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Comment by stef25 | Hacker News Reader

> The problem here is: in the transition from the -NG to the -MAX, Boeing added this protection and didn’t tell anyone

Totally out of my depth here, but how is it possible that they "didn't tell anyone" about a feature so crucial, how could pilots not know how the plane's computer will behave in such a fundamental, non edge-case scenario?

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Boeing specifically marketed this as not requiring new training. So they did everything they could to hide it in order to satisfy marketing’s desire.

The FAA signed off. I guess the lawyers will argue if it was legal or not.

But it's right there in the Terms of Service box! The pilot probably just clicked right through it without reading.

Wow. Your statement is so true on so many levels and applicable to so many industries that it seems unfair that I can only upvote you once.

That's what I'm wondering about. How did Boeing get away without disclosing it. Or if they disclose it how did FAA agree that this is not something that pilots need to know and train for. If they signed off on Boeing not letting pilots know FAA should be on the hook for this too.

A stick Pusher is not a new feature, if you're right, and the FAA signed off that operators of the NG did not need any supplementary training and that caused the accident that will be very ugly for Boeing

MCAS is not a stick pusher -- and that's the crux of the problem. A stick pusher would essentially adjust the elevator to pitch the plane down. MCAS trims the stabilizer. At full stabilizer travel you may not have sufficient elevator authority to overcome the downward pitch. Put another way, you can counteract the stick pusher by pulling back (although this may take a strong pilot). If you don't catch MCAS doing something stupid and the plane ends up fully trimmed down you cannot counteract this by pulling back.

Here's some other examples of what happens when the stabilizer is trimmed without the pilot knowing. Sadly Capt. VanderBurgh died a couple years ago. Had he not it would have been fascinating to see his take on the Lion Air wreck.

https://www.youtube.com/watch?v=WfNBmZy1Yuc

And here's a quick overview of how the stabilizer functions in a 737:

https://www.youtube.com/watch?v=l62NvkRWa5E

The first video is fantastic (and terrifying). Thanks for sharing.

> At full stabilizer travel you may not have sufficient elevator authority to overcome the downward pitch.

Do the stabilizer trim tabs really have more authority than the elevator? That seems odd.

They aren’t “trim tabs” — it moves the entire stabilizer.

This article from 2013 outlines the differences quite nicely. It's a subtle point (that I hadn't appreciated before reading the article), but trim on a Cessna (on which many pilots learn) and modern jet work quite differently, though you rarely ever notice the difference in normal ops (because you never go massively out of trim). The author speculates that several accidents might be attributable to this.

Do you really understand how your trim works? Many do not, and why it matters. Alex Fisher - GAPAN

https://www.skybrary.aero/bookshelf/books/2627.pdf

I only really appreciated this while reading while reading about https://en.wikipedia.org/wiki/Alaska_Airlines_Flight_261

It's quite an interesting one if you are into that kind of thing.

Well an MD-80 does use trim tabs.

Not really. It trims via a moving stabiliser. You are confused because it has flying elevators and the pilot only directly controls the servo tabs which look a lot like trim tabs.

Ah yes, brain fart. Tabs for the elevator, jackscrew for the stabilizer.

Ah ok, that make sense.

Yes (but it's not tabs)

> Totally out of my depth here, but how is it possible that they "didn't tell anyone" about a feature so crucial, how could pilots not know how the plane's computer will behave in such a fundamental, non edge-case scenario?

I couldn't even hazard a guess. Boeing told the Brazilian authorities about it. See the table on Page 18 under MCAS.

http://www.anac.gov.br/assuntos/setor-regulado/profissionais...

For comparison here's the FAA equivalent:

http://fsims.faa.gov/wdocs/fsb/b-737_rev%2014.pdf

Note that MCAS is distinctly absent from the difference tables (also) on page 18.

The FAA signed off on the NG to MAX training as "B" level differences. This means no sim time necessary[1] (although apparently American Airlines doesn't have a MAX sim in the first place). The Brazilians were made aware of MCAS and thought that also qualified as a "B" level difference. The question then is: did the FAA think MCAS constituted a C or D level difference?

1: See the table on page 14 for a list of the FAA definition of different categories of differences

Boeing can self certify[0]. I'm not sure where this enhancement/feature would fall but they seem to have at least some latitude.

[0] https://www.seattletimes.com/business/boeing-aerospace/faa-e...

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