[tzs]

You obviously know way more than I do about this, so maybe I completely misunderstood but what I saw on one of those YouTube videos by a 737 pilot was something like that before MCAS when you got a runaway trim situation and followed the procedure and got trim back to normal and had the trim system turned off, most of the time you could turn the trim system back on and the problem would not come back. Runaway trim was usually due to some transient problem that would be gone after the system was turned off and on.

In those cases where the problem does come back it is not any worse so you can just try again. You might go a whole flight doing this if you are getting enough time between the problem recurring for restarting the trim stab system to be less annoying than not having it.

With MCAS, if I understood correctly, it would add a bias to the trim that was not reset when you restarted the trim system. Each cycle of turning it off and back on that was harmless pre-MCAS would increase that bias until you reached a point where the trim down bias was more than you could counter.

This wouldn't matter for the flights that crashed, because they didn't get to the turn it off stage, but it seems likely it would have eventually happened to some crew that did follow the procedure including the emergency directive.

The impression I got from this, and from watching many episodes of "Air Emergency", is that there is the official way to operate a plane as documented by the manufacturer, and then the unofficial way that pilots actually use that comes from combining the official way with the pilot's mental models of how the systems work to cover what can actually be done. So if the official way says to turn something off, but does not say "and then keep it off until a mechanic checks it out", and the pilot's mental model says that it is safe to turn back on, then they might turn it back on.

MCAS invalidated the mental model pilots had for the stab trim system, but the MAX-specific training they got and the emergency directive after the first crash did not do anything to tell pilots that their model was invalid.

When documenting a system and training people to use it, you really need to take into account the mental model they will have of the system. It is their mental model that people actually use to guide their interaction with the system.

[secabeen]

The second flight totally did get to the turn-it-off stage. They turned it off multiple times. Unfortunately, during that process, they neglected to manage their throttle, leaving it at near full takeoff thrust the whole time, and reached a very high speed. Thus, while they could turn off MACS, they were unable to manually restore trim with MACS off due to the strong physical forces on the stabilizer. There was probably a recovery path for them where they lowered speed, then corrected the trim manually, but they didn't see it.

Some day, there will be an AdmiralCloudberg write up on it, but he hasn't done it yet: https://www.reddit.com/r/AdmiralCloudberg/

[wolverine876]

In the Internet era, I can directly access reports from leading experts. Why would I be interested in a reddit user?

[secabeen]

Because he does a pretty good job of producing nice summaries. Why not both?

[wolverine876]

I have no idea if this person knows what they are talking about, either in total or for any particular bit of information. Also, why create a filter between me and the original report - a great benefit of the Internet is disintermediation.

Personally, I find original reports from experts to be much more clear than the attempts of others to digest them for me. To write clearly, you need to know what you are talking about. The better you know, the more clearly you write.