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The pitch-up moment caused by the engine placement only occurs at high AoA. Basically, as you start to pitch up more and more, eventually the engine nacelles 'catch the wind' forward of the wings and CoM, and try to flip the nose up. For whatever reason, the basic facts of this whole debacle are consistently wrong in reporting and discussion. Common falsehoods: * The 737MAX is inherently unstable. No, it is a stable aircraft as required by FAA regulation that all passenger aircraft be as such. Nothing about the airframe or design is even that remarkable, beyond the simple fact that it is different from the older models. * MCAS is anti-stall. No, MCAS does exactly what it's named for, to modify and augment the 'feel' of the flight stick for the pilot. Because Boeing uses a 'pilot in the loop' design approach, the physical feedback from the flight stick is critical to safe flying, and cannot be faked. What MCAS does is make is to that you have to apply a proportional amount of force on the flight stick to pitch up a given amount (regulations define how far from linear this relationship can be). With the new engine placement, it would not require as much force as you would expect from earlier planes, which could lead to pilots approaching stall conditions unknowingly without that feedback. There are still anti-stall measures to prevent an actual stall (e.g. stick shakers), but the intuitive 'feel' of flying would be lost. The initial MCAS design was reasonable, but test pilots felt that it wasn't strong enough, so it was changed. It is at this point, with no critical review being triggered, that the failure took place. The stronger system put pilots in a catastrophic situation in the event of a malfunction, and two independent flight crews failed to overcome the error. |