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You're having a completely different discussion unrelated to what I wrote. Fly-by-wire doesn't imply an inherently unstable airframe.

throwanem 8y ago
What do you mean by “inherently unstable” here? We’re talking about an automatic trim system, in the case of the MAX 8; you seem to be reasoning from that to an airframe like that of the F-16 or B-2, that’s effectively unmanageable without its FBW, and I don’t understand how you’re getting there.
- pedrocr 8y ago
  If you read my post I was only explaining how the OP didn't actually know the things he was claiming. We just don't know how much inherent instability there is in the airframe that then needs to be fixed by fly-by-wire. It's surely not F-16 level but there is clearly a flight regime where things get hairy. The point is we don't know. The OP was claiming everything was fine on the instability issue without knowing how much there is and how much actually exists in common airliner designs.
  - cmurf 8y ago
    Based on my reading of FAR 25, specifically 25.143 - 25.207, I can't agree it's possible for a transport category airplane to be unstable. So I guess I don't know what you mean by "inherent instability". I also am unconvinced computers can mediate an airplanes aerodynamic design in order to achieve certification under this part, because again by my reading the expectation is that we (pilots and the flying public) can expect static and dynamic stability dynamic stability, laterally and longitudinally. Can computer control improve these behaviors? Sure. Can computer control make them more linear or consistent or docile or predictable? All of those things. But is it acceptable to have negative static stability, naturally occurring by design, in transport category aircraft moderated by computer? I don't think that's allowed but then I'm not deep diving these regulations either. What happens if all computer control goes down? You can't fall back on the natural static and dynamic stability of the airplane, because it doesn't exhibit those characteristics? That seems bad and not consistent with FAR 25. However, if any such computer safeguards can be disabled (either by the computer itself becoming confused and removing those safeguards; or by pilots disabling them explicitly), I expect the pilot to be trained as part of the type certification requirements, how the airplane behaves when the safeguards are enabled and disabled. I'm quite sure Boeing knows the answer to these questions. I suspect the FAA knows the answer to these questions. And when you say "we don't know" I think you mean HNer's engaged in discussion, who maybe aren't even aware that quite a lot of things are mandatory for obtaining transport category aircraft airworthiness certification.
    - pedrocr 8y ago
      > I'm quite sure Boeing knows the answer to these questions. I suspect the FAA knows the answer to these questions. And when you say "we don't know" I think you mean HNer's engaged in discussion, who maybe aren't even aware that quite a lot of things are mandatory for obtaining transport category aircraft airworthiness certification. I mean that the general public doesn't know that indeed. And thanks for the references, I'll look them up. The original post I replied to was specifically saying that what you are now describing is not allowed (these types of planes being unstable) should be acceptable.

throwanem 8y ago

What do you mean by “inherently unstable” here? We’re talking about an automatic trim system, in the case of the MAX 8; you seem to be reasoning from that to an airframe like that of the F-16 or B-2, that’s effectively unmanageable without its FBW, and I don’t understand how you’re getting there.

pedrocr 8y ago
If you read my post I was only explaining how the OP didn't actually know the things he was claiming. We just don't know how much inherent instability there is in the airframe that then needs to be fixed by fly-by-wire. It's surely not F-16 level but there is clearly a flight regime where things get hairy. The point is we don't know. The OP was claiming everything was fine on the instability issue without knowing how much there is and how much actually exists in common airliner designs.
- cmurf 8y ago
  Based on my reading of FAR 25, specifically 25.143 - 25.207, I can't agree it's possible for a transport category airplane to be unstable. So I guess I don't know what you mean by "inherent instability". I also am unconvinced computers can mediate an airplanes aerodynamic design in order to achieve certification under this part, because again by my reading the expectation is that we (pilots and the flying public) can expect static and dynamic stability dynamic stability, laterally and longitudinally. Can computer control improve these behaviors? Sure. Can computer control make them more linear or consistent or docile or predictable? All of those things. But is it acceptable to have negative static stability, naturally occurring by design, in transport category aircraft moderated by computer? I don't think that's allowed but then I'm not deep diving these regulations either. What happens if all computer control goes down? You can't fall back on the natural static and dynamic stability of the airplane, because it doesn't exhibit those characteristics? That seems bad and not consistent with FAR 25. However, if any such computer safeguards can be disabled (either by the computer itself becoming confused and removing those safeguards; or by pilots disabling them explicitly), I expect the pilot to be trained as part of the type certification requirements, how the airplane behaves when the safeguards are enabled and disabled. I'm quite sure Boeing knows the answer to these questions. I suspect the FAA knows the answer to these questions. And when you say "we don't know" I think you mean HNer's engaged in discussion, who maybe aren't even aware that quite a lot of things are mandatory for obtaining transport category aircraft airworthiness certification.
  - pedrocr 8y ago
    > I'm quite sure Boeing knows the answer to these questions. I suspect the FAA knows the answer to these questions. And when you say "we don't know" I think you mean HNer's engaged in discussion, who maybe aren't even aware that quite a lot of things are mandatory for obtaining transport category aircraft airworthiness certification. I mean that the general public doesn't know that indeed. And thanks for the references, I'll look them up. The original post I replied to was specifically saying that what you are now describing is not allowed (these types of planes being unstable) should be acceptable.