7 ms·

According to this article https://spectrum.ieee.org/aerospace/aviation/how-the-boeing-737-max-disaster-looks-to-a-software-developer https://spectrum.ieee.org/a

by m12k 7y ago

According to this article https://spectrum.ieee.org/aerospace/aviation/how-the-boeing-737-max-disaster-looks-to-a-software-developer https://spectrum.ieee.org/aerospace/aviation/how-the-boeing-... the engines had become so big that they had to be moved in front of the wings instead of under them, which in turn causes the plane to pitch up when the engines apply power (which was not the case with previous 737s). Whether this can be called 'unstable' or not seems to be a matter of definition. Sure it's not unstable the same way that something that completely randomly pitches up or down is. But if you got to drive a car that veers to the left at high speeds and to the right at low speeds, and needs a computer to compensate (oh, and this computer compensates based on sensor readings that may be faulty, so you might need to disable the computer's compensation and compensate yourself), would you perhaps call that unstable compared to a normal car? Doesn't seem unreasonable to me. It's unstable in the sense that controls that are normally independent are not.

_ph_ 7y ago
All aircrafts with engines below the wings have the pitch up tendency at high angles of attack. The only problem with that is, that it is required that the stick force the pilot needs to apply need to increase with high angles of attack. Therefore, most airplanes have some way of augmenting the stick forces in those domains.
- 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?
- mannykannot 7y ago
  All airliners have a cylindrical body projecting forwards from the wing, which produces a pitch-up moment at high angles of attack: it is called the fuselage [1]. Normally, the effects of all such destabilizing aspects of the design are compensated for by the correct positioning and sizing of the wing and horizontal stabilizer. Most airliners now flying do have additional augmentation of their handling for a variety of reasons. Earlier versions of the 737 already had the elevator feel and centering unit and mach trim, but they are much less aggressive than MCAS, and, most importantly, their presence and full scope of effect was not obscured in order to avoid the cost of training pilots for their failure modes. Furthermore, in other cases where handling and stability augmentation is powerful, adequate redundancy is provided (even the 737 feel and centering unit has more redundancy than MCAS.) The presence of handling augmentation devices on other airliners does not justify the way Boeing introduced MCAS. [1] https://leehamnews.com/2018/11/30/bjorns-corner-pitch-stability-part-2/ https://leehamnews.com/2018/11/30/bjorns-corner-pitch-stabil...
  - Excel_Wizard 7y ago
    I really appreciate the increasing levels of technical accuracy the further this thread grows. Hacker News draws in subject matter experts like moths to a flame. Reminds me of the old futurama quote- technically correct is the best kind of correct.
cwingrav 7y ago
Exactly. This is what is not coming out in most of these discussions. 1. Boeing took a great design and by moving the engine forward towards the nose of the plane and on a different position of the wing than what all previous testing had been on, changed how that design worked. 2. Boeing then skipped necessary testing, claiming it was an incremental change, not something that effected the overall design. They did this to keep costs low. Boeing cut corners for costs and the FAA allowed them, in large part because they don't have the people power to do a lot of this testing. People died and faith has been lost in the US's oversight of airlines. This is a clear problem.
coin 7y ago
That article is just an opinion piece by self proclaimed expert who is just a recreational pilot.
- briandear 7y ago
  “Recreational pilot?” He has over 2000 hours of flight time and 30 years of flight experience including time in type for the 757. That’s 1700 more total hours than the copilot of the Ethiopian Airlines flight. The author isn’t a recreational pilot: https://en.m.wikipedia.org/wiki/Pilot_certification_in_the_United_States#Pilot_certificates https://en.m.wikipedia.org/wiki/Pilot_certification_in_the_U... It wouldn’t matter if Chuck Yeager wrote the article; people have their minds made up. It’s either “Boeing should die because ‘big US corporation’ or “The crash pilots were a victim of deficient, assembly-line third world airline training.” The answer is more nuanced and in the middle of those extremes.
  - wglb 7y ago
    Agree with your point. Both things can be true: 1) Many pilots are trained in assembly-ling training 2) Boing did not do adequate testing. Often discorse seems to follow the lines of bracket tournaments: only one thing can be true.
  - gizmo385 7y ago
    I'm assuming this is a typo, but I'll just mention it: Sullenberger has 20,000+ hours of flight experience, not 2,000 :)
    - FabHK 7y ago
      I presume GP was referring to the other article linked above, not Sully's. https://www.boldmethod.com/learn-to-fly/aerodynamics/3-types-of-static-and-dynamic-stability-in-aircraft/ https://www.boldmethod.com/learn-to-fly/aerodynamics/3-types...
Gravityloss 7y ago
The pitchup cause is aerodynamic because the fan shroud is so big. Moving the engines backwards or forward doesn't affect thrust related pitch moment.
- ncmncm 7y ago
  Right: you have the underslung engine pitch effect from the engine thrust, and in the MAX an additional pitch force applied by airflow under the big-ass fan shroud. It was this latter force tbat would be unfamiliar to 737 pilots, and that MCAS was supposed to hide. It didn't vary with engine thrust, but only with airspeed at a high attack angle.

by m12k 7y ago

_ph_ 7y ago

All aircrafts with engines below the wings have the pitch up tendency at high angles of attack. The only problem with that is, that it is required that the stick force the pilot needs to apply need to increase with high angles of attack. Therefore, most airplanes have some way of augmenting the stick forces in those domains.

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?
mannykannot 7y ago
All airliners have a cylindrical body projecting forwards from the wing, which produces a pitch-up moment at high angles of attack: it is called the fuselage [1]. Normally, the effects of all such destabilizing aspects of the design are compensated for by the correct positioning and sizing of the wing and horizontal stabilizer. Most airliners now flying do have additional augmentation of their handling for a variety of reasons. Earlier versions of the 737 already had the elevator feel and centering unit and mach trim, but they are much less aggressive than MCAS, and, most importantly, their presence and full scope of effect was not obscured in order to avoid the cost of training pilots for their failure modes. Furthermore, in other cases where handling and stability augmentation is powerful, adequate redundancy is provided (even the 737 feel and centering unit has more redundancy than MCAS.) The presence of handling augmentation devices on other airliners does not justify the way Boeing introduced MCAS. [1] https://leehamnews.com/2018/11/30/bjorns-corner-pitch-stability-part-2/ https://leehamnews.com/2018/11/30/bjorns-corner-pitch-stabil...
- Excel_Wizard 7y ago
  I really appreciate the increasing levels of technical accuracy the further this thread grows. Hacker News draws in subject matter experts like moths to a flame. Reminds me of the old futurama quote- technically correct is the best kind of correct.

cwingrav 7y ago

Exactly. This is what is not coming out in most of these discussions. 1. Boeing took a great design and by moving the engine forward towards the nose of the plane and on a different position of the wing than what all previous testing had been on, changed how that design worked. 2. Boeing then skipped necessary testing, claiming it was an incremental change, not something that effected the overall design. They did this to keep costs low. Boeing cut corners for costs and the FAA allowed them, in large part because they don't have the people power to do a lot of this testing. People died and faith has been lost in the US's oversight of airlines. This is a clear problem.

coin 7y ago

That article is just an opinion piece by self proclaimed expert who is just a recreational pilot.

briandear 7y ago
“Recreational pilot?” He has over 2000 hours of flight time and 30 years of flight experience including time in type for the 757. That’s 1700 more total hours than the copilot of the Ethiopian Airlines flight. The author isn’t a recreational pilot: https://en.m.wikipedia.org/wiki/Pilot_certification_in_the_United_States#Pilot_certificates https://en.m.wikipedia.org/wiki/Pilot_certification_in_the_U... It wouldn’t matter if Chuck Yeager wrote the article; people have their minds made up. It’s either “Boeing should die because ‘big US corporation’ or “The crash pilots were a victim of deficient, assembly-line third world airline training.” The answer is more nuanced and in the middle of those extremes.
- wglb 7y ago
  Agree with your point. Both things can be true: 1) Many pilots are trained in assembly-ling training 2) Boing did not do adequate testing. Often discorse seems to follow the lines of bracket tournaments: only one thing can be true.
- gizmo385 7y ago
  I'm assuming this is a typo, but I'll just mention it: Sullenberger has 20,000+ hours of flight experience, not 2,000 :)
  - FabHK 7y ago
    I presume GP was referring to the other article linked above, not Sully's. https://www.boldmethod.com/learn-to-fly/aerodynamics/3-types-of-static-and-dynamic-stability-in-aircraft/ https://www.boldmethod.com/learn-to-fly/aerodynamics/3-types...

Gravityloss 7y ago

The pitchup cause is aerodynamic because the fan shroud is so big. Moving the engines backwards or forward doesn't affect thrust related pitch moment.

ncmncm 7y ago
Right: you have the underslung engine pitch effect from the engine thrust, and in the MAX an additional pitch force applied by airflow under the big-ass fan shroud. It was this latter force tbat would be unfamiliar to 737 pilots, and that MCAS was supposed to hide. It didn't vary with engine thrust, but only with airspeed at a high attack angle.