This is a bit of an urban myth that keeps flying around. The plane is perfectly stable without MCAS, but would have had different handling characteristics than the previous version. The purpose of MCAS was to modify the handling to match the previous plane, so pilots' type ratings would carry over with minimal retraining. At its heart, it was a (poorly implemented) kludge in an attempt to sidestep a lot of paperwork.

You're mostly right, but it wasn't just to make it handle like the previous versions, it was to satisfy FAA certification rules on how it handled.

This is my understanding too, though Boeing hasn't really come out and said it definitively. Specifically:

> MCAS was devised to fulfill an airworthiness certification condition in 14 CFR 25.173 and 14 CFR 25.175. In high angle-of-attack (AoA) flight configuration, it is required that stick force/g (the stick force necessary to produce (hold) an incremental normal acceleration of 1g) and stick movement/g (ditto mutatis mutandis) must increase (or at least not decrease) with an increase in AoA (thanks to Clive Leyman for this formulation). I understand that in flight test, in which “wind-up turns” were conducted (a turn with increasing angle of bank; an increasing angle of bank means ceteris paribus increasing AoA), this condition was not fulfilled.

-- https://abnormaldistribution.org/index.php/2019/04/30/ieee-s...

@throwaway201103: “You're mostly right, but it wasn't just to make it handle like the previous versions, it was to satisfy FAA certification rules on how it handled.”

And also to stop the plane dropping out of the sky!

Boeing PR must be out in force.

@HPsquared “.. The plane is perfectly stable without MCAS ..”

NO, no, no it isn't .. doesn't matter how many different ways you re-arrange the word salad. MCAS is a software kludge to compensate for an unstable airframe

I'm not so sure I'd call it a 'kludge'.

The implementation is certainly not "elegant". It's like this random batch job that triggers when some conditions are satisfied and does its own thing without regard for anything else, sure. Not integrated with the auto-pilot, does not display anything in the panel.

We have plenty of aircraft (even though most are military) which are aerodynamically unstable, and require fly-by-wire software to even be flyable. We don't think those are kludges. That's not too far from what Boeing has to do.

It could have been either a fancier fly-by-wire software in the Airbus philosophy that wouldn't let pilots exceed parameters(retrofitting it might have been a challenge, and possibly even more dangerous). Or it could be this non-elegant solution that behaves like an old-school stick pusher. Both could have worked.

The problem is that they did this while trusting a single sensor. And not informed anyone.

The 737 MAX is aerodynamically stable. MCAS is primarily concerned with making the forces one needs to apply on the stick at high angles of attack comparable to earlier generations of 737.

An interesting question is going to be as to what happens elsewhere in the world; I'd be intrigued to see if Boeing ends up certifying the 737 MAX with MCAS as an optional feature with additional difference training required, as some things mentioned suggest that it's not implausible that this will happen.

Have an up vote for logic!

Airbus had a crash because their fly by wire averaged the input from both yokes vs doing what Boeing does which is to allow both to feel that the other is pushing on the yoke and the pilots where not trained to understand that.

Well, they have ejection seats.

Precisely. An inherently unstable airframe with no passengers and with crew able to eject is perfectly acceptable. An airliner, not so much.