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>You keep insisting that we don’t know if this airplane is unstable. Because we don't. You've only corrected a strawman, that we know this plane isn't a fully

>You keep insisting that we don’t know if this airplane is unstable. Because we don't. You've only corrected a strawman, that we know this plane isn't a fully aerodynamic unstable design like an F-16. No one thought that. I never even specifically said aerodynamically unstable, just that we don't know the level of instability MCAS is correcting for to know if it is unusual for an airliner design. From what I had read I had assumed it was actually a power-on issue and not aerodynamic. But as it turns out the issue is indeed apparently aerodynamic from the engine shape generating more lift at higher angle of attack and thus leading to pitch up. And no, we don't know. We the public at least. We don't know how big the flight envelope is where MCAS is needed or how much of an issue it would be if MCAS didn't exist. From public information we don't know if without MCAS the plane wouldn't enter an unrecoverable pitch up attitude in normal flying conditions. That's not knowing if the plane isn't inherently unstable.

mikeash 8y ago
Just a few comments up, your response to the idea of being totally aerodynamically unstable was “hopefully yes.” Not “of course it’s stable, that’s obvious.” https://news.ycombinator.com/item?id=19556911 https://news.ycombinator.com/item?id=19556911 I’m not responding to a straw man, I’m responding to what you wrote. If it’s not what you meant, well, I can’t really do anything about that.
- deleted 8y ago
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- pedrocr 8y ago
  Well, quote the whole thing then: Hopefully yes, but we don't know how much instability there is. If you assume the answer there's no point having the discussion. I was making the point that we don't know what was actually certified. Having F-117 level instability is obviously not the case but apparently the instability it does have is actually aerodynamic so your argument doesn't even work. The actual opposite is happening. The non-fly-by-wire design was modified with an automatic control adjustment to fix an aerodynamic issue. All your argument proves is that the MAX didn't magically turn the 737 into an inherently unstable design which is not relevant to the issue of if the MCAS is covering up a large or small instability.
  - mikeash 8y ago
    Ok. “Hopefully” did not convey that to me. It means you leave open the other possibility, not that it’s obviously not the case. In which case I interpreted your statement as saying it could have F-117 level instability. If that’s not what you meant then carry on.
    - pedrocr 8y ago
      That part of the comment was about the second sentence, that if we just assume everything is fine there's no point in the discussion. F-117 instability would be a ridiculous level though. That we can safely assume. My conjecture from what I've read so far: - Boeing did a good enough job with the airframe that the plane flies mostly fine but there are no miracles and some specific angle-of-attack and airspeed situations make it fall outside the rules. - To bring the plane back into compliance they implemented a fairly simplistic solution in MCAS. Because the situation that is being fixed was fairly benign they didn't engineer it with triple redundancies. - The lack of redundancy was understood to not be a big issue because the worst possible outcome is runaway trim and that's something pilots already train for anyway. - The MCAS runaway trim presents itself in such a unique way that even experience pilots that are already warned about issues with the plane fail to diagnose it. It's in this context that I would then like to know how unusual this situation is. If modern airliners all have these kinds of issues solved by control software then this is just another failure mode that needs to be fixed. If instead it was indeed unusual to fix the aerodynamic issue with control systems then the discussion about Boeing forcing through with the 737 instead of redesigning becomes more important.
      - salawat 8y ago
        I made a post upthread about it, but no, this isn't actually unusual, particularly for Boeing. Their design philosophy generally revolves around granting the pilot maximum authority over the plane. There have been various models that have had the same type of compensator built into their controls. The 727-300 I believe is the most notorious I've come across. McDonnell Douglas also implemented something similar one of their DC' family of jets. I believe it was either the 9, 10, or 11. In that case it was to accommodate a smaller tailplane to decrease drag.

mikeash 8y ago

Just a few comments up, your response to the idea of being totally aerodynamically unstable was “hopefully yes.” Not “of course it’s stable, that’s obvious.” https://news.ycombinator.com/item?id=19556911 https://news.ycombinator.com/item?id=19556911 I’m not responding to a straw man, I’m responding to what you wrote. If it’s not what you meant, well, I can’t really do anything about that.

deleted 8y ago
[deleted]
pedrocr 8y ago
Well, quote the whole thing then: Hopefully yes, but we don't know how much instability there is. If you assume the answer there's no point having the discussion. I was making the point that we don't know what was actually certified. Having F-117 level instability is obviously not the case but apparently the instability it does have is actually aerodynamic so your argument doesn't even work. The actual opposite is happening. The non-fly-by-wire design was modified with an automatic control adjustment to fix an aerodynamic issue. All your argument proves is that the MAX didn't magically turn the 737 into an inherently unstable design which is not relevant to the issue of if the MCAS is covering up a large or small instability.
- mikeash 8y ago
  Ok. “Hopefully” did not convey that to me. It means you leave open the other possibility, not that it’s obviously not the case. In which case I interpreted your statement as saying it could have F-117 level instability. If that’s not what you meant then carry on.
  - pedrocr 8y ago
    That part of the comment was about the second sentence, that if we just assume everything is fine there's no point in the discussion. F-117 instability would be a ridiculous level though. That we can safely assume. My conjecture from what I've read so far: - Boeing did a good enough job with the airframe that the plane flies mostly fine but there are no miracles and some specific angle-of-attack and airspeed situations make it fall outside the rules. - To bring the plane back into compliance they implemented a fairly simplistic solution in MCAS. Because the situation that is being fixed was fairly benign they didn't engineer it with triple redundancies. - The lack of redundancy was understood to not be a big issue because the worst possible outcome is runaway trim and that's something pilots already train for anyway. - The MCAS runaway trim presents itself in such a unique way that even experience pilots that are already warned about issues with the plane fail to diagnose it. It's in this context that I would then like to know how unusual this situation is. If modern airliners all have these kinds of issues solved by control software then this is just another failure mode that needs to be fixed. If instead it was indeed unusual to fix the aerodynamic issue with control systems then the discussion about Boeing forcing through with the 737 instead of redesigning becomes more important.
    - salawat 8y ago
      I made a post upthread about it, but no, this isn't actually unusual, particularly for Boeing. Their design philosophy generally revolves around granting the pilot maximum authority over the plane. There have been various models that have had the same type of compensator built into their controls. The 727-300 I believe is the most notorious I've come across. McDonnell Douglas also implemented something similar one of their DC' family of jets. I believe it was either the 9, 10, or 11. In that case it was to accommodate a smaller tailplane to decrease drag.