> My understanding is that they had to change some things about the plane due to efficiency regulations, which should have resulted in a new plane which would r

> My understanding is that they had to change some things about the plane due to efficiency regulations, which should have resulted in a new plane which would require flight lessons. They kept the things that they had to to escape FAA definitions of "different plane" and MCAS made the resulting monstrosity airworthy. Again, not really. They wanted to put more efficient engines on the 737 to compete with the A320Neo, but keep the 737’s basic height in order to be compatible with the huge number of gates at old regional airports that are essentially built for the exceptionally low shape of the 737. Compatibility with those airports is essentially a competitive advantage for Boeing, as it’s what’s been keeping Airbus from eating their lunch with more efficient planes. But the new engines were essentially too big for the low slung 737, so they moved them super far out from under the wing, making the plane basically unstable. Yeah, if they’d solved this by redesigning the plane to have more room under the wings it would have moved them out from under the 737’s grandfathered approval and thus required a lot more certification certainty and new training – but it would also have messed up the fundamental sales pitch of “buy this instead of the A320 Neo so that you don’t have to renovate your gates at every regional airport you fly to”. the big engines moved so far forward creates the inverted stick forces that aren’t a training issue – no amount of training would have permitted them. It’s a Fundamental Airworthiness Requirement that they not occur, and the MCAS exists explicitly to make them not occur. eg) > Engineers observed a tendency for the plane’s nose to pitch upward during a specific extreme maneuver. After other efforts to fix the problem failed, the solution they arrived at was a piece of software — the Maneuvering Characteristics Augmentation System (MCAS) — that would move a powerful control surface at the tail to push the airplane’s nose down. Again, that “tendency for the plane’s nose to pitch upward” isn’t a training issue – it’s flat out illegal for commercial planes to have that characteristic: https://www.law.cornell.edu/cfr/text/14/25.175 https://www.law.cornell.edu/cfr/text/14/25.175 So the MCAS is just an airworthiness issue caused by the legacy market Boeing was trying to continue to be compatible with. They were careful not to call attention to the MCAS to avoid the FAA adding training requirements that would have upset Southwest, is the main thing where that factors in.