5 ms·

Using those Positive, Neutral, and Negative stability terms from PuffinBlue's comment, is the problem that we want positive stability, but the MAX without MCAS

by bret6 7y ago

Using those Positive, Neutral, and Negative stability terms from PuffinBlue's comment, is the problem that we want positive stability, but the MAX without MCAS exhibits negative stability at a certain part of its flight envelope? That is, if the pilot is holding the airplane with a constant amount of force and the airplane is climbing at a certain angle, some turbulence or other disturbance could cause the nose to pitch up, and then continue pitching up?

sokoloff 7y ago
No. The Max exhibits positive pitch stability in all flight regimes. What it fails to do is have sufficiently linearly increasing stick forces (required by certification) at high pitch. It’s still aerodynamically stable (not neutral or divergent).
- bumby 7y ago
  Just trying to understand the nuances of terminology here, but does increasing thrust count as a 'disturbance' in defining the stability regime? Meaning if the plane is under increased power and continues to pitch up to the point of stalling, is that an indicator of a negative stability regime?
  - sokoloff 7y ago
    If it would continually increase pitch without bound, that would be described as negatively stable (a ball sitting on top of an upside down capital U. If it "didn't care", that would be neutrally stable (a ball sitting on a horizontal plane). If it tended to reach stability (even if at a different pitch than originally), that's positive stability (a ball sitting in an upright U). The 737-Max does pitch up with increased thrust (as does any jet airplane with a low center of thrust), but it still reaches a stable pitch. The problem in hand flight is that the controls on the Max don't get "progressively stiffer" in a linear fashion with increasing pitch. They still require positive force, but the force required to pull 2.0 G is less than double the force required to pull 1.5 G (as an example; I don't know the particulars of the Max stick force gradient curve). I have not read anything to suggest that a normally trimmed, MCAS-disabled Max would pitch itself into a stall from thrust application. That would be bad, but is also almost surely far from the actual situation.
    - bumby 7y ago
      Thanks for the clarification. Do you know if there are rules/regulations regarding the control inputs? I.e. is linear feedback necessary or just a preferred/expected feel? Without much knowledge of the system, wouldn't a non- linear input be the "expected" feel of a purely mechanical system because the drag increases proportionately to the square of air speed?
      - sokoloff 7y ago
        Yes, there are certification requirements spelled out in FAR 25.175. https://www.law.cornell.edu/cfr/text/14/25.175 https://www.law.cornell.edu/cfr/text/14/25.175 The requirements have the form of "In configuration <XYZ>, the stick force curve must have a stable slope at all speeds within a range which is the greater of <range definition> above and below the trim speed." It is this linear stick force curve requirement that was failing that MCAS was implemented to address. The airplane is still aerodynamically stable.
    - ScottBurson 7y ago
      I am fairly sure I recall reading somewhere that because the engines on the Max are mounted farther forward than previously, there is a region of high AoA and high thrust where the pitch-up force increases with AoA. I can't vouch for this. It's possible I misunderstood it; it's possible the source was misinformed, though I seem to recall it was written by a pilot. But I have trouble imagining what lesser problem would have required such a heavy hand as MCAS to fix. If it were really just a matter of the controls going light, surely that could have been cured with motors that were not so powerful as to be almost impossible to overcome.
    - mlyle 7y ago
      > I have not read anything to suggest that a normally trimmed, MCAS-disabled Max would pitch itself into a stall from thrust application. That would be bad, but is also almost surely far from the actual situation. ?? Most aircraft will do this. Even docile trainers. https://www.youtube.com/watch?v=DZfOtvjJR-I https://www.youtube.com/watch?v=DZfOtvjJR-I [edit: watch this instead https://youtu.be/6RtVdmsx4qU?t=71 https://youtu.be/6RtVdmsx4qU?t=71 ] There's been a couple 747 crashes due to this exact scenario. Or regarding the 737 in particular, to quote "Mike734" in 2007: > In the B-737 too much nose up trim can make a go-around very exciting. The under wing engines create a very large pitch up when adding full power for a go-around. The pilot has to really push hard to stop the jet from pitching too far nose up. and "Cac737": > Now in the Boeings for example, B737 and bigger with underslung engines, this situation is aggravated even more because when you push the power levers forward for go-around thrust, due to where they are and their thrust lines, that act alone will cause the nose to rise very noticeably, and if you are light you can actually find yourself pushing forward on the control column on a go around. now trim "back" as you say and forgetting you did so, can find yourself in a very nosehigh attitude if not careful. https://www.airlinepilotforums.com/hangar-talk/17446-trim-landing.html https://www.airlinepilotforums.com/hangar-talk/17446-trim-la...

by bret6 7y ago

sokoloff 7y ago

No. The Max exhibits positive pitch stability in all flight regimes. What it fails to do is have sufficiently linearly increasing stick forces (required by certification) at high pitch. It’s still aerodynamically stable (not neutral or divergent).

bumby 7y ago
Just trying to understand the nuances of terminology here, but does increasing thrust count as a 'disturbance' in defining the stability regime? Meaning if the plane is under increased power and continues to pitch up to the point of stalling, is that an indicator of a negative stability regime?
- sokoloff 7y ago
  If it would continually increase pitch without bound, that would be described as negatively stable (a ball sitting on top of an upside down capital U. If it "didn't care", that would be neutrally stable (a ball sitting on a horizontal plane). If it tended to reach stability (even if at a different pitch than originally), that's positive stability (a ball sitting in an upright U). The 737-Max does pitch up with increased thrust (as does any jet airplane with a low center of thrust), but it still reaches a stable pitch. The problem in hand flight is that the controls on the Max don't get "progressively stiffer" in a linear fashion with increasing pitch. They still require positive force, but the force required to pull 2.0 G is less than double the force required to pull 1.5 G (as an example; I don't know the particulars of the Max stick force gradient curve). I have not read anything to suggest that a normally trimmed, MCAS-disabled Max would pitch itself into a stall from thrust application. That would be bad, but is also almost surely far from the actual situation.
  - bumby 7y ago
    Thanks for the clarification. Do you know if there are rules/regulations regarding the control inputs? I.e. is linear feedback necessary or just a preferred/expected feel? Without much knowledge of the system, wouldn't a non- linear input be the "expected" feel of a purely mechanical system because the drag increases proportionately to the square of air speed?
    - sokoloff 7y ago
      Yes, there are certification requirements spelled out in FAR 25.175. https://www.law.cornell.edu/cfr/text/14/25.175 https://www.law.cornell.edu/cfr/text/14/25.175 The requirements have the form of "In configuration <XYZ>, the stick force curve must have a stable slope at all speeds within a range which is the greater of <range definition> above and below the trim speed." It is this linear stick force curve requirement that was failing that MCAS was implemented to address. The airplane is still aerodynamically stable.
  - ScottBurson 7y ago
    I am fairly sure I recall reading somewhere that because the engines on the Max are mounted farther forward than previously, there is a region of high AoA and high thrust where the pitch-up force increases with AoA. I can't vouch for this. It's possible I misunderstood it; it's possible the source was misinformed, though I seem to recall it was written by a pilot. But I have trouble imagining what lesser problem would have required such a heavy hand as MCAS to fix. If it were really just a matter of the controls going light, surely that could have been cured with motors that were not so powerful as to be almost impossible to overcome.
  - mlyle 7y ago
    > I have not read anything to suggest that a normally trimmed, MCAS-disabled Max would pitch itself into a stall from thrust application. That would be bad, but is also almost surely far from the actual situation. ?? Most aircraft will do this. Even docile trainers. https://www.youtube.com/watch?v=DZfOtvjJR-I https://www.youtube.com/watch?v=DZfOtvjJR-I [edit: watch this instead https://youtu.be/6RtVdmsx4qU?t=71 https://youtu.be/6RtVdmsx4qU?t=71 ] There's been a couple 747 crashes due to this exact scenario. Or regarding the 737 in particular, to quote "Mike734" in 2007: > In the B-737 too much nose up trim can make a go-around very exciting. The under wing engines create a very large pitch up when adding full power for a go-around. The pilot has to really push hard to stop the jet from pitching too far nose up. and "Cac737": > Now in the Boeings for example, B737 and bigger with underslung engines, this situation is aggravated even more because when you push the power levers forward for go-around thrust, due to where they are and their thrust lines, that act alone will cause the nose to rise very noticeably, and if you are light you can actually find yourself pushing forward on the control column on a go around. now trim "back" as you say and forgetting you did so, can find yourself in a very nosehigh attitude if not careful. https://www.airlinepilotforums.com/hangar-talk/17446-trim-landing.html https://www.airlinepilotforums.com/hangar-talk/17446-trim-la...