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> 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

> 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
- mannykannot 7y ago
  I am simply pointing out what can and cannot be deduced just from these CFRs. Evidence showing directly that the 737 MAX (without MCAS) was found to be divergent within the operational scope of the CFRs (which excludes, for example, CofG well behind the aft limit), would be more persuasive than these specific CFRs themselves. Your comment about earlier 737s is informative, as the time taken to return to trim speed is not a parameter in the CFRs in question. Given that response time, could there have been any plausible doubt that the 737 MAX would require some additional form of stability augmentation? The story, as commonly told, is that MCAS was a response to an unanticipated problem, but now I am wondering if Boeing knew from the start that it would likely have to withold information in order to keep its 'no new training' pledge. While we are on the topic of what the CFRs say, they also do not support the often-repeated claim that the purpose of MCAS is as a stall-prevention device, which is where this thread started.

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

mannykannot 7y ago
I am simply pointing out what can and cannot be deduced just from these CFRs. Evidence showing directly that the 737 MAX (without MCAS) was found to be divergent within the operational scope of the CFRs (which excludes, for example, CofG well behind the aft limit), would be more persuasive than these specific CFRs themselves. Your comment about earlier 737s is informative, as the time taken to return to trim speed is not a parameter in the CFRs in question. Given that response time, could there have been any plausible doubt that the 737 MAX would require some additional form of stability augmentation? The story, as commonly told, is that MCAS was a response to an unanticipated problem, but now I am wondering if Boeing knew from the start that it would likely have to withold information in order to keep its 'no new training' pledge. While we are on the topic of what the CFRs say, they also do not support the often-repeated claim that the purpose of MCAS is as a stall-prevention device, which is where this thread started.