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Comment by robertkoss | Hacker News Reader

Sounds like a perfect setup to replicate the MCAS disaster. https://en.wikipedia.org/wiki/Maneuvering_Characteristics_Au...

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the counterpoint is that this is a system that would only be engaged at cruising altitudes, and which would probably be manually turned on. the problem with mcas (aside from using a single sensor for flight control) was that it had large effects at low speed and problems near the ground don't give you time to react

You might not have time to react to a failure in this system, given how quickly it alters the wing geometry.

The issue with the MCAS-fiasco was that it (by design) was not communicated to the pilots, so when it failed, they didn't know immediately what to do. If they had known, they would have cut out the trim motor and left it off and trimmed manually until landing. In the accident flight, the crew repeatedly turned on and off the electric trim and never figured out why it started moving the trim every time they turned it on again (though, yeah, it's an open question why they did this more than one cycle....)

The second accident flight shows knowledge of the system wasn't enough. On that flight, they did turn off electric trim, but weren't able to manually trim because the force required to manually trim from the limit was too much. So they turned electric trim back on, and shortly after they stopped touching the electric trim inputs, MCAS resumed setting the wrong trim.

IMHO, the second accident had a better chance of survival, but it wasn't enough. IIRC, the flight before the first accident also had erroneous MCAS activation, and the flight crew did turn off electric trim and did it manual, but it wasn't treated as a must fix maintenance item, because MCAS was hidden.

There really should have been a separate shutoff for MCAS apart from the electric trim switches. Limited activation authority will hopefully be sufficient, but doesn't satisfy my airchair aerospace engineering demands.

counterpoint: the primary problem wasn't that mcas was hard to turn off well. it was that it was a critical system without any redundancy and therefore failed about 100x too often. if mcas had been based off of 3 sensors like it obviously needed to be it would have failed (and known that it failed so it could turn itself off instead of pitching down the plane) one a decade or so rather than a few times a year.

If you're advocating against the existence of feedback control systems in aerospace, you're going to have a bad time.

Is this now the default comment for any improvement in airline technology?

Of course. It makes people feel better about a system they know nothing about

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