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Comment by raverbashing | Hacker News Reader

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

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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

> 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.

Ah ok, that make sense.

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.

Yes (but it's not tabs)

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

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