3 ms·

Not quite. The issue is that, due to the engine placement and its effect on center of gravity, the aircraft will tend past a certain angle of attack to start no

Not quite. The issue is that, due to the engine placement and its effect on center of gravity, the aircraft will tend past a certain angle of attack to start nosing into a stall, rather than away from one. MCAS was added to counteract this highly counterintuitive and dangerous tendency by automatically driving the horizontal stabilizer to nose the aircraft back toward level flight. As I understand it, the 737 MAX was the first commercial airframe ever approved by the FAA with this sudden inflection point in behavior when approaching a stall. (There's a term for it, something like "positive rate characteristic", but I forget. Perhaps someone who knows it will mention it here.) Seeing how well that worked out, one hopes it'll also be the last such airframe approved.

stephen_g 6y ago
They're pretty adamant that it's not an anti-stall feature - those normally have 'stick pushers' or shakers.
- throwanem 6y ago
  Considering that both crashes occurred because of MCAS acting as intended on the basis of invalid data from a failed AoA sensor with no backup, I feel like that would be a surprising lie for Boeing to have told. edit: And, having checked the Wikipedia page for the system, I find myself surprised that that is in fact a lie Boeing has told. They may as well go ahead and try to rebrand the damned thing, I guess. On top of everything else they've done, I can't see how it would make their problems any worse... also edit: Via Wikipedia, 14 CFR 25.203: "No abnormal nose-up pitching may occur. The longitudinal [pitch] control force must be positive up to and throughout the stall." This is the characteristic the MAX lacks due to the engine placement. If you want to add an active system to correct for that, the AoA sensor is what you use to drive it, since that's what tells you the difference between the direction of airflow and the angle of the wing. That's how you know you're approaching a stall, and need to add a negative pitching moment. But if that sensor jams, which being a mechanical device it can do, and you don't have input from another sensor so you can see the disagreement, you risk spurious activation of the system when it isn't needed, potentially forcing the aircraft out of level flight. I get that it's in Boeing's interest to claim MCAS isn't an anti-stall system. Why would they say anything else? To do so would implicitly admit not only that they built an aircraft that would fail dangerously in a common situation, but also that the FAA let them get away with selling it and selling it as type-identical with prior versions lacking the fail-dangerous characteristic. Neither admission would serve their interests. But, equally, why should we take them at their word on this, or indeed anything? It's pretty clear that their corporate interests have ceased to align with ours as potential airline passengers. It's generous unto foolishness, in light of that, to give them the benefit of any doubt whatsoever. https://www.law.cornell.edu/cfr/text/14/25.203 https://www.law.cornell.edu/cfr/text/14/25.203
- ashtonkem 6y ago
  They were pretty desperate to maintain the 737 type certification, to reduce airline adoption cost. So the question is: are they right, or are they stretching the truth via motivated reasoning?
  - throwanem 6y ago
    This is a remarkably generous framing of the question. Why exclude the possibility that they were simply lying? Oh, I'm sure the engineers didn't like it, and said so. I believe the news has reported to that effect, based on internal emails. But when did people with golden parachutes start deserving the benefit of the doubt, in this industry or any other?
catalogia 6y ago
> the aircraft will tend past a certain angle of attack to start nosing into a stall, rather than away from one. Virtually all aircraft will do that, including the original 737. Different aircraft do this at different points; provided the pilot is trained for the aircraft, it's not a fundamental problem. The problem for the MAX is they wanted pilots to be able to treat it like an airplane it wasn't.
- throwanem 6y ago
  https://www.law.cornell.edu/cfr/text/14/25.203 https://www.law.cornell.edu/cfr/text/14/25.203 seems to say otherwise - specifically, that you shouldn't be able to stall a certifiable aircraft unless you maintain a positive (ie nose-up, pulling back) control force "up to and throughout the stall". If that were true of the 737 MAX without MCAS, then MCAS wouldn't need to exist. All you'd need to do to avoid a stall would be to stop pulling back on the yoke, and let the aircraft settle on its own. Stick shakers exist to warn the pilot that they need to stop pulling, and the 737 MAX has one. But evidently the stick shaker isn't enough, because the 737 MAX also has MCAS, which operates to actively deflect the nose back down as the aircraft approaches stall. That shouldn't be necessary. That's the thing that other airframes, including prior marks of the 737, do not do. In those airframes, you don't need to push the nose down to get away from a stall. You only need to stop pulling the nose up. MCAS exists to try to replicate this behavior in the 737 MAX, but that MCAS needs to exist for that purpose is enough to demonstrate that the 737 MAX does not behave that way by default. I agree that the falsely shared type certification, and the consequent lack of training on the MAX's type-unique and dangerous stall behavior without MCAS, is a problem. Where we disagree is that I don't consider acceptable a design that exhibits that behavior at all in the absence of an active control system that can fail or be turned off. Neither do the relevant regulations for certification - I linked them above; they make no mention of an exception for an airframe with something equivalent to MCAS - and neither, historically, has the FAA. Not, at least, until the 737 MAX was itself type-certified, and the way that process played out is itself highly suggestive of significant corruption, in the form of regulatory capture if nothing else.

stephen_g 6y ago

They're pretty adamant that it's not an anti-stall feature - those normally have 'stick pushers' or shakers.

throwanem 6y ago
Considering that both crashes occurred because of MCAS acting as intended on the basis of invalid data from a failed AoA sensor with no backup, I feel like that would be a surprising lie for Boeing to have told. edit: And, having checked the Wikipedia page for the system, I find myself surprised that that is in fact a lie Boeing has told. They may as well go ahead and try to rebrand the damned thing, I guess. On top of everything else they've done, I can't see how it would make their problems any worse... also edit: Via Wikipedia, 14 CFR 25.203: "No abnormal nose-up pitching may occur. The longitudinal [pitch] control force must be positive up to and throughout the stall." This is the characteristic the MAX lacks due to the engine placement. If you want to add an active system to correct for that, the AoA sensor is what you use to drive it, since that's what tells you the difference between the direction of airflow and the angle of the wing. That's how you know you're approaching a stall, and need to add a negative pitching moment. But if that sensor jams, which being a mechanical device it can do, and you don't have input from another sensor so you can see the disagreement, you risk spurious activation of the system when it isn't needed, potentially forcing the aircraft out of level flight. I get that it's in Boeing's interest to claim MCAS isn't an anti-stall system. Why would they say anything else? To do so would implicitly admit not only that they built an aircraft that would fail dangerously in a common situation, but also that the FAA let them get away with selling it and selling it as type-identical with prior versions lacking the fail-dangerous characteristic. Neither admission would serve their interests. But, equally, why should we take them at their word on this, or indeed anything? It's pretty clear that their corporate interests have ceased to align with ours as potential airline passengers. It's generous unto foolishness, in light of that, to give them the benefit of any doubt whatsoever. https://www.law.cornell.edu/cfr/text/14/25.203 https://www.law.cornell.edu/cfr/text/14/25.203
ashtonkem 6y ago
They were pretty desperate to maintain the 737 type certification, to reduce airline adoption cost. So the question is: are they right, or are they stretching the truth via motivated reasoning?
- throwanem 6y ago
  This is a remarkably generous framing of the question. Why exclude the possibility that they were simply lying? Oh, I'm sure the engineers didn't like it, and said so. I believe the news has reported to that effect, based on internal emails. But when did people with golden parachutes start deserving the benefit of the doubt, in this industry or any other?

catalogia 6y ago

> the aircraft will tend past a certain angle of attack to start nosing into a stall, rather than away from one. Virtually all aircraft will do that, including the original 737. Different aircraft do this at different points; provided the pilot is trained for the aircraft, it's not a fundamental problem. The problem for the MAX is they wanted pilots to be able to treat it like an airplane it wasn't.

throwanem 6y ago
https://www.law.cornell.edu/cfr/text/14/25.203 https://www.law.cornell.edu/cfr/text/14/25.203 seems to say otherwise - specifically, that you shouldn't be able to stall a certifiable aircraft unless you maintain a positive (ie nose-up, pulling back) control force "up to and throughout the stall". If that were true of the 737 MAX without MCAS, then MCAS wouldn't need to exist. All you'd need to do to avoid a stall would be to stop pulling back on the yoke, and let the aircraft settle on its own. Stick shakers exist to warn the pilot that they need to stop pulling, and the 737 MAX has one. But evidently the stick shaker isn't enough, because the 737 MAX also has MCAS, which operates to actively deflect the nose back down as the aircraft approaches stall. That shouldn't be necessary. That's the thing that other airframes, including prior marks of the 737, do not do. In those airframes, you don't need to push the nose down to get away from a stall. You only need to stop pulling the nose up. MCAS exists to try to replicate this behavior in the 737 MAX, but that MCAS needs to exist for that purpose is enough to demonstrate that the 737 MAX does not behave that way by default. I agree that the falsely shared type certification, and the consequent lack of training on the MAX's type-unique and dangerous stall behavior without MCAS, is a problem. Where we disagree is that I don't consider acceptable a design that exhibits that behavior at all in the absence of an active control system that can fail or be turned off. Neither do the relevant regulations for certification - I linked them above; they make no mention of an exception for an airframe with something equivalent to MCAS - and neither, historically, has the FAA. Not, at least, until the 737 MAX was itself type-certified, and the way that process played out is itself highly suggestive of significant corruption, in the form of regulatory capture if nothing else.