Also linked in this article is the original FAA table of differences between the Max and other 737 models. It makes no metion of MCAS.
[1]: https://www.ethiopianairlines.com/EAA
[2]: https://www.cae.com/news-events/press-releases/caes-boeing-7...
27 - FLIGHT CONTROLS STABILIZER TRIM: Stab Trim cutout switches panel
I'm not a real-world airliner pilot (I just play with them in simulators), so I can't comment on whether it's reasonable to expect pilots to recognize the situation and react accordingly in this new failure mode. Without knowledge of the existence of the MCAS system and how it can fail, with uncommanded nose-down, it's unclear that pilots would think there were any factors they recognize that would cause a runaway trim situation and think, "oh, I need to run the runaway trim checklist." I think that's the crux of the argument "any 737 pilot should have been competent to save the plane because they should have recognized runaway stab trim" versus "the set of conditions that caused this problem involved systems that pilots weren't trained on, so it's not reasonable for them to recognize what was happening and realize they can fall back on different training that they did have."
Anyway... comment I was replying to seemed to imply the existence of the stab trim cutout switches implied that the MCAS system was made known to pilots, which is why I pointed out those switches had been there forever.
I mean, yes, the trim system was being driven by input from a single nonredundant sensor, which is a violation of the kind of engineering for failure process that airliners should be designed in. But at the same time that trim system is, by design, not strong enough to overpower pilot input to the other control surfaces. I mean, it's supposed to have been possible for the pilot to just fly the aircraft to a landing in the face of outrageous trim problems.
But the actual pilots didn't, not because they weren't strong enough to push the yoke but because they simply didn't know what to do and didn't have the presence of mind to just yank hard on the controls. Somehow the culture of "just fly the ?!@#! plane" got lost in all those training iPads.
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.”
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.
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".
> 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
See https://www.satcom.guru/2018/11/stabilizer-trim.html (where i took this image from) for further details.
"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."
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.
That is incorrect for the 737 and many airliners. Elevator input is way less strong than trim. Trim moves the entire surface, not just the elevator control surface.
I had the same misconception, my brain was translating GA aircraft knowledge to airliners.
Some light GA aircraft have systems with jackscrews that deflect the horizontal stabilizer as in airliners. These systems are more efficient aerodynamically, but can overpower pilot input, at least in theory. The aero trim system (in prior paragraph) cannot.
No this is not true. In near stall situations for which the MCAS is designed, a full deflection of the elevators is not enough to counter the stabiliser effects. I'm not sure about the entire flight envelope, but I guarantee there are other situations where the elevator cannot counter the stabiliser.
The right way to "regain control" isn't to overpower the stabiliser by inputs to other flight control surfaces, but to disable automatic stabiliser trim and manually trim the aircraft.