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Comment by EngineerDude | Hacker News Reader

As a few posters mentioned, an interesting question is why the plane needs MCAS in the first place. I read somewhere that the max 8 got new engines which were bigger than engines on previous 737 series, and that makes the plane nose up. Any kid who ever made a model plane understands the idea of CG - if it pitches up you change something, maybe move the wings back. Is it possible that an industry leader shipped a plane that’s not even freakin balanced, tried to fix it in software(!) to avoid an expensive redesign, screwed up the software and maybe the safety analysis, and then didn’t even manage to mention to the pilots that this software hack even existed? This is hard for me to fathom. If the Ethiopian report is right, it might even be that the procedure to turn off MCAS didn’t even work. I hope I am misunderstanding something here.

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The new, more economical engines are physically bigger, so in order to maintain the required ground clearance the engines were moved forward and up. This gave the plane a tendency to nose-up in certain extreme circumstances. MCAS was supposed to make the plane behave more like previous 737 models, in order to fulfill the "common type" requirement that would allow 737 pilots to fly it without getting qualified for a new aircraft type.

The plane _is_ balanced; it's not like Boeing built an aircraft that falls out of the sky if you slightly nudge the yoke wrong. (Which wouldn't even be unusual; plenty of modern aircraft are completely uncontrollable without computer assistance.) Yes, the problem could've been solved with a re-design of the aircraft- lengthening the landing gear struts, modifying the wings, etc. - but that would negate the point of using an existing and well-tested design.

There's A LOT of guilt to go around in this story, but no one can actually be casting doubt on the abilities of Boeing engineers in the fundamentals of aircraft design.

You're partway there. The design isn't inherently unstable under normal conditions, it just becomes unstable under slightly different extreme conditions than the previous 737. Rather than training pilots on this difference, Boeing tried to emulate the old behaviour in software.

OK, so maybe it’s unfair to say that MCAS (as a concept) is a hack, since it could be seen as a good-faith effort to provide a predictable aircraft?

Not quite, without MCAS the flight behavior is quite predictable, it just doesn't match the existing type certification and it falls outside of the allowed parameters for this class of planes.

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