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Comment by mannykannot | Hacker News Reader

Thanks for the documents! I searched through item 1 for 'trim', and while I found some trim failure reports, and the complaint about Boeing's nondisclosure of MCAS in its documentation, I did not notice any concerns over disabling a runaway trim situation.

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Not a pilot here (developer) but doesn't "nose down" imply trim? If so, one of those pilot reports in the Dallas News story does say that they experienced uncommanded nose down. Both pilots were aware of MCAS and discussed it before the flight. Another one who also knew of MCAS reported uncommanded leveling-off at the wrong altitude. Is that not trim, too?

"Nose down" does not imply a trim adjustment, as the primary control in pitch is performed by the elevator, while trim adjustment, on this airplane, is done by rotating the horizontal stabilizer. From the fact that the pitch changed, we cannot tell whether it was commanded by the autopilot or MCAS, but as MCAS is supposed to be disabled when the autopilot is engaged, and this occurred when the autopilot was engaged, it appears less likely that it was MCAS than it was the autopilot.

Furthermore, the signature feature of the MCAS failure that downed the Lion Air flight was runaway trim, and there is no indication in this report that the crew had to take any action to stop it by disabling MCAS (setting STAB TRIM CUTOUT to CUTOUT, the documented procedure for doing so), or to restore the correct trim manually (as far as I know, it would not revert to the correct setting simply because MCAS was disabled.)

Not sure what that means though.

As far as I am aware (totally a layperson), there have been potentially only 3 potential 737-MAX MCAS trim issue occurrences:

1. The Lion Air flight prior to the fatal lion air flight

2. The Lion air flight which crashed

3. The Ethiopian flight which crashed

Of course I don't think any of those 3 are absolutely confirmed, but I do think it seems likely the three had MCAS trim related issues. If the success rate for pilots dealing with this issue just 1 in 3, does that say anything regarding the MCAS issue difficulty/complexity or the skill/training of pilots?

From the article's title, one might get the impression that pilots have been saying that MCAS is dangerous. What I was trying to do was distinguish between possible MCAS incidents and safety concerns, complaints about Boeing's nondisclosure of MCAS, and pitch-down incidents that had other causes (incidents of the last case seem to be related to the autopilot, rather than MCAS.)

The latest crash certainly looks suspicious, but we don't really know yet, and the Lion Air crash occurred before the details about how MCAS could behave on sensor failure had been revealed. One would certainly hope that after this was revealed, all Max-8 pilots would be well prepared to recognize the situation and handle it, which might suggest that there is something more (or different) about this case (which could be worse news for Boeing).

> One would certainly hope that after this was revealed, all Max-8 pilots would be well prepared to recognize the situation and handle it

Far from certain, becaus Boeing is selling and the companies indeed treat it as "the same plane like old" even if it obviously isn't:

https://news.ycombinator.com/item?id=19374386

There certainly seems to be a regrettable attempt to diminish the differences between this model and its predecessors in that case. With regard to the MCAS specifically, once the cause of the Lion Air crash was determined, the FAA issued a new Airworthiness Directive [1], and I would be surprised if this information was not sent to all operators globally.

For obvious reasons, pilots tend to take a keen interest in matters of aviation safety, so I wonder if this was not simply a repeat of the Lion Air crash (which, as I said elsewhere, might be even worse news for Boeing.)

[1] https://rgl.faa.gov/Regulatory_and_Guidance_Library/rgad.nsf...

From the database, #1597286, "agressive" uncommanded nose down.

Day 3 of 3 departing in a MAX 8 after a long overnight. I was well rested and had discussed the recent MAX 8 MCAS guidance with the Captain. On departure, we had strong crosswinds (gusts > 30 knots) directly off the right wing, however, no LLWS or Micro-burst activity was reported at the field. After verifying LNAV, selecting gear and flaps up, I set "UP" speed. The aircraft accelerated normally and the Captain engaged the "A" autopilot after reaching set speed. Within two to three seconds the aircraft pitched nose down bringing the VSI to approximately 1,200 to 1,500 FPM. I called "descending" just prior to the GPWS sounding "don't sink, don't sink." The Captain immediately disconnected the autopilot and pitched into a climb. The remainder of the flight was uneventful. We discussed the departure at length and I reviewed in my mind our automation setup and flight profile but can't think of any reason the aircraft would pitch nose down so aggressively.

This sounds very similar to the MCAS problems on the other flights, and these pilots were aware of MCAS (this is post Lion Air).

MCAS is not active with autopilot engaged, according to Boeing.

"should not", not "is not". That's why boeing promise a software fix ...

Is your statement true, and that is what they are fixing with the software update, or is that just your conjecture?

I have not read in detail yet about the proposed update to MCAS they are working on

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