling with the cutout switches after being used, it might not be procedural, although the memory items do not mention that explicitly. They do explicitly mention not reenabling the A/P and A/T. If not fiddling with them is so important, it should have been explicitly mentioned.
And, in any case, procedures are written by humans, and can and have been modified a lot of times in the past to make them better.