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While the effect of pushing the nose down is an all-too observable effect, that its purpose is stall prevention is not directly observable. According to this a

While the effect of pushing the nose down is an all-too observable effect, that its purpose is stall prevention is not directly observable. According to this article[1], the intended purpose was to restore the back-force needed to put the airplane in a stall, as the effect of the new engines was to make the control feel lighter at high AofA. The article also discusses the certification requirements with regard to acceptable handling characteristics. FWIW, Boeing CEO Dennis Muilenburg is quoted in [2] as saying that "When you take a look at the original design of the MCAS system. I think in some cases, in the media, it has been reported or described as an anti-stall system, which it is not. It's a system that's designed to provide handling qualities for the pilot that meet pilot preferences. "We want the airplane to behave in the air similar to the previous generation of 737s. That's the preferred pilot feel for the airplane, and MCAS is designed to provide those kinds of handling qualities at a high angle of attack." Of course, the elephant in the room not being mentioned here is whether Boeing's primary motive for replicating the handling characteristics of earlier versions of 737s was an overriding concern with avoiding pilot training. [1] https://leehamnews.com/2019/02/15/bjorns-corner-pitch-stability-part-10-wrap-up/ https://leehamnews.com/2019/02/15/bjorns-corner-pitch-stabil... [2] https://www.businessinsider.com/boeings-ceo-on-why-737-max-pilots-not-told-of-mcas-2019-4 https://www.businessinsider.com/boeings-ceo-on-why-737-max-p...

roelschroeven 8y ago
One of the requirements for an airplane to receive a certification is that the pilot needs to put more and more pressure on the stick to achieve higher and higher angle of attack. If at some point the airplane starts to pitch to higher AoA without extra pressure from the pilot, that's a big no. Reason: otherwise it would be all too easy to inadvertently enter a stall condition. Boeing 737 MAX without MCAS violates that requirement (caused by the lift generated by large engine nacelles in front of the wings). MCAS is indeed designed to restore that required back-force. Back-force that is required to prevent stalls. I suppose one could debate whether the stall prevention effect of MCAS is direct or indirect. But I don't think it's debatable that it is designed to comply with a certification requirement that's intended to help prevent stalls.
- mannykannot 8y ago
  You have a point, though, from what I have read, I do not think the airplane would become unstable prior to the stall, merely less statically stable than its predecessors. Boeing may be denying that MCAS has a stall prevention purpose only because to say so might put it in the critical systems category, requiring additional training. If so, this would apply as much to the fix as it does to the original version. To acknowledge that it has a stall prevention function might require an additional AofA sensor as well as additional training.

roelschroeven 8y ago

One of the requirements for an airplane to receive a certification is that the pilot needs to put more and more pressure on the stick to achieve higher and higher angle of attack. If at some point the airplane starts to pitch to higher AoA without extra pressure from the pilot, that's a big no. Reason: otherwise it would be all too easy to inadvertently enter a stall condition. Boeing 737 MAX without MCAS violates that requirement (caused by the lift generated by large engine nacelles in front of the wings). MCAS is indeed designed to restore that required back-force. Back-force that is required to prevent stalls. I suppose one could debate whether the stall prevention effect of MCAS is direct or indirect. But I don't think it's debatable that it is designed to comply with a certification requirement that's intended to help prevent stalls.

mannykannot 8y ago
You have a point, though, from what I have read, I do not think the airplane would become unstable prior to the stall, merely less statically stable than its predecessors. Boeing may be denying that MCAS has a stall prevention purpose only because to say so might put it in the critical systems category, requiring additional training. If so, this would apply as much to the fix as it does to the original version. To acknowledge that it has a stall prevention function might require an additional AofA sensor as well as additional training.