sgrd0q

Incredibly, the Ethiopian crew did just that. I refer you to page 26 of the preliminary report. Look at the graph at time 5:43:15 - two short clicks. That is it. Then the MCAS dials in nose down trim and it is over:

https://assets.documentcloud.org/doc...X-Ethiopia.pdf

I am not going to label them as competent or incompetent - all I know is that they sure knew how to trim and did it all the time under normal circumstances. Then when the forces were too great and the feed back loop non-existent they all of a sudden could not do it.

Same report, page 11, quote:

At 05:40:27, the Captain advised the First-Officer to trim up with him.

Why on earth would you want someone to click on their thumb switches with you unless you believe the trim on your side is not working? And why does he think it is not working? Surely because the feed back loop is distorted - the correction is small, he does not feel it and he gives up. This is my interpretation, you are welcome to disagree.

Adlerdriver

You know this how? It's entirely possible that the training pipeline for these pilots (ab initio?) put them in that airline cockpit with minimal hand flying experience. Follow that with an automation heavy recurrent training regimen, a restrictive company auto-pilot policy and daily flying routine that involves little to no hand flying. It's very possible that these pilots rarely needed to trim.
Rather than build some complex feedback loop theory, isn't it more likely that they simply weren't familiar with the flight regime they were in and failed to take appropriate action?

I also fail to understand why you chose to open your comments with the statement that "everyone" is deficient. You appear to think this feedback loop myth is waiting to bite us all. What data are you using to make these broad generalizations?

trip

That's a stupid analogy. A better analogy-The S&W misfired because of a poorly designed firing pin. While attempting to clear the mis-fire the S&W discharged killing the user.

Excargodog

https://www.ntsb.gov/investigations/...ts/AAR0906.pdf

Or two:

https://www.ntsb.gov/about/employmen...03FA147&akey=1

JohnBurke

You understand neither the report, nor the aircraft, nor the system.

The control wheel nose-up trim attempt at 5:43:11 occurred as pilot-action. This is very significant, because at 5:40:35, nearly three minutes prior, the crew had already used the stab trim cutoff switches. Once the stab trim cutoff switches have been moved to the cutoff position, stab trim may not e re-engaged.

The recording of pilot electric trim input at 5:43:11, with subsequent increase from 2.1 to 2.3 units, is evidence that the crew re-engaged the pitch trim motors.

The pitch trim then moved nose down from 2.3 units to 1.0 unit. THIS WOULD NOT HAVE OCCURRED IF THE CREW HAD NOT RE-ESTABLISHED PITCH TRIM MOTOR CAPABILITY BY RESTORING THE POSITION OF THE STAB TRIM SWITCHES.

Read that again in case you didn't get it.

Once the stab trim motors have been cut off, the stab trim motor cutoff switches must NOT be restored. The crew restored them, thus enabling the trim to run away again.

At the same time, they continued to make the trim force problem WORSE by accelerating.

You don't know that.

Clearly they had one mission with unwanted trim motion: cutoff the stab trim motors and leave them cutoff. That singular action would have prevented this mishap, had they been left in the cutoff position, and had the crew not continued to accelerate.

As stated previously, it's possible to make a salvageable situation unsalvagable, and that's exactly what the crew did.

It is your interpretation. It's wrong.

The captain made several requests of the F/O. Trim with me. Pitch with me. Pitch with me.

The crew didn't trim in the wrong direction. The issue of a "feedback loop" is irrelevant. The controls were heavy nose down, due to an out of trim condition. They made an attempt to restore it, with minor success, but the let the trim run nose down again. They attempted to retrim, cut it off, and would have been required to maintain back pressure based on an out of trim condition, but it was manageable and flyable at that point. Only two things could have made it worse: re-engage the stab trim motors, and increase airspeed.

The crew did both. And it killed them.

Not at all.

The 737 Max didn't misfire, nor was it poorly designed. First and foremost, a sensor failure occurred: the crew recognized the sensor failure, and already had notification in their flight manual of the potential complications of an AoA failure; this notification had been in their flight manuals for several months.

The procedure for unwanted stab trim, regardless of the cause, is the same. It's fully applicable here. It's not new. It's been in play for decades. It's spelled out in the report, in case you don't know it. The single most important element of that procedure is a memory item: stab trim cutout switches CUTOUT.

Once those switches are placed in cutout, they are not to be restored. The crew restored stab trim operation, knowing that stab trim motion was nose down, and that it was uncommanded.

This was not a faulty design or aircraft: this was a faulty pilot action, and a fatal one. Accelerating beyond Vmo in the aircraft sealed it. No chance of recovery, and they rode it into the ground with the overspeed clackers going off the entire time.

No, if a man robs a liquor store with a S&W handgun and shoots the clerk, it doesn't matter if he fumbled the safety at the time, and it doesn't really matter if the pistol has a mechanical problem and discharges without his finger on the trigger: it's his action, robbing that store, that killed the clerk, and it won't help him a bit to try to pawn it off on the pistol.

A pilot in command has the ultimate responsibility for the safe outcome of the flight. He knows that malfunctions can occur in the aircraft; it's quite literally all we train to do, is handle them. Ultimately, however, our first job is to fly the aircraft, and we have procedures to do this; the procedure was violated in this case, and it was that violation that allowed the trim to decrease further nose down, and on top of that, the crew allowed the aircraft to continue to accelerate, thus increasing the nose-down force as the flight diverged farther and farther from it's trimmed speed. Additionally, leaving power in and flying it beyond it's maximum operating speed, entirely out of the operating envelope, eventually prevented any possibility of recovery.

If you want to pick nits about a pistol analogy, the pilots were robbing the liquor store, and held the pistol on the clerk. They engaged the safety (stab trim cutoff switches), and at that stage, there was no chance of shooting the clerk...not until they disengaged the safety (restored the stab trim motors), and set the ball rolling. Everything they did thereafter only sealed the fate, and that's pilot action, not a manufacturer failure.

sgrd0q

I am not exactly sure why you are arguing as I essentially said the same thing in my first post. They re-engaged the trim motors. I am assuming they kept accelerating and probably realized they could not maintain level flight in that configuration due to the control column pressure, and they could not move the trim wheel manually, again presumably because of the aerodynamic forces involved at that speed.

I am not sure where the disconnect is.

If we put aside whether they should or should not deviate from a procedure, whether they should or should not exceed Vmo, and everything else they may or may not have done right - at some point they found themselves with electric stab trim restored. Right or wrong, they did it. We can analyze and debate what they did right and wrong prior to that point, but lets put that aside, as it detracts from my main point - which is that after the electric trim was restored, at that point one would assume they would aggressively trim with the thumb switches (and in the process override MCAS), instead they barely did anything - two clicks at around 5:43:11 and 5:43:15. That in my mind is incredible. Then the MCAS commands nose down trim at what looks like 5:43:20 for about six seconds and shortly after they lose control.

I agree they did not trim in the wrong direction, but they did not trim much at all to relieve the pressure. This is before they cut off the trim. This is exactly where the feedback loop is relevant.

By the way, when they cut off the stab trim motors, they were so out of trim that it is not a given, based on what we know, that the plane was flyable, no matter the speed. You can hold the pressure for so long until your muscles give out.

JohnBurke

No need to assume. It's a fact.

We really can't put those things aside, because in context, they're everything. They're the reason everyone is dead. They're the cause of the loss of the aircraft.

They didn't "find themselves" with trim restored. That sounds too much like a trumpian alternate fact.

They restored trim. They'd put the genie in the bottle, and they let it out. It cost them their lives.

The aircraft was accelerating the entire time. Control forces were getting heavier. The overspeed clacker was sounding. Power was pushed forward; altitude loss, airspeed gain, the distraction of differing cockpit indications, and a perception that trim was not working; with increasing nose down trim, and an increasing nose down force, at some point it was all they could do to pull back on the control column, both of them, and their only other effort to leverage a nose-up pitching force came in the form of a significant power application. Ironically, it only made things worse. We know the result.

Adlerdriver

But the speed does matter. It’s the main reason the aircraft became uncontrollable. Had they stayed in the flight envelope, any trim setting, even an extreme one would have still been flyable. It wasn’t until that trim setting was combined with a speed well beyond limits that the aircraft was un-flyable. So saying “no matter the speed” as if the trim position alone doomed the aircraft makes absolutely no sense.

sgrd0q

Sure, I will agree with you generally. But I said when THEY cut off the stab trim, in that particular scenario I am not sure if the plane was recoverable, no matter what they did with the speed going forward. This was at 5:40:35 – stab trim was cut out, the speed on the left looks like 300 kts (unreliable due to faulty AOA) and the right is about 325 from what I can see on the graph (the font is small). So they are within the flight envelope (just) and presumably the manual trim wheel did not work. How much do they need to slow down for the trim wheel to work? Do they have enough altitude and terrain clearance? Do they have the muscle strength to see this through? If you know that, good for you; to me it is not obvious. Maybe, maybe not.

I should add that we are going off on a tangent here. They didn't even try to slow down, so this is all highly hypothetical. My initial point was about trimming or failure to trim in extreme conditions.

Adlerdriver

You do realize that with normal hydraulic pressure (as was the case here), the control column forces are artificially created by a feel computer and the flight controls are still hydraulically actuated, right? The pilots aren't actually working against the control forces created by airflow over the control surfaces.