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.”
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.
> 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
I think you should perhaps reconsider your line of argument in relation to your own footnote.
What you are saying is directly contradicted by the article unless you apply a very unconventional reading to the word "authority". What you're saying is also contradicted by just looking at an image of a 737's tail arrangement. Lastly, you're claiming that two pilots fell out of the sky to their deaths and it simply never occurred to them to try pulling back hard. I find that not just implausible, but bordering on poor taste.
I appreciate your style note, but I think you should really follow your own advice (look at the style of your own comment). If you don't fly the airplane in question for a living, it's perhaps best not to make categorical statements that turn out to be incorrect. As others have pointed out, stabilizer (not elevator) trim on large aircraft works differently from what you might expect from looking at smaller planes. People don't always realize the whole stabilizer can move, because it moves slowly and never on the ground, and you can't see it from the airplane's windows. The stabilizer has a substantial range of motion because it needs to maintain aerodynamic efficiency while offsetting the range of off-center of mass forces at the edges of the flight envelope. Runaway trim on the stabilizer can result in loss of control authority by the pilot.
A comment from a pilot citing the 737 flight manual [1] contradicts this:
"Excessive air loads on the stabilizer may require effort by both pilots to correct mis-trim. In extreme cases it may be necessary to aerodynamically relieve the air loads to allow manual trimming. Accelerate or decelerate towards the in-trim speed while attempting to trim manually."
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.
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".
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.”