IIRC, on a high AoA the lift generated by the larger engines moves the center of lift ahead of the center of gravity, making the plane unstable. This is what MCAS was arguably designed to prevent, forcing the plane behave like a 737 with smaller engines.
Without MCAS, this would be a crappy experience for the pilots, but, with proper training, they'd be able to fly it just like you and me can safely drive cars like a Reliant Robin (just never think about hitting the brakes in a curve). If you feel the plane wanting to point up a bit too enthusiastically, you can push the stick forward (or adjust the trim) and make it more cooperative.
It just turns out that, with MCAS, little training, and a defective AoA sensor, the experience was lethally crappy.
> Without MCAS, this would be a crappy experience for the pilots, but, with proper training, they'd be able to fly it just like you and me can safely drive cars like a Reliant Robin (just never think about hitting the brakes in a curve). If you feel the plane wanting to point up a bit too enthusiastically, you can push the stick forward (or adjust the trim) and make it more cooperative.
That's the worst part of this, the plane is perfectly flyable without MCAS but they applied it anyways because it would have required recertifying pilots because that difference is enough that it might have required a new type certificate for pilots to fly. So all this trouble and the deaths are because Boeing couldn't make an appealing aircraft with the old 737 body and handling so they took a shortcut to make few 100M more.
In the 737 Max, the engine nacelles themselves can, at high angles of attack, work as a wing and produce lift. And the lift they produce is well ahead of the wing’s center of lift, meaning the nacelles will cause the 737 Max at a high angle of attack to go to a higher angle of attack. This is aerodynamic malpractice of the worst kind.
That's just the with it feeling wrong near stall at high angle-of-attacks.
Theoretically yes, you can train pilots to deal with it. Hell, it would have been safer to just ignore the issue and not train pilots than the clusterfuck of the original MCAS implementation.
But it's not that it doesn't just feel wrong compared to the 737-NG. It feels wrong compared to every single certified aircraft.
The FAA has strict rules on how all aircraft must feel when approaching stall. You can't certify an aircraft without meeting this feel requirement. So the 737-MAX simply can't be certified without MCAS or some other fix.
The actual problem is Boeing made more changes from the 737 base and the checks for this changes were nod done properly because FAA didn't do it's job. Now if FAA and other international agencies check everything from scratch you will find all this hidden problems.
Now imagine you are hired to check the plane systems, would you sign on subsystem X because it worked fine in the old 737 or use your brain and experience andcx flag all potential issues you see.
Is the plane not safe to fly in general AND it needs pilots trained to fly it specifically?