737 Driver
. . . On the NG, the left switch disables the main electric (pilot actuated) trim and the right switch disable all automatic inputs (Speed Trim/Mach Trim/Autopilot Trim). We used to try to identify the offending system and actuate these switches separately in the runaway stab trim procedure depending on the situation. Somewhere along the way, Boeing changed their philosophy on the procedure (probably part of a larger trend of moving away from “troubleshooting” type actions) and now we always use both switches when necessary. I have previously speculated that two switches were retained in the MAX for both redundancy against possible relay welding and to simply harmonize the procedural aspects of runaway trim between the NG and the MAX. That is, both aircraft have two switches (for different reasons) and you always use both switches at the same time.
This is a very revealing quote. I can see how the procedures evolve, or de-evolve. But I can also see how it could lead even attentive crews down a very dark garden path.
#845 on the parallel thread
we have promise of a further probing into the AV H's list and circuits. I haven't woken up yet.
737 Driver's comments about the need to know levels I concede completely though as I say, if I were young . . .
FrequentSLF #843 on parallel thread. Quote:
I am not a pilot, but I have eletrical and automation background.
Based on the wirings that are available on the net the systems are as stated.
MCAS is not stopped by column switches!
MCAS cannot be disabled without cutting off manual electrical trim, which means only wheel cranking can be used if CUT OFF switches are used.
CUT OFF switches are connected in series, and renamed PRI and B/U, either one will CUTOFF all electrical controls (manual thumb on control column, autopilot, STS, MCAS), while on NG one switch will cut off automatic trim, while the other whole cut off the electrics.
Well, I found it hard to reconcile the MAX system as described in the AV H's question list - with the outline wiring we'd all been lead to believe was simply a series BackUp configuration. It was too late last night to trace circuits, but 737 Driver did protest it doesn't matter about the details, as long as the crew take the right actions. I have never thought like that and if I'd been on type, (and 40 years younger) I would have known exactly what that circuit did. At least, I hope I would, given I'd got the first clue that something like MCAS was lurking in the background. But then, I'd spent sproghood on the electronics workbench.
Now to another issue in the Herald's list. They go back to the black box taking in good AoA data and corrupting it -
in both if not all, cases. Certainly, the chance failure of three vanes stretches the old credulity, though the balance weight hypothesis in the ET flight is very compelling. But along a pure logic line, I would have looked very carefully at the prior reports - in addition to the three major vane issues. The more I read the more I'm not satisfied the Herald's suggestion is not correct. It leaves me with a deep concern about what I've described as a ghost in the machine.
Apart from the digital errors fitting nicely with the three vane positions - they take the very rapid change of angle as being more electronic than a pendulous swing. But as I say, the latter is good fault modelling. It's just that slight angle change from a long steady error state, to a slightly different long steady state. That's odd.