5 ms·

I don't agree. I think the lift from the nacelles results in decreasing stick backpressure while the nose is being raised in some circumstances, which is both a

I don't agree. I think the lift from the nacelles results in decreasing stick backpressure while the nose is being raised in some circumstances, which is both a handling issue and an airworthiness violation. It's an airworthiness violation because it exacerbates rather than discourages stalls. If the pilot continues pulling back on the stick with reduced backpressure the plane would stall, therefore MCAS is (indirectly) an anti-stall device. It's hard to know what the real issue is, because Boeing isn't forthcoming about it. But your claim that the airframe is fundamentally aerodynamically unstable ("needed for hands-off stability") is a very extreme claim that I don't think is supported.

mlyle 7y ago
I think it's odd that you describe a failing of 14 CFR 25.173-- the stability airworthiness requirement-- as not being "fundamentally aerodynamically unstable". The stick force inversions you describe are just a symptom of divergent longitudinal stability. If you are trimmed stick-neutral for climb, deceleration doesn't cause sufficient nose drop, which in turn causes further deceleration and the aircraft does not return to the trim speed. Both test methods are described in 25.175 and were problematic for the 737 MAX.
- sokoloff 7y ago
  My understanding is that the stick force did not actually invert, but merely failed to increase sufficiently. It’s possible the first derivative of stick force inverted, but an airplane can have positive longitude stability and still fail the stick force curve certification requirements.
- cjbprime 7y ago
  My understanding is that a plane can have non-monotonically increasing backpressure without being aerodynamically unstable, and the difference between the two is all about what the plane does when the stick is released. I don't believe the MAX will ever "stall itself" even with MCAS disabled, as previous planes with aerodynamic instability would if you disable that correction. It requires the pilot to fly into the stall, which is easier than it ought to be due to the placement of the engines relative to the center of gravity.
- mannykannot 7y ago
  > I think it's odd that you describe a failing of 14 CFR 25.173-- the stability airworthiness requirement-- as not being "fundamentally aerodynamically unstable." One should look at the actual requirement, not just the title, and for convenience, I have copied it below. As you can see, the issue in this requirement is not one of fundamental aerodynamic stability; this is a handling characterisics issue (and also, 14 CFR 25.175, which does require static lonitudinal stability, also requires more than that (a "stable slope" of the stick force curve in several different cases), and so can be failed by an airplane that is nevertheless statically longitudinally stable.) 14 CFR 25.173: Under the conditions specified in §25.175, the characteristics of the elevator control forces (including friction) must be as follows: (a) A pull must be required to obtain and maintain speeds below the specified trim speed, and a push must be required to obtain and maintain speeds above the specified trim speed. This must be shown at any speed that can be obtained except speeds higher than the landing gear or wing flap operating limit speeds or VFC/MFC, whichever is appropriate, or lower than the minimum speed for steady unstalled flight. (b) The airspeed must return to within 10 percent of the original trim speed for the climb, approach, and landing conditions specified in §25.175 (a), (c), and (d), and must return to within 7.5 percent of the original trim speed for the cruising condition specified in §25.175(b), when the control force is slowly released from any speed within the range specified in paragraph (a) of this section. (c) The average gradient of the stable slope of the stick force versus speed curve may not be less than 1 pound for each 6 knots. (d) Within the free return speed range specified in paragraph (b) of this section, it is permissible for the airplane, without control forces, to stabilize on speeds above or below the desired trim speeds if exceptional attention on the part of the pilot is not required to return to and maintain the desired trim speed and altitude.
  - mlyle 7y ago
    I think it's hilarious that I discuss the test methods, cite the CFR, and mention the actual test results, and in turn you paste back the CFR to me that I cited. Thanks! 737's with smaller engines have very close to neutral pitch stability when at high AOA and high thrust. It takes 5ever to get back to trim speed. 737 MAX is even worse; depending on CG it can be divergent. Gesticulating about stick forces and "handling" is obfuscation and misses the point. Yes, that's fucked up, too. The descriptions of allowing you to pull "harder" assume you're not trimmed for the climb and are holding a constant pull, which uh, some crews do, but autopilot won't (it'll trim off those forces), and lazy crews with weak arms like me won't either. :P

mlyle 7y ago

I think it's odd that you describe a failing of 14 CFR 25.173-- the stability airworthiness requirement-- as not being "fundamentally aerodynamically unstable". The stick force inversions you describe are just a symptom of divergent longitudinal stability. If you are trimmed stick-neutral for climb, deceleration doesn't cause sufficient nose drop, which in turn causes further deceleration and the aircraft does not return to the trim speed. Both test methods are described in 25.175 and were problematic for the 737 MAX.

sokoloff 7y ago
My understanding is that the stick force did not actually invert, but merely failed to increase sufficiently. It’s possible the first derivative of stick force inverted, but an airplane can have positive longitude stability and still fail the stick force curve certification requirements.
cjbprime 7y ago
My understanding is that a plane can have non-monotonically increasing backpressure without being aerodynamically unstable, and the difference between the two is all about what the plane does when the stick is released. I don't believe the MAX will ever "stall itself" even with MCAS disabled, as previous planes with aerodynamic instability would if you disable that correction. It requires the pilot to fly into the stall, which is easier than it ought to be due to the placement of the engines relative to the center of gravity.
mannykannot 7y ago
> I think it's odd that you describe a failing of 14 CFR 25.173-- the stability airworthiness requirement-- as not being "fundamentally aerodynamically unstable." One should look at the actual requirement, not just the title, and for convenience, I have copied it below. As you can see, the issue in this requirement is not one of fundamental aerodynamic stability; this is a handling characterisics issue (and also, 14 CFR 25.175, which does require static lonitudinal stability, also requires more than that (a "stable slope" of the stick force curve in several different cases), and so can be failed by an airplane that is nevertheless statically longitudinally stable.) 14 CFR 25.173: Under the conditions specified in §25.175, the characteristics of the elevator control forces (including friction) must be as follows: (a) A pull must be required to obtain and maintain speeds below the specified trim speed, and a push must be required to obtain and maintain speeds above the specified trim speed. This must be shown at any speed that can be obtained except speeds higher than the landing gear or wing flap operating limit speeds or VFC/MFC, whichever is appropriate, or lower than the minimum speed for steady unstalled flight. (b) The airspeed must return to within 10 percent of the original trim speed for the climb, approach, and landing conditions specified in §25.175 (a), (c), and (d), and must return to within 7.5 percent of the original trim speed for the cruising condition specified in §25.175(b), when the control force is slowly released from any speed within the range specified in paragraph (a) of this section. (c) The average gradient of the stable slope of the stick force versus speed curve may not be less than 1 pound for each 6 knots. (d) Within the free return speed range specified in paragraph (b) of this section, it is permissible for the airplane, without control forces, to stabilize on speeds above or below the desired trim speeds if exceptional attention on the part of the pilot is not required to return to and maintain the desired trim speed and altitude.
- mlyle 7y ago
  I think it's hilarious that I discuss the test methods, cite the CFR, and mention the actual test results, and in turn you paste back the CFR to me that I cited. Thanks! 737's with smaller engines have very close to neutral pitch stability when at high AOA and high thrust. It takes 5ever to get back to trim speed. 737 MAX is even worse; depending on CG it can be divergent. Gesticulating about stick forces and "handling" is obfuscation and misses the point. Yes, that's fucked up, too. The descriptions of allowing you to pull "harder" assume you're not trimmed for the climb and are holding a constant pull, which uh, some crews do, but autopilot won't (it'll trim off those forces), and lazy crews with weak arms like me won't either. :P