It's not aerodynamically unstable. It's aerodynamically different than the 737-800 but Boeing decided instead of new pilot training and possibly re-certification, they would address the difference in it's flight characteristics by automatically adjusting the elevator trim when the aircraft is in a specific off-nominal situation such that the 737 MAX 8 feels and behaves just like the 737-800 (MCAS). That's not a bad engineering design decision IMO, but the follow-on decisions (or mistakes) made by Boeing (one AoA truth source instead of two, no extra training, zero indications, basically don't tell anyone about it) is the problem. Once those problems are fixed, no 737 MAX 8 will ever fail due to MCAS again. In fact it's going to be one of the - if not the - most scrutinized (and thereby safer) aircraft ever made.

I exaggerate slightly for [black] humor, but the point is hanging those huge engines so far forward puts its center of thrust uncomfortably ahead of its center of gravity, making it naturally want to pitch nose-up in flight.

All the rest—dodgy sensors, crappy software kludge, no pilot retraining—is hacks laid on top of that initial flaw just to pretend it isn’t there. A lot of us here build complex systems for a living; if that isn’t a “smell”, what is?

(Aerodynamically unstable design is a thing, btw, e.g. Typhoon. But that’s Military, which can afford fly-by-wire software that works.)

A stable design might’ve lifted the airframe further from the ground to give those huge engines the clearance they need under the wing, keeping them to the COG and thus ensuring the plane continues to fly naturally level as per its original design. (Any air geeks want to check me on that?)

But obviously that would’ve cost more, and now Boeing’s run by short-term profit chasers, not adult engineers, the attitude is “What can we get away with?”, not “What do we need to build?” That alone’s reason not to trust their bloody product any more. Not least cos what else might they be tempted to cut corners on?

I mean, a shitty iPhone will screw up your day; a shitty Boeing will screw up the rest of your life.

This is why Corporate Manslaughter laws should include hanging the Board by its heels.

Is it unstable? I thought the problem was that they didn't want to require retraining so they built in a flight control system that made it seem like the previous generation plane from a flight dynamics pov. That system would only take inputs from a single sensor on any given flight. If that sensor had issues then the flight control system would misbehave. If they had gone the retraining route and not included MCAS, then the plane would be fine. At least that was my understanding.

I suspect they’d need to teach those pilots to add a lot of trim. Putting those big engines well ahead of the CoG makes them throw its nose up; it’s basic Newtonian levers.

Not that any plane flies perfectly level at all speeds, which is why they need trim in the first place. But presumably there’s a “sweet spot” which the original 737 was designed to nail in normal flight, which subsequent revisions have moved further and further away from.

At some point you really need an adult in the room to stand up and declare “no more”. But by accounts McDonnell-Douglas already showed all the grown-ups the door. That’s the nice thing about responsibility: just disseminate it enough, and no-one’s to blame.

I think most aircraft are unstable or have quirks outside their approved flight envelope. For passenger aircraft you want it to be docile and predictable inside that. The MAX far as I've gleaned naturally wants to pitch up at high angles of attack. And that the problem is more severe than they expected when they initially designed the thing.

The MCAS might have been okay if the behavior matched initial expectations.

Are you sure ? I though it wanted to pitch up when engines where at full throttle, not because of angles of attack.

I think what I've read is the engine nacelles create lift at higher angles of attack. Because the nacelles are larger and located more forward than in older models. That's the source of the extra lift. Might also be the engines also contribute.

On the other hand I'm just a bystander in all this.

Also consider the position of the thrust point relative to the plane’s center of gravity. Being positioned below and ahead of the CoG, the engines naturally try to push the plane around it, i.e. nose-up.

Mind you, my physics ed stopped after high school (though even that much is enough to do a double-take at MAX). This looks like a much better explanation:

https://www.quora.com/Where-should-the-center-of-mass-lift-a...

It pitches up with throttle, due to the position of the engines. Angle of attack sensors feed MCAS, so it can detect the pitching up and adjust the elevator trim to counteract it.