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Comment by js2 | Hacker News Reader

The runaway stabilizer trim procedure is how it was expected to be handled:

https://www.flightglobal.com/news/articles/faa-order-tells-h...

https://www.youtube.com/watch?v=3pPRuFHR1co

The specifics of MCAS weren't out there because Boeing expected the pre-existing procedure to have been sufficient. That procedure wasn't sufficient, or the Lion and Ethiopian Air pilots were somehow not aware of it, or they forgot their training in a stressful situation.

To be clear: the planes should not have put these pilots into the situation they were in, but unless the recovery procedure doesn't work, these planes should not have crashed.

[One thing I've been wondering: somehow the pilots were able to bring the nose back up... it was only (apparently) after multiple nose downs that they eventually failed to recover the plane. What was going on there? It's as if they understood how to correct the stabilizer trim, but weren't aware of the cut-off switches.]

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somehow the pilots were able to bring the nose back up... it was only (apparently) after multiple nose downs that they eventually failed to recover the plane. What was going on there?

The MCAS activated several times, each time ratcheting the stabilizer down by 2.5 degrees. Every time the pilots tried to correct for it, it would engage again. That is what caused the characteristic up/down flight pattern.

There are a lot of details in this story: https://www.seattletimes.com/business/boeing-aerospace/faile...

I've read that but it still leaves me confused. Say trim is at 0°. MCAS sets it to nose down pitch of 2.5°. Pilot pulls back yoke. Why doesn't pilot then also look over at the stab indicator, see that it's at 2.5° nose down and reset it with the electric trim switch back to 0°? If the pilot had done that, when MCAS runs again, doesn't it set the stab back to 2.5° nose down? So the pilot and MCAS should be fighting over 2.5° of trim.

What it seems like happened is MCAS ran the stabilizer to 2.5°, but the pilot didn't reset it back to 0, but just back enough he could counter with the elevator. So MCAS keeps cranking the stab each time a bit further nose down and the pilot keeps only partially countering MCAS, instead of running the stab fully back to 0.

The pilot was clearly aware stab trim had something to do with the situation because there were stab adjustments from the pilot. What I can't make sense of is the pilot only partially countering MCAS. I realize the Lion pilots didn't know about MCAS and so their mental model must have been flawed. I'm even more confused about the Ethiopian Air pilots who should have known about MCAS.

edit: also this - https://news.ycombinator.com/item?id=19442160

One thing I've been wondering: somehow the pilots were able to bring the nose back up... it was only (apparently) after multiple nose downs that they eventually failed to recover the plane. What was going on there? It's as if they understood how to correct the stabilizer trim, but weren't aware of the cut-off switches.

My understanding is that every time the MCAS activated, its effect was incremental. It could command only a limited deflection of the relevant control surface each time -- less than 1 degree? -- but the cumulative effect of multiple activations would eventually exceed the ability of the manual controls to override it.

From the reports I've read it was 2.5 degrees, even though Boeing had produced safety docs to the FAA only mentioning their initial design when they were only applying 0.6 degrees per activation.

That procedure wasn't sufficient

Winner, winner, chicken dinner. The existing runaway stabilizer trim checklist does not fit the MCAS failure to a T. The procedure dictates that you stop if the trimming action stops when you counter it with the switches (MCAS pauses when you do this). Unfortunately when you're taught to only think inside the box, you'll go as far down that checklist as you need to and no further.

Beyond that, just after takeoff is one of the busiest part of the flight. In fact, if you look at the graphs you'll see that the stick shaker on the left side was going crazy almost immediately after takeoff (and before MCAS kicked in as a result of retracting the flaps). The pilots of flight 610 were able to reign in MCAS with the trim buttons and almost certainly continued to do so in order to focus on the other failures and their relevant checklists (stick shaker, unreliable airspeed, elevator feel system non-op).

Yep, to me, that looks an awful lot like some sort of hardware failure. Something to look for in the subsequent and final reports.

Runaway stabilizer feels nothing like MCAS. They likely didn't associate the training they received with the problem at hand.

Can you explain how they are different?

Both cause the stabilizer to tilt. Both cause the stabilizer wheels next to the pilot/co-pilot to turn. If the plane is pitching down, and you pull back the yoke and that still doesn't recover, don't you at some point check the stabilizer angle? If it's wrong, you use the stabilizer trim switch on the yoke under your thumb to correct it. Won't you see the stabilizer wheel turn when you do that? If you let go and the wheel immediately starts turning again the wrong way (pitching the plane down), doesn't that look like runaway stabilizer?

Also, how did the off-duty pilot figure it out if they are nothing alike?

BTW, when you write "runaway stabilizer feels nothing like MCAS" are you speaking from personal experience?

Runaway stabilizer causes a more dramatic sudden movement than MCAS. MCAS's start, wait 5 seconds, re-start, motion is more gradual. For example you could look at the wheel and it has stopped, only to re-start after you look away (add noise cancelling Bose headphones and you may not hear it)

Pilots are trained well for runaway stabilizer, MCAS was essentially the same thing, but it didn't FEEL like the same thing, so a stressed pilot's mind may not immediately go to that solution/memory item.

Basically, the pilots would get more and more confused as their plane went more and more out of trim.

Now things are starting to make sense.

Boeing claimed (and F.A.A agreed) that pilots didn't need training because they already had training for a memory procedure which would have solved the issue.

They both appear to have overlooked is the MCAS symptoms were sufficiently different to the runaway stabiliser scenarios pilots trained on, that pilots are having problems knowing which procedure to apply.

That might be the ultimate answer to why the accidents did happen. That while pilots should be well trained to deal with runaway stabilizers, they didn't recognize the situation in time (which probably was extremely short with the second crash). Which can only be explained that the way the events unfolded, distracted the pilots from dealing with a runaway stabilizer. The third man might just have not been "distracted" by piloting the airplane and thus could see it.

Besides obviously making the sensors used by MCAS truely redundant and limiting its extreme behavior of moving the trim up to the stops, a large part of fixing the MAX might be just having a big warning light for MCAS operating.

Also, how did the off-duty pilot figure it out if they are nothing alike?

My assumption is that the jupmseat guy took a very generous interpretation of the checklist. But there may have been more panic about the trim on the earlier flight. If you compare trim inputs on flight 610 to the previous flight, the pilots on the earlier flight were making much shorter, more rapid inputs than the pilots on flight 610. The plane also pitched down far more dramatically than on flight 610.

The graphs for flight 610 seem to indicate that the pilots thought they had the trim situation under control (and they did right up until they didn't).

https://reports.aviation-safety.net/2018/20181029-0_B38M_PK-...

Thank you for the preliminary report link. It’s the first I’ve seen it.

A runaway stabilizer is a single motion. The control surface moves, you react by shutting down the motor or fighting it by hand, and the situation should calm down. A malfunctioning MCAS will feel like multiple different runaways, in various directions. Once you react to the first, the system could be start pulling things the other way. Rather than a steady pull one way or another, you get into a back-and-forth fight.

My guess would be perspective, two kinds.

First he’s gonna be behind the throttle quadrant. The stabilizer wheels are roughly in the center of his vision, so he will notice them moving and when they start and stop. The pilots have them down near their thighs, so they’re not even necessarily in their peripheral vision. They will be less likely to notice every time they start and stop.

Second the jumpseat guy has no flight responsibilities. He can focus on thinking about what’s wrong without dealing with all of the other stuff you’re doing on departure. Radio calls, navigation, performance monitoring, configuration changes or being agitated by the stick shaker.

Pilot priorities are: aviate, navigate, communicate. If you listen to incident recordings often pilots tell tower to wait and do not respond immediately. So radio calls are always low priority compared to actual flying. But I agree with point of view argument.

> The specifics of MCAS weren't out there because Boeing expected the pre-existing procedure to have been sufficient.

This was argued by Boeing to the FAA, who accepted the reasoning.

It should be noted that, if it had been determined the pre-existing procedure wasn't sufficient, then all pilots on the 737 Max would've had to undergo re-training. This would have been expensive, and probably hurt adoption of the new airframe.

all pilots on the 737 Max would've had to undergo re-training. This would have been expensive, and probably hurt adoption of the new airframe.

Am I the only one hoping that in the end, the costs for Boeing will be more expensive than the global re-training would have been?

It's not the cost of the training; that would have been borne by the airlines, not Boeing, I think. The problem is that if this plane required extra training, this would have been an additional cost to the airlines, meaning they might not have bought the plane in the first place, and would have bought the competing plane from Airbus. Also, not only is it an additional cost, it means they would have had to have a different type-rating for this plane (a pilot rated for a regular 737 wouldn't be allowed to fly this version).

> This would have been expensive, and probably hurt adoption of the new airframe.

What alternative did the airlines have though? The A320neo? Cause that would've required re-training too.

Probably Boeing should've pushed back against the airlines: we've made you a more fuel efficient 737, but it's going to require re-training.

Someone or someones may have thought: "MCAS is the only thing that requires retraining? Can't we fudge that?"

And the thing is, it still seems like MCAS is a reasonable solution, if poorly implemented.

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