4 ms·

Without MCAS a MAX has flight characteristics that would not pass FAA certification.

by danharaj 7y ago

foldr 7y ago
I don't believe that's true. What's your source?
- tzs 7y ago
  There are specific regulations on how the stick force must change in response to various flight conditions. Without MCAS the stick force curve of the 737-MAX does not satisfy those regulations. Do an HN search on "MCAS stick force curve" (without the quotes)[1] and you can find some discussion of this with cites to and quotes from specific sections of the regulations. [1] https://hn.algolia.com/?query=MCAS%20stick%20force%20curve&sort=byDate&prefix&page=0&dateRange=all&type=comment https://hn.algolia.com/?query=MCAS%20stick%20force%20curve&s...
  - foldr 7y ago
    There is no source in those comments for the claim that the 737-MAX would not satisfy these regulations without MCAS. Note that the actual regulations are a tad more complex than the incomplete sentence quoted: https://books.google.co.uk/books?id=5tAhOC4KC8MC&pg=PA200&lpg=PA200&dq=%22%22The+stick+force+curve+must+have+a+stable+slope%22%22&source=bl&ots=_B6qqMDW-K&sig=ACfU3U1jvCJhRgh2GLNa_3uPSPUBdWUvWA&hl=en&sa=X&ved=2ahUKEwijvpCNqa_iAhWIUt8KHT_1AagQ6AEwAnoECAoQAQ#v=onepage&q=%22%22The%20stick%20force%20curve%20must%20have%20a%20stable%20slope%22%22&f=false https://books.google.co.uk/books?id=5tAhOC4KC8MC&pg=PA200&lp...
    - danharaj 7y ago
      You're right, I thought I read a post that had a citation, but I cannot find one.
    - salawat 7y ago
      I'm one of the originators and principle repeaters of the theory, and the various facets that go into it are admittedly built up via synthesis of many different slices of life. The principle source that moved me in the direction that the MAX may not be certifiable without MCAS was from this interview from the Royal Aeronautics Society with D. P. Davies, a well known test pilot for the U.K. ARB, and the past woes that Boeing has had to navigate with their designs to be flown in the U.K. https://www.aerosociety.com/news/audio-the-d-p-davies-interview-on-the-boeing-747-the-trident-vc10-one-eleven-the-boeing-727/ https://www.aerosociety.com/news/audio-the-d-p-davies-interv... The 727 had difficulties getting certified for use in British airspace due to high lift device interactions that tended to cause undesired extra pitch sensitivity near critical AoA. The 727 was eventually certified under protest conditional to the addition of a stick pusher to remedy the behavior. The approach was preferred because it both made it harder to stall the plane, and made it readily apparent to the pilot that "thar be dragons here". This makes apparent to me, that at a minimum, it is favorable to design a system to cope with aberrant behavior in such a way that the pilot is immediately aware of the fact they are in a perilous part of the flight envelope. The certification was controversial amongst test pilots because it was seen as as the beginning of a slippery slope that would eventually normalize airframes that don't actually present sound aerodynamic characteristics without excessive assistance from systems that are liable to break. Also note, the FAA never required Boeing to install stick-pushers, and several deep stalls did end up occurring. Since then, many aircraft have become notorious for accidents/difficulties caused where this slippery slope approach to aerodynamics was embraced. See the MD-11 LSAS, and now the MAX 8. Both of these were 'hacks' put in place to make aerodynamic changes to the airframe, while minimizing retraining requirements. The sales points being that the new plane flew just like the old plane. I'm not sure that MCAS is first and foremost an artificial feel system. I've categorized it as more of a "certification hack" in light of whistleblower testimony, and conclusions I've come to doing calculations and attempting to cobble together simulations myself. I do know, however, that if the aerodynamics are so severe near critical AoA, that mere elevator actuation pressure adjustments are no longer adequate to prevent the plane from flinging itself past critical AoA at the merest nudge, and that the entire horizontal stabilizer needs to be moved to ensure the nose is brought back down in short order, I am not confident that the raw math to draw out the stick force curve will end up drawing a compliant curve. And I know that it is not acceptable for a pilot familiar with a sluggishly handling aircraft to just hop into a jitterbug and be on their way without training; I have a hard time being sold that having the same plane go from "smooth and steady" to "put your nose in the air, and stall it like you just don't care" in the same civil transport airframe should be acceptable either. That seems a clear violation of the intent of that design directive. If there's info/precedent/experience missing you can offer to refine or refute it, that'd be awesome. I'm just grabbing on to as many facts as I can run down, throwing them against reality as best I can find equations or regulation to describe it, and trying to see what sticks. I'm more interested in finding the truth than my theory being right.

Without MCAS a MAX has flight characteristics that would not pass FAA certification.

foldr 7y ago

I don't believe that's true. What's your source?

tzs 7y ago
There are specific regulations on how the stick force must change in response to various flight conditions. Without MCAS the stick force curve of the 737-MAX does not satisfy those regulations. Do an HN search on "MCAS stick force curve" (without the quotes)[1] and you can find some discussion of this with cites to and quotes from specific sections of the regulations. [1] https://hn.algolia.com/?query=MCAS%20stick%20force%20curve&sort=byDate&prefix&page=0&dateRange=all&type=comment https://hn.algolia.com/?query=MCAS%20stick%20force%20curve&s...
- foldr 7y ago
  There is no source in those comments for the claim that the 737-MAX would not satisfy these regulations without MCAS. Note that the actual regulations are a tad more complex than the incomplete sentence quoted: https://books.google.co.uk/books?id=5tAhOC4KC8MC&pg=PA200&lpg=PA200&dq=%22%22The+stick+force+curve+must+have+a+stable+slope%22%22&source=bl&ots=_B6qqMDW-K&sig=ACfU3U1jvCJhRgh2GLNa_3uPSPUBdWUvWA&hl=en&sa=X&ved=2ahUKEwijvpCNqa_iAhWIUt8KHT_1AagQ6AEwAnoECAoQAQ#v=onepage&q=%22%22The%20stick%20force%20curve%20must%20have%20a%20stable%20slope%22%22&f=false https://books.google.co.uk/books?id=5tAhOC4KC8MC&pg=PA200&lp...
  - danharaj 7y ago
    You're right, I thought I read a post that had a citation, but I cannot find one.
  - salawat 7y ago
    I'm one of the originators and principle repeaters of the theory, and the various facets that go into it are admittedly built up via synthesis of many different slices of life. The principle source that moved me in the direction that the MAX may not be certifiable without MCAS was from this interview from the Royal Aeronautics Society with D. P. Davies, a well known test pilot for the U.K. ARB, and the past woes that Boeing has had to navigate with their designs to be flown in the U.K. https://www.aerosociety.com/news/audio-the-d-p-davies-interview-on-the-boeing-747-the-trident-vc10-one-eleven-the-boeing-727/ https://www.aerosociety.com/news/audio-the-d-p-davies-interv... The 727 had difficulties getting certified for use in British airspace due to high lift device interactions that tended to cause undesired extra pitch sensitivity near critical AoA. The 727 was eventually certified under protest conditional to the addition of a stick pusher to remedy the behavior. The approach was preferred because it both made it harder to stall the plane, and made it readily apparent to the pilot that "thar be dragons here". This makes apparent to me, that at a minimum, it is favorable to design a system to cope with aberrant behavior in such a way that the pilot is immediately aware of the fact they are in a perilous part of the flight envelope. The certification was controversial amongst test pilots because it was seen as as the beginning of a slippery slope that would eventually normalize airframes that don't actually present sound aerodynamic characteristics without excessive assistance from systems that are liable to break. Also note, the FAA never required Boeing to install stick-pushers, and several deep stalls did end up occurring. Since then, many aircraft have become notorious for accidents/difficulties caused where this slippery slope approach to aerodynamics was embraced. See the MD-11 LSAS, and now the MAX 8. Both of these were 'hacks' put in place to make aerodynamic changes to the airframe, while minimizing retraining requirements. The sales points being that the new plane flew just like the old plane. I'm not sure that MCAS is first and foremost an artificial feel system. I've categorized it as more of a "certification hack" in light of whistleblower testimony, and conclusions I've come to doing calculations and attempting to cobble together simulations myself. I do know, however, that if the aerodynamics are so severe near critical AoA, that mere elevator actuation pressure adjustments are no longer adequate to prevent the plane from flinging itself past critical AoA at the merest nudge, and that the entire horizontal stabilizer needs to be moved to ensure the nose is brought back down in short order, I am not confident that the raw math to draw out the stick force curve will end up drawing a compliant curve. And I know that it is not acceptable for a pilot familiar with a sluggishly handling aircraft to just hop into a jitterbug and be on their way without training; I have a hard time being sold that having the same plane go from "smooth and steady" to "put your nose in the air, and stall it like you just don't care" in the same civil transport airframe should be acceptable either. That seems a clear violation of the intent of that design directive. If there's info/precedent/experience missing you can offer to refine or refute it, that'd be awesome. I'm just grabbing on to as many facts as I can run down, throwing them against reality as best I can find equations or regulation to describe it, and trying to see what sticks. I'm more interested in finding the truth than my theory being right.