3 ms·

Here is some non pay-walled content: https://qz.com/1718506/boeings-737-max-mcas-has-better-designed-military-forerunner/ https://qz.com/1718506/boeings-737-max

Here is some non pay-walled content: https://qz.com/1718506/boeings-737-max-mcas-has-better-designed-military-forerunner/ https://qz.com/1718506/boeings-737-max-mcas-has-better-desig... The key quote from there: Notably, pilots of the KC-46 are able to override the MCAS—or Maneuvering Characteristics Augmentation System—simply by pulling on the controls, the Journal reports. It quotes an Air Force procurement chief as saying, “We have better sensor data. But most importantly, when the pilot grabs the stick, the pilot is completely in control.”

cjbprime 7y ago
It's weird to see an article writing about this without mentioning the regulatory and training aspect. Previous MAX MCAS *couldn't" allow the pilot to override with the stick, because previous MCAS both needed to prevent a pilot-induced stall and needed to require no training or knowledge from the pilot in order to maintain the type rating. A better way to write the article would be "Boeing and the airlines have given up on trying to do MCAS without telling pilots anything about it."
- mlyle 7y ago
  > previous MCAS both needed to prevent a pilot-induced stall MCAS is there mostly because of certification requirements for hands-off stability. It is not a mechanism to prevent the pilot from stalling the aircraft.
  - cjbprime 7y ago
    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.
  - msbarnett 7y ago
    > MCAS is there mostly because of certification requirements for hands-off stability I don’t believe this is correct. During testing the MAX was discovered to have an inverted force curve on the stick approaching a stall — as the aircraft approaches stall angle, forces flip around and it becomes easier to pull the stick back (into the stall) than forward (out of the stall angle). This violates a fundamental airworthiness requirement — commercial aircraft cannot be certified as airworthy if the stick forces invert. MCAS “solves” this by commanding the stabilizers down as the airplane approaches stall — effectively using the stabilizer to put the forces on the stick that are “missing” due to the MAX’s aerodynamics, ensuring the force curve never inverts. MCAS is entirely about generating force on the stick to meet airworthiness regs, not hands-off stability. It actually can’t disengage when the pilot pulls back on the stick, as that would defeat its entire purpose re: the airworthiness force-on-stick requirement. The WSJ kind of muddled things here. The KC-46A does mid-air refueling, and its semi-similar MCAS system is about hands-off stability — compensating automatically as the weight and balance of the plane dynamically changes as fuel is offloaded. The pilot can override it with the stick because its purpose isn’t to provide stick force.
    - GhettoMaestro 7y ago
      Is there a summarized list somewhere of fundamental airworthiness requirements such as the one you listed?
      - mlyle 7y ago
        It's 14 CFR 25.173 and 14 CFR 25.175.
        GhettoMaestro 7y ago
        Thanks

cjbprime 7y ago

It's weird to see an article writing about this without mentioning the regulatory and training aspect. Previous MAX MCAS *couldn't" allow the pilot to override with the stick, because previous MCAS both needed to prevent a pilot-induced stall and needed to require no training or knowledge from the pilot in order to maintain the type rating. A better way to write the article would be "Boeing and the airlines have given up on trying to do MCAS without telling pilots anything about it."

mlyle 7y ago
> previous MCAS both needed to prevent a pilot-induced stall MCAS is there mostly because of certification requirements for hands-off stability. It is not a mechanism to prevent the pilot from stalling the aircraft.
- cjbprime 7y ago
  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.
- msbarnett 7y ago
  > MCAS is there mostly because of certification requirements for hands-off stability I don’t believe this is correct. During testing the MAX was discovered to have an inverted force curve on the stick approaching a stall — as the aircraft approaches stall angle, forces flip around and it becomes easier to pull the stick back (into the stall) than forward (out of the stall angle). This violates a fundamental airworthiness requirement — commercial aircraft cannot be certified as airworthy if the stick forces invert. MCAS “solves” this by commanding the stabilizers down as the airplane approaches stall — effectively using the stabilizer to put the forces on the stick that are “missing” due to the MAX’s aerodynamics, ensuring the force curve never inverts. MCAS is entirely about generating force on the stick to meet airworthiness regs, not hands-off stability. It actually can’t disengage when the pilot pulls back on the stick, as that would defeat its entire purpose re: the airworthiness force-on-stick requirement. The WSJ kind of muddled things here. The KC-46A does mid-air refueling, and its semi-similar MCAS system is about hands-off stability — compensating automatically as the weight and balance of the plane dynamically changes as fuel is offloaded. The pilot can override it with the stick because its purpose isn’t to provide stick force.
  - GhettoMaestro 7y ago
    Is there a summarized list somewhere of fundamental airworthiness requirements such as the one you listed?
    - mlyle 7y ago
      It's 14 CFR 25.173 and 14 CFR 25.175.
      - GhettoMaestro 7y ago
        Thanks