I'm starting to think that disabling MCAS isn't even a sane option because then the pilots are exposed to unexpected aerodynamic characteristics due to the plane's weird design. Does this make sense?
Ethiopian Airlines 737 MAX Investigation Preliminary Report
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Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report
#32Indeed looks like they did disable MCAS using the stabilizer trim cutout switches, but the aircraft was trimmed nose down when they did so. Aerodynamic loading prevented manual trimming using the trim wheels. They eventually turned electric trim back on, as they didn't really have any other options, but didn't trim back enough, and left it turned on. At this point the right airspeed indicator (which should be more re…
Yeah, from the report: > At 05:40:35, the First-Officer called out “stab trim cut-out” two times. Captain agreed and First Officer confirmed stab trim cut-out. [...] > At 05:41:46, the Captain asked the First-Officer if the trim is functional. The First-Officer has replied that the trim was not working and asked if he could try it manually. The Captain told him to try. > At 05:41:54, the First-Officer replied that it…
Interesting, that when they turned the electric trim back on, they only momentarily blipped the switches to trim up, and the stabilizer did move. Then five seconds later MCAS pitched down again and it was over.
Wonder why didn't they trim up more when they turned the electric trim back on. On the surface it appears that was their last chance to salvage the situation.
Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report
#33> Also, the airspeed, altitude and flight director pitch bar values from the left side noted deviating from the corresponding right side values. The left side values were lower than the right side values until near the end of the recording
Hmm. Seems not just faulty AoA sensors? Perhaps some major malfunction in the flight computer? I've felt this entire time there's been a rush to claim 'faulty AoA sensors' because 'sensors' are known to fail, and it's an easy scapegoat.
My question is, does the flight recorder have raw inputs from the sensor, or just what the flight computer is reporting from the sensor? Seems like the latter.
Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report
#34This isn't only MCAS issue anymore... If there are issues with manually rectifying trim after it went haywire because of any reason, that might be a problem. Interesting to find out was that an issue with older 737 models, just without severe runaway trim events.
> If there are issues with manually rectifying trim after it went haywire because of any reason, that might be a problem. It's not so much a "problem" as it is "physics". If the stabilizers are forcing the nose down, and the pilot pulls back on the stick (engaging the elevators) to pull the nose up, the forces exerted on the stabilizer are such that, in extreme cases, human force is insufficient to move the manual wh…
From the way you describe it, if the stabilizer reaches the point where human force is insufficient to bring it back then they're toast since MCAS is tied into the electronic stabilizer.
How did this scenario get missed during testing? You'd think they would do extra testing around the new changes.
Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report
#35This isn't only MCAS issue anymore... If there are issues with manually rectifying trim after it went haywire because of any reason, that might be a problem. Interesting to find out was that an issue with older 737 models, just without severe runaway trim events.
Doubtful. Older models don't have the oversized, forward-mounted engines, and thus don't have as much of an AoA problem as the MAX does in typical flight conditions. That's the reason the MAX should have been a separate classification with specific hands-on training.
The MAX is stable in normal flight. It's only when you are pitched up does the engine placement make it want to pitch up even more (and it is only a problem at a speed that is low enough that the normal stabilizer control doesn't have enough control authority to push it back) hence MCAS forcing the nose down every so often via the trim. A normal flight does include pitching up but it does not include pitching up at such a low speed that a little more pitch would quickly cause a stall. MCAS (or at least the nose down function) is specifically designed for an edge case (in that it should be sitting there watching, not actually doing anything during a normal flight).
edited
Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report
#36> The crew performed runaway stabilizer checklist and put the stab trim cutout switch to cutout position and confirmed that the manual trim operation was not working. This is very damning to Boeing. It's precisely the procedure they prescribed and intended to mitigate possible MCAS caused trim runaway. In light of the fact that they put this in place as a justification for moving the engines forward without reclassif…
This means that Boeing is now 100% liable for the crash, as there was nothing the airline could've done in this scenario.
It also means that the aircraft will have to be re-designed and re-classified before it ever flies again (it's now a very clear design error, not a training or communication error)
Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report
#37Earlier quoted context omitted.
Per the article a combination of things put them in that situation. Not just MCAS. Why is everyone so wedded to the narrative that this is the fault of MCAS and nothing else? Complex systems that have millions of man hours spent making them safer don't usually have back to back failures because of one faulty subsystem. We've incrementally refined commercial air travel to the point where single source accidents are ba…
MCAS issues nose-down, speed increases and nose pitches down. Nose never pitches back up. Plane crashes. Sure, maybe some complex combination of maneuvers could have helped them out of this but there's really everything pointing the blame at this one system. Pilots did what the boeing manual suggested (even included at the end of this report).
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Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report
#38This shouldn't be marked as a dupe since it's the actual report. It sounds like they did everything right. There are two fateful moments: >At 05:41:46, the Captain asked the First-Officer if the trim is functional. The First-Officer has replied that the trim was not working and asked if he could try it manually. The Captain told him to try. At 05:41:54, the First-Officer replied that it is not working At this point t…
The report was the basis for this article and discussion yesterday: https://news.ycombinator.com/item?id=19570378 . From Hacker News' perspective, since the report itself was prominently linked in the article and its contents were discussed directly in the thread many times, it qualifies as a duplicate.
You'd just noted and moved comments to this submission from another, which is confusing.
(emailed to hn@ also)
Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report
#39Indeed looks like they did disable MCAS using the stabilizer trim cutout switches, but the aircraft was trimmed nose down when they did so. Aerodynamic loading prevented manual trimming using the trim wheels. They eventually turned electric trim back on, as they didn't really have any other options, but didn't trim back enough, and left it turned on. At this point the right airspeed indicator (which should be more re…
> Aerodynamic loading prevented manual trimming using the trim wheels. Are the trim wheels mechanically connected to the trim mechanism in the back? Is there no way to trim the horizontal stabilizer electrically without also enabling MCAS? What would happen if the pilots continually pressed the trim up switch on the yoke? That should've prevented MCAS from activating, right? I thought Boeing airplanes were built such…
Yes they are manually connected but not mechanically assisted in any large (maybe any) part and the nose down trim and full back on the yoke made the manual wheel impossible to use. Thumbing the switch will work to disengage the mcas temporarily yes.
I think the only way to survive this is to use the electrical trim assist to get to level flight and then shut off the system. After it’s level you can go ahead and trim using the manual wheel. Well the other way to survive this is to not have Boeing design the mcas to read from one sensor only and also have mcas operate in a narrow authority.
Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report
#40This isn't only MCAS issue anymore... If there are issues with manually rectifying trim after it went haywire because of any reason, that might be a problem. Interesting to find out was that an issue with older 737 models, just without severe runaway trim events.
> If there are issues with manually rectifying trim after it went haywire because of any reason, that might be a problem. It's not so much a "problem" as it is "physics". If the stabilizers are forcing the nose down, and the pilot pulls back on the stick (engaging the elevators) to pull the nose up, the forces exerted on the stabilizer are such that, in extreme cases, human force is insufficient to move the manual wh…
> This is extremely well documented by Boeing going back decades, and it presents solutions of either: 1) reengaging the electronic stabilizer trim and trimming electronically back to a point where the forces are manageable, then cutting electronic trim again, or 2) relaxing the stick, letting the nose go the way the stabilizers want it to, which slackens the force, and makes cranking the wheel manually doable. Great if you're stabilizers are trimmed severely nose down at a height of 40,000 feet; less great at a height of 1,000 feet.
They went for option 1 in this case, with the MCAS kicking in 5 seconds later.
> This all seems to add up to Boeing's narrative of "The MCAS is safe, pilots just need to know to hit the stab trim cutout" being grossly insufficient.
Yeah, and even that is understating it in my opinion.