Originally Posted by
yoko1
However, with the existence of four distinct speeds and the fact that the "fast" automatic was now faster than the "slow" Main Electric, there was some concern that pilots might misidentify the malfunctioning system, use the wrong cutout, and aggravate the problem.
I agree, they could misidentify the malfunctioning system, but if the F/O kept his hand on the cutout switch after reenabling the wrong system, they could immediately disable trim again at the first sign of a runaway. On the other hand, MCAS running intermittently, and apparently randomly for somebody not knowing how it works, and being reset by the thumb switches, adds even more confusion to the scenario. The pilots might assume the runaway is caused by the thumb switches, not by the automatic systems, since the runaway starts 5 seconds after using the thumb switches. Horrible system. I know it wasn't Boeing's intention, but it's like it was designed to create as much confusion as possible.
Now, about those blips towards the end of the Ethiopian flight, now that we know that using either of the cutout switches disables recording the thumb switch trim commands on the FDR, that could be one possible explanation for the blips. It could be that, while trimming with the thumb switches at the end, they were also playing with the cutout switches and intermittently disabling trim as a result. For example they could have been experimenting with the backup cutout switch, thinking it might be analogous to the autopilot cutout switch on the NG.