41–50 of 347 posts
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MCAS forced the trim to a point where (because of aerodynamics) it was impossible to manually change the trim even after disabling MCAS, and was designed in such a way that it was impossible to engage the electrical trim controls (which could have saved the plane in this case) without also reenabling MCAS.
Flaps out supposedly would have kept MCAS off but the pilots weren't expected to know that.
The design of this system, from what I've heard so far sounds insane . For some background, systems intended to automatically override pilot input upon detecting an impending stall have been used on large, complex aircraft for a long time. The most common mechanism is a stick pusher, which mechanically pushes forward on the stick to mimic what a pilot should do about an impending stall. The force of a typical stick p…
> 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…
And, the crew left the power setting at climb throughout almost the entire sequence, so they oversped the airframe.
And, when they (against procedure) (apparently) turned the electric trim system back on, they did not use the yoke manual trim switch to fix the nose down trim problem -- which is a big mystery.
The MCAS design inhibits its FCC trim down output when manual trim is utilized. So, if they had just continuously pushed the nose up manual trim switch on the yoke until they got the trim to neutral, then pulled the cutout (either the first time or second time), the accident could have been avoided.
History shows this is not a good human factors design on the part of Boeing, but the crew does not look good, either, IMO.
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> 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…
I thought all jets had to be capable of performing non powered glides (as in no electrical power, not just engine power). I guess that's no longer a requirement.
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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 s…
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?
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> 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…
Yeah, they basically had to go with option 1. They were only 1,000 feet above the ground, the maneuvering room to let the nose dip, crank, dip, crank, etc until properly trimmed just wasn't there.
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It's true there were aggravating factors, like the inability to manually rectify trim, which (would seem to me) would be a aerodynamic or design issue. The point still stands, the root cause was MCAS, without it this would never have happened. MCAS would not have pitched them into a dive, and even if they were pitched into a dive by some non MCAS system, they would have been able to turn on electric trim and recover.…
Likewise had Boeing designed a control surfaces system such that the pilot always has more control authority than the trim this also would not have happened. I didn't say that MCAS wasn't the lions share root cause here. It's just not as simple as the "hurr durr, MCAS bad" that a lot of people keep trying to portray the more nuanced statements of various regulatory agencies as being equivalent to. MCAS might be bad b…
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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)