5 ms·

Not true. https://www.seattletimes.com/business/boeing-aerospace/failed-certification-faa-missed-safety-issues-in-the-737-max-system-implicated-in-the-lion-air

by tafda 8y ago

Not true. https://www.seattletimes.com/business/boeing-aerospace/failed-certification-faa-missed-safety-issues-in-the-737-max-system-implicated-in-the-lion-air-crash/ https://www.seattletimes.com/business/boeing-aerospace/faile... Quote: The discrepancy over this number is magnified by another element in the System Safety Analysis: The limit of the system’s authority to move the tail applies each time MCAS is triggered. And it can be triggered multiple times, as it was on the Lion Air flight. One current FAA safety engineer said that every time the pilots on the Lion Air flight reset the switches on their control columns to pull the nose back up, MCAS would have kicked in again and “allowed new increments of 2.5 degrees.” “So once they pushed a couple of times, they were at full stop,” meaning at the full extent of the tail swivel, he said. Peter Lemme, a former Boeing flight controls engineer who is now an avionics and satellite-communications consultant, said that because MCAS reset each time it was used, “it effectively has unlimited authority.”

ajross 8y ago
Yes true.[1] You're misunderstanding that quote. It's talking about "authority" in the sense of software's authorization to move the trim tab, not the aerodynamic authority of the tailplane. The 737 has a 60 year old, very conventional elevator with a trim tab. The tab changes the resting angle of the overall elevator, and thus the "center point" of tailplane force on the airframe. Being out of trim means that the aircraft might naturally want to pitch up or down, and forces the pilot to apply to force to the stick (which moves the elevator, not the trim tab!) to correct it. This stick force can be significant and surprising, but it's designed to (and the certification surely requires it to) be achievable by a pilot under all conditions. That is: it's not supposed to be possible for any trim failure on a 737 to render an aircraft uncontrollable, and I don't see any assertion in the analysis of the MAX 8 that changes that. At the end of the day, what happened here is that the aircraft had a runaway trim failure. That was a failure that was understood and forseen (albeit not under software control) in the aircraft's original design, and the redundancy to treat it was the pilot's physical ability to override the trim. This was supposed to have been a recoverable failure within the cockpit, and it wasn't. [1] Style note: why do people do this on the internet? Ah hah! You fool! You are WRONG! Stuff is complicated folks, be nice, assume competence, assume lack of malice, try to teach and not argue.
- tzakrajs 8y ago
  Your footnote is really appreciated. I think there is a lot of trauma in this world and coping is a daily struggle for many of us. My hat is off to you.
  - wenc 8y ago
    I don't know -- in discourse, it's perfectly fine to dispute ideas (it's the foundation of Western civilization and of analytical thought, and is a means to knowledge and wisdom). If we cannot argue in a marketplace of ideas, we are prevented from having hard but necessary conversations. We need to adhere to parameters of civility of course, but to me, a necessary freedom is the freedom to disagree/dispute. Some ideas are truly wrong and they need to be put through the crucible. That said, I've always been told to always "address the ideas, not the person" (never attack someone's character) and to adopt a pose of "curiosity".
- speakeron 8y ago
  The 737 isn't trimmed with a trim tab; the whole stabilizer is moved (rotated) by a jackscrew driven by an electric motor.
- zkms 8y ago
  > You're misunderstanding that quote. It's talking about "authority" in the sense of software's authorization to move the trim tab, not the aerodynamic authority of the tailplane. > The 737 has a 60 year old, very conventional elevator with a trim tab. The tab changes the resting angle of the overall elevator, and thus the "center point" of tailplane force on the airframe. Being out of trim means that the aircraft might naturally want to pitch up or down, and forces the pilot to apply to force to the stick (which moves the elevator, not the trim tab!) to correct it. I don't think so. The 737 has a trimmable horizontal stabilizer. Stab trim moves (and MCAS moves) the big and leading-edge airfoil and not a trailing-edge tab on a trailing-edge elevator control surface: https://my.mixtape.moe/tkyyzt.png https://my.mixtape.moe/tkyyzt.png See https://www.satcom.guru/2018/11/stabilizer-trim.html https://www.satcom.guru/2018/11/stabilizer-trim.html (where i took this image from) for further details.
- 8y ago

by tafda 8y ago

ajross 8y ago

Yes true.[1] You're misunderstanding that quote. It's talking about "authority" in the sense of software's authorization to move the trim tab, not the aerodynamic authority of the tailplane. The 737 has a 60 year old, very conventional elevator with a trim tab. The tab changes the resting angle of the overall elevator, and thus the "center point" of tailplane force on the airframe. Being out of trim means that the aircraft might naturally want to pitch up or down, and forces the pilot to apply to force to the stick (which moves the elevator, not the trim tab!) to correct it. This stick force can be significant and surprising, but it's designed to (and the certification surely requires it to) be achievable by a pilot under all conditions. That is: it's not supposed to be possible for any trim failure on a 737 to render an aircraft uncontrollable, and I don't see any assertion in the analysis of the MAX 8 that changes that. At the end of the day, what happened here is that the aircraft had a runaway trim failure. That was a failure that was understood and forseen (albeit not under software control) in the aircraft's original design, and the redundancy to treat it was the pilot's physical ability to override the trim. This was supposed to have been a recoverable failure within the cockpit, and it wasn't. [1] Style note: why do people do this on the internet? Ah hah! You fool! You are WRONG! Stuff is complicated folks, be nice, assume competence, assume lack of malice, try to teach and not argue.

tzakrajs 8y ago
Your footnote is really appreciated. I think there is a lot of trauma in this world and coping is a daily struggle for many of us. My hat is off to you.
- wenc 8y ago
  I don't know -- in discourse, it's perfectly fine to dispute ideas (it's the foundation of Western civilization and of analytical thought, and is a means to knowledge and wisdom). If we cannot argue in a marketplace of ideas, we are prevented from having hard but necessary conversations. We need to adhere to parameters of civility of course, but to me, a necessary freedom is the freedom to disagree/dispute. Some ideas are truly wrong and they need to be put through the crucible. That said, I've always been told to always "address the ideas, not the person" (never attack someone's character) and to adopt a pose of "curiosity".
speakeron 8y ago
The 737 isn't trimmed with a trim tab; the whole stabilizer is moved (rotated) by a jackscrew driven by an electric motor.
zkms 8y ago
> You're misunderstanding that quote. It's talking about "authority" in the sense of software's authorization to move the trim tab, not the aerodynamic authority of the tailplane. > The 737 has a 60 year old, very conventional elevator with a trim tab. The tab changes the resting angle of the overall elevator, and thus the "center point" of tailplane force on the airframe. Being out of trim means that the aircraft might naturally want to pitch up or down, and forces the pilot to apply to force to the stick (which moves the elevator, not the trim tab!) to correct it. I don't think so. The 737 has a trimmable horizontal stabilizer. Stab trim moves (and MCAS moves) the big and leading-edge airfoil and not a trailing-edge tab on a trailing-edge elevator control surface: https://my.mixtape.moe/tkyyzt.png https://my.mixtape.moe/tkyyzt.png See https://www.satcom.guru/2018/11/stabilizer-trim.html https://www.satcom.guru/2018/11/stabilizer-trim.html (where i took this image from) for further details.
8y ago