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> Because the 737 Max had been outfitted with larger new engines that could cause its nose to pitch dangerously skyward I'm stretching to the limit of my under

> Because the 737 Max had been outfitted with larger new engines that could cause its nose to pitch dangerously skyward I'm stretching to the limit of my understanding of the matter here but I'll try outline why that statement is misleading. To put it in basic terms, you want to avoid a situation where an aircraft can enter an 'accelerated stall'. I believe this to be the correct term. In the original documents to the FAA I think the exact quote of the wording was: > MCAS “was added to address potential nose-up pitching moment at high angles of attack at high airspeeds outside the normal flight envelope.” Specifically this was in a banked downward spiral, known as a 'wind-up turn' where instead of maintaining constant resistance on the control column, at a certain point the resistance to pulling back would lessen and this could allow entering an accelerated stall situation. NOTE - from there I think MCAS ballooned into other areas of control, but I believe that the original intent of MCAS. The reason this possibility arose was because the engines were moved forward, altering their thrush relationship to the center of gravity of the aircraft and, as I understand it, by the design of the engine pods themselves (flat bottomed and can add lift). Under normal flight conditions an airliner would never enter a wind up turn, but it is a test that must be passed as part of the certification process, as far as I understand it. So yes, there are circumstances where the characteristics of the 737 Max can make it easier to enter a dangerous nose up pitch. Note only 'easier' i.e. less resistance on the control column so pulling back with the same force in the same circumstance through the duration of a wind-up turn could allow it to be easier to enter a stall state. But in normal flight and under conditions an airliner would fly, the nose up pitching moment caused by the engines would be entirely 'normal' and in line with what many airliners do (i.e. control column resistance would remain linear) but would have caused the handling to be different is very specific circumstances and so would have needed a new type rating and pilot training to address. MCAS sought to avoid the need for that type rating by automatically applying trim to prevent any situation arising, even outside the normal flight parameters, that would cause different handling characteristics. Again, none of this relates to stability/instability as was my original point. Sources: I read about this level of detail here quite a long time ago: https://www.seattletimes.com/seattle-news/times-watchdog/the-inside-story-of-mcas-how-boeings-737-max-system-gained-power-and-lost-safeguards/ https://www.seattletimes.com/seattle-news/times-watchdog/the... In looking up this article I found this aviation.stackexchange.com answer that also comments on this: https://aviation.stackexchange.com/questions/66799/what-is-mcas-trying-to-fix-on-b737-max https://aviation.stackexchange.com/questions/66799/what-is-m...

beerandt 7y ago
An accelerated stall is one of the most basic safety training exercises done. It's fundamentally familiarizing the pilot with the stall envelope of the aircraft. The pilot should have to increasingly fight the stick to approach the stall. If the operating characteristics of the plane allow it to approach an accelerated stall with only an increase in thrust, then that is inherently not a stable behavior.
- PuffinBlue 7y ago
  This doesn't have anything to do with inherent instability. There's not enough information around that I can find to give a concrete answer as to what was found under high speed, high bank angle and high AoA as would be tested in a Wind-Up Turn. Some things I've seem mention it's a simple 'stick-force-per-g' test. Other things I've read suggest there was maybe flow separation under these conditions, which would point to accelerated stall. What we can say is accelerated stalls are stalls that occur at higher airspeeds than the aircraft would otherwise normally stall due to increased g load. In the specific example I mentioned above, I believe (I can find scant info on this) that additional handling augmentation was required because of the non-linear force on the control column when conducting the wind-up turn. To be certified I believe the 737 Max needed to return a linear force input requirement throughout the entire wind-up turn. Another comment has pointed out that it is a basic requirement for all aircraft to have this linear feedback curve, so this wasn't just specifically to make the 737 Max handle like other 737 under this particular scenario (though apparently it did evolve to that under other conditions but I'd have to look into that further to be sure). Again, this is not about inherent instability. There are some comments out there on the web I've seen that point to there being some issues at slow speeds and high AoA. I'd love to get more info on both things if anyone has it. But really, we have to use the correct language for the field we are talking about and that means understanding stability/instability are specifically defined and relate to a very specific thing.
  - beerandt 7y ago
    >There are some comments out there on the web I've seen that point to there being some issues at slow speeds and high AoA. 1)This is precisely how you induce the accelerated stall I mentioned. Slow down, nose up till you get to the bottom of the energy curve, then accelerate while continuing to nose up. Hold this straight back, or bank to one side, and you stall. It's one of the most basic training procedures there is, although maybe not for 737s. 2) If accelerating causes nose up on it's own (without pulling on the stick) while near the bottom of the energy curve, you could have a run away condition. So are you saying a run-away condition can still be "stable" in aeronautical engineering terms? Because I'm pretty sure that's wrong. You might be studying the wrong books re: the technical language.

beerandt 7y ago

An accelerated stall is one of the most basic safety training exercises done. It's fundamentally familiarizing the pilot with the stall envelope of the aircraft. The pilot should have to increasingly fight the stick to approach the stall. If the operating characteristics of the plane allow it to approach an accelerated stall with only an increase in thrust, then that is inherently not a stable behavior.

PuffinBlue 7y ago
This doesn't have anything to do with inherent instability. There's not enough information around that I can find to give a concrete answer as to what was found under high speed, high bank angle and high AoA as would be tested in a Wind-Up Turn. Some things I've seem mention it's a simple 'stick-force-per-g' test. Other things I've read suggest there was maybe flow separation under these conditions, which would point to accelerated stall. What we can say is accelerated stalls are stalls that occur at higher airspeeds than the aircraft would otherwise normally stall due to increased g load. In the specific example I mentioned above, I believe (I can find scant info on this) that additional handling augmentation was required because of the non-linear force on the control column when conducting the wind-up turn. To be certified I believe the 737 Max needed to return a linear force input requirement throughout the entire wind-up turn. Another comment has pointed out that it is a basic requirement for all aircraft to have this linear feedback curve, so this wasn't just specifically to make the 737 Max handle like other 737 under this particular scenario (though apparently it did evolve to that under other conditions but I'd have to look into that further to be sure). Again, this is not about inherent instability. There are some comments out there on the web I've seen that point to there being some issues at slow speeds and high AoA. I'd love to get more info on both things if anyone has it. But really, we have to use the correct language for the field we are talking about and that means understanding stability/instability are specifically defined and relate to a very specific thing.
- beerandt 7y ago
  >There are some comments out there on the web I've seen that point to there being some issues at slow speeds and high AoA. 1)This is precisely how you induce the accelerated stall I mentioned. Slow down, nose up till you get to the bottom of the energy curve, then accelerate while continuing to nose up. Hold this straight back, or bank to one side, and you stall. It's one of the most basic training procedures there is, although maybe not for 737s. 2) If accelerating causes nose up on it's own (without pulling on the stick) while near the bottom of the energy curve, you could have a run away condition. So are you saying a run-away condition can still be "stable" in aeronautical engineering terms? Because I'm pretty sure that's wrong. You might be studying the wrong books re: the technical language.