Lion Air crash: Jakarta Boeing 737 'had prior instrument error'

[KL27x]

^Same as scrawny male pilots. Hit the gym.

[Bud]

i have a better idea: pedals. Leg force is much bigger.

[chris_leyson]

I'm assuming, perhaps incorrectly, that the trim wheels are mechanically connected to the stabilizer jack screws so that one turn on the trim wheel corresponds to one turn on the jackscrew.

Mentour Pilot has acknowledged that the forces on the stabilizer can be so great that it might be possible that manual trim cannot be performed

Under abnormal, worst case, stabilizer loading conditions how much torque do you need to put onto the stabilizer jack screws ? Quite a lot I would have thought, but to make matters worse frictional losses along the entire length of the mechanical link would increase under high load conditions, so more torque would be required on the trim wheels that at the jackscrews.
Just trying to get my head around the losses in a mechanical system at maximum load. I don't think trim wheels were designed to work at these sort of loads, not only does friction increase but stiction comes into play as well.

[langwadt]

I'm assuming, perhaps incorrectly, that the trim wheels are mechanically connected to the stabilizer jack screws so that one turn on the trim wheel corresponds to one turn on the jackscrew.

Mentour Pilot has acknowledged that the forces on the stabilizer can be so great that it might be possible that manual trim cannot be performed

Under abnormal, worst case, stabilizer loading conditions how much torque do you need to put onto the stabilizer jack screws ? Quite a lot I would have thought, but to make matters worse frictional losses along the entire length of the mechanical link would increase under high load conditions, so more torque would be required on the trim wheels that at the jackscrews.
Just trying to get my head around the losses in a mechanical system at maximum load. I don't think trim wheels were designed to work at these sort of loads, not only does friction increase but stiction comes into play as well.

the trim has a build in clutch so you can override it just by grabbing a trim wheel so the forces required can't be that high
or the clutch would slip under automatic trim as well

https://youtu.be/cQirIH_DuAs

[Homer J Simpson]

[chris_leyson]

@langwadt, forgot about the grab and hold thing, thanks. So it isn't a simple 1:1 mechanical linkage. That makes sense because I couldn't see it working mechanically on an aircraft that size. So it's all servo driven and the pilots have no direct control of air surfaces, it's just haptic feedback. More control systems and more rabbit holes.
EDIT: Probably wrong about the haptic feedback, I was second guessing.

[djacobow]

@langwadt, forgot about the grab and hold thing, thanks. So it isn't a simple 1:1 mechanical linkage. That makes sense because I couldn't see it working mechanically on an aircraft that size. So it's all servo driven and the pilots have no direct control of air surfaces, it's just haptic feedback. More control systems and more rabbit holes.
EDIT: Probably wrong about the haptic feedback, I was second guessing.

Pretty sure it *is* a mechanical linkage, and of course it's not 1:1. It's many to 1, which you can see in the videos when the motor is turning the jackscrew to move the stabilizer a few degrees, and the wheel is going round and round.

Still, even with the mechanical advantage, it might be difficult or maybe impossible. To me, this new information makes a lot more sense to me than MCAS-pushed-the-nose-down-and-we-crashed scenario by itself ever did. Not being able to turn the trim at all or fast enough makes more sense than pilots forgot to disable the electric trim.

[langwadt]

@langwadt, forgot about the grab and hold thing, thanks. So it isn't a simple 1:1 mechanical linkage. That makes sense because I couldn't see it working mechanically on an aircraft that size. So it's all servo driven and the pilots have no direct control of air surfaces, it's just haptic feedback. More control systems and more rabbit holes.
EDIT: Probably wrong about the haptic feedback, I was second guessing.

Pretty sure it *is* a mechanical linkage, and of course it's not 1:1. It's many to 1, which you can see in the videos when the motor is turning the jackscrew to move the stabilizer a few degrees, and the wheel is going round and round.

Still, even with the mechanical advantage, it might be difficult or maybe impossible. To me, this new information makes a lot more sense to me than MCAS-pushed-the-nose-down-and-we-crashed scenario by itself ever did. Not being able to turn the trim at all or fast enough makes more sense than pilots forgot to disable the electric trim.

if the clutch is so that you can override it just by holding it, then the electric motor couldn't turn the trim either
the clutch would just slip. so afaikt if the forces were so high that you couldn't turn it by hand you had reached
a point where the force on the trim was so high the electric couldn't turn it either

[floobydust]

"Boeing confirmed to The Washington Post that it had found a second software problem that the Federal Aviation Administration has ordered fixed - separate from the anti-stall system under investigation in the two crashes, and that had led to the aircraft's worldwide grounding.
That additional problem pertains to software affecting flaps and other flight stabilization hardware and is therefore classified as critical to flight safety, said two officials who spoke on condition of anonymity to discuss the ongoing probe."
"... Boeing called the additional problem "relatively minor" 

The CEO's little message was lame and not a single mistake admitted by Boeing. Some spin about 'we own it' for lower risk design, and a bad sensor being the cause of it all. Wow.

[TerraHertz]

https://www.zerohedge.com/news/2019-04-04/we-own-it-boeing-admits-its-software-was-behind-737-max-crashes-says-sorry-lives
Extract:

Several hours after Ethopian investigators found that the crash of Ethiopian Airlines Flight 302 was not the result of pilot error (hence, it was the result of Boeing error), and demanded a full review of the Boeing 737 Max flight control system, just after 3pm, Boeing CEO Dennis Muilenburg took to social media where in what passed as an attempt at a a "heartfelt" apology, the CEO of the most important, for the Dow Jones, company said that Boeing was "sorry for the lives lost" and essentially admitted that it was the company's software that was responsible for the crashes, saying that "with the release of the preliminary report of Ethiopian Airlines Flight 302accident investigation, it's apparent that in both flight the Maneuvering Characteristics Augmentation System, known as MCAS, activated in response to erroneous angle of attack information."

Here are Muilenburg's prepared remarks:

    We at Boeing are sorry for the lives lost in the Boeing 737 MAX accidents. These tragedies continue to weigh heavily on our hearts and minds and we extend our sympathies to the loved ones of the passengers and crew on board Liion Air Flight 610 and Ethiopian Airlines Flight 302. All of us feel the immense gravity of these events across our company and recognize the devastation to the families and friends of the loved one who perished. The full details of what happened in these two accidents will be issued by the government authorities in the final reports.

    But with the release of the preliminary report of Ethiopian Airlines Flight 302accident investigation, it's apparent that in both flight the Maneuvering Characteristics Augmentation System, known as MCAS, activated in response to erroneous angle of attack information.

    The history of our industry shows most accident are caused by a chain of events. This again is the case here, and we know we can break one of those chain links in these two accidents. As pilots have told us, erroneous activation of the MCAS function can add to what is already a high workload environment. It's our responsibility to eliminate this risk. We own it, and we know how to do it.

[chris_leyson]

N:1 trim wheel to jack screw ratio makes sense from a mechanical advantage point of view but then you have to turn the trim wheels N times faster than the motors that could be driving one or both jack screws and try to fly the aircraft at the same time.

[GeorgeOfTheJungle]

Preliminary report:
http://www.ecaa.gov.et/documents/20435/0/Preliminary+Report+B737-800MAX+%2C(ET-AVJ).pdf

At 05:39:55, Autopilot disengaged,
At 05:39:57, the Captain advised again the First-Officer to request to maintain runway heading and that they are having flight control problems.
At 05:40:00 shortly after the autopilot disengaged, the FDR recorded an automatic aircraft nose down (AND) activated for 9.0 seconds and pitch trim moved from 4.60 to 2.1 units. The climb was arrested and the aircraft descended slightly.
At 05:40:03 Ground Proximity Warning System (GPWS) “DON’T SINK” alerts occurred.
At 05:40:05, the First-Officer reported to ATC that they were unable to maintain SHALA 1A and requested runway heading which was approved by ATC.
At 05:40:06, left and right flap position reached a recorded value of 0.019 degrees which remained until the end of the recording.
The column moved aft and a positive climb was re-established during the automatic AND motion.
At 05:40:12, approximately three seconds after AND stabilizer motion ends, electric trim (from pilot activated switches on the yoke) in the Aircraft nose up (ANU) direction is recorded on the DFDR and the stabilizer moved in the ANU direction to 2.4 units. The Aircraft pitch attitude remained about the same as the back pressure on the column increased.
At 05:40:20, approximately five seconds after the end of the ANU stabilizer motion, a second instance of automatic AND stabilizer trim occurred and the stabilizer moved down and reached 0.4 units.

Sorry guys, but this pilot... should have flipped the cutout switches just after restoring (*) trim between 5:40:12..5:40:20 and they would all be alive now. It's sad.

And that, unlike the captain, the "rookie" pilot were the first to diagnose the problem correctly... says a lot. To me it seems as if the captain had not read and comprehended TB19, and the FO being only the "rookie" copilot was not in charge although he might have been better prepared to face the situation.

"Ethiopian Airlines pilot of doomed flight didn’t take training on 737 MAX simulator"
https://duckduckgo.com/?q=Yared+Mulugeta+737+max+simulator

(*) barely, to 2.4 units instead of the more proper 4.6 they had before.

[dzseki]

Sorry guys, but this pilot... should have flipped the cutout switches just after restoring (*) trim between 5:40:12..5:40:20 and they would all be alive now. It's sad.

So do if they would have not take off at all...
That point you refer was actually 3 minutes before the crash, and they did cut off trim after that.

[GeorgeOfTheJungle]

Why when I say that the pilot screwed up there is someone who understands that I'm trying to exculpate Boeing?

[Towger]

It is not clear if the pressure on the stabiliser was too high and the motor clutch slipped when power was turned back on and the pilot attempted to adjust it.

[tooki]

I don't know the details of the published procedures then in effect.  Under exactly what circumstances do they require putting those switches into cutout position, and what other things are supposed to be tried first?  How much were the official procedures modified after the Lion Air crash?

The Mentour Pilot video about this literally performs the entire “runaway stabilizer trim” checklist in a 737 simulator and then explains each step in detail.

[SkyMaster]

100% sure.

My point is that those switches are probably inputs to a logic system, who will later decide what to do based on 20 other inputs, all controlled by software

GeorgeOfTheJungle is 1000% correct (yes one thousand). These switchs cut the electrical power to the Horizontal Stabilizer trim mechanism.

Still 10000000% correct?

Of course yes still 1000000000000% correct 

You do not cite your source, but your text reads that there was a "trim command" without any movement of the Horizontal Stabilizer; because the electrical power to the Horizontal Stabilizer trim mechanism had been removed by the STAB TRIM Switches being in the CUT-OUT position.

 

[SkyMaster]

It's not entirely clear what they are saying. If they are saying that attempts to trim the aircraft with the trim wheel did not work after the trim motors were put in cutout, then Boeing has a much bigger problem on its hands than MCAS.

If they are saying that manual trim switches on the yoke didn't work after the trim motors were put into cutout, then, yeah, duh. The question then becomes whether the pilots tried to trim the aircraft with the wheel, and if they did, then was there just not enough time to make the adjustment, etc.

As a pilot myself, I would not be particularly scared of an automated system that moves the trim, as long as I could disable it. On the other hand, there's no way in hell I'd launch in an aircraft where I felt that manual trim might not work.

The mechanical trim system of the Boeing 737 have been installed in more than 10 000 Boeing 737 since the '60s.

 

[djacobow]

It's not entirely clear what they are saying. If they are saying that attempts to trim the aircraft with the trim wheel did not work after the trim motors were put in cutout, then Boeing has a much bigger problem on its hands than MCAS.

If they are saying that manual trim switches on the yoke didn't work after the trim motors were put into cutout, then, yeah, duh. The question then becomes whether the pilots tried to trim the aircraft with the wheel, and if they did, then was there just not enough time to make the adjustment, etc.

As a pilot myself, I would not be particularly scared of an automated system that moves the trim, as long as I could disable it. On the other hand, there's no way in hell I'd launch in an aircraft where I felt that manual trim might not work.

The mechanical trim system of the Boeing 737 have been installed in more than 10 000 Boeing 737 since the '60s.

 

Indeed. It would be surprising if the discovered a new failure mode or if the max system is different enough to have a new failure mode. But it is surprising that pilots cannot handle a runaway trim situation, too, so we have to be open to surprising things.

It could also be that not a lot of people have tried to pull many 737s out of deep dive with significant nose-down trim. So despite the many overall aircraft hours, this could be a relatively unexplored flight regime.

[TerraHertz]

[quote


[/quote]

Umm... I recall that the crash investigators early on reported that they found the trim jack screw in the wreckage was at end of travel. So the MCAS system had driven it right to the end, before the pilots did (or didn't?) think to turn off power to the MCAS system.
Supposedly they could then have wound it back manually with those wheels on the central console. But either they didn't think of that in time, or they tried and it didn't work. At all, because the jack screw was still at end of travel in the wreckage.

The question that occurs to me - is there a clutch between those manual winding wheels, and the actual jack screw?

Because when something drives a worm screw hard against its end stop, such mechanisms tend to lock up hard.
Maybe the jack screw was jammed, and attempts to wind it back manually failed because a clutch was slipping?

That would be a whole 'nother level of design incompetence on Boeing's part.
I can imagine no one ever thinking to try doing that during system testing. Why would it ever be jammed hard against the end?
Or maybe they thought 'better put in a clutch, so the stupid pilots don't keep winding it to the end stop, and jam it.'
Never thinking 'what if it's already jammed, and they need to un-jam it?'

Or maybe the pilots, unfamiliar with the whole MCAS system and the manual trim wheel specifically, were just trying to turn it the wrong way?

[floobydust]

The mechanical trim system of the Boeing 737 have been installed in more than 10 000 Boeing 737 since the '60s.

This is precisely the problem - decades of small changes to a design leads to cover ups on the newly introduced safety violations.

Speed, payload and the stabilizer size increased, surely the jackscrew loads are higher.
Anyone do the evaluation if the antiquated manual trim system is realistic?
The wheels seem to be a dinosaur safety item, something giving "coverage" for many possible failures but do they work at all? How many times have they been used successfully in the past 60 years?

You want to be the engineer telling his boss "um, the trim wheels need to be twice the size, they are too small". This is when you're told "don't worry about it, the project is behind schedule and nobody really uses manual trim anyway, there's no room in the cockpit" so it gets swept under the rug.

[langwadt]

[quote

Umm... I recall that the crash investigators early on reported that they found the trim jack screw in the wreckage was at end of travel. So the MCAS system had driven it right to the end, before the pilots did (or didn't?) think to turn off power to the MCAS system.
Supposedly they could then have wound it back manually with those wheels on the central console. But either they didn't think of that in time, or they tried and it didn't work. At all, because the jack screw was still at end of travel in the wreckage.

The question that occurs to me - is there a clutch between those manual winding wheels, and the actual jack screw?

Because when something drives a worm screw hard against its end stop, such mechanisms tend to lock up hard.
Maybe the jack screw was jammed, and attempts to wind it back manually failed because a clutch was slipping?

That would be a whole 'nother level of design incompetence on Boeing's part.
I can imagine no one ever thinking to try doing that during system testing. Why would it ever be jammed hard against the end?
Or maybe they thought 'better put in a clutch, so the stupid pilots don't keep winding it to the end stop, and jam it.'
Never thinking 'what if it's already jammed, and they need to un-jam it?'

Or maybe the pilots, unfamiliar with the whole MCAS system and the manual trim wheel specifically, were just trying to turn it the wrong way?
[/quote]

afaiu the trim wheels are connected to the jack screw with cables, I image the clutch being between the electric motor
and the jackscrew, so that by grabbing the wheels the motor cannot move the jack screw

[TerraHertz]

afaiu the trim wheels are connected to the jack screw with cables, I image the clutch being between the electric motor
and the jackscrew, so that by grabbing the wheels the motor cannot move the jack screw

Yes, there's definitely a clutch between the motor and jackscrew. Demonstrated by being able to halt auto-trim by grabbing and holding the heel.
My question is whether there is _another_ clutch between the wheel and the jackscrew. Or maybe just a potential for the cables to slip. Those have to be a continuous loop, like a belt, since the manual wheel can take many turns.
A combination of jammed jack screw, and slipping manual adjustment clutch (or cables) might have featured in these accidents,

Hmm... that cable loop has to run between the cockpit and the tail, which makes it quite long. I wonder... in a steep nose-down angle, maybe there's less droop in the cable loop and more of its own weight causing cable lengthening. Allowing it to slip? Suppose it just slips on the trim wheel pulley (or whatever) ONLY if the plane is already steeply nose down? (And the jackscrew is jammed.)

[SkyMaster]

Yes, there's definitely a clutch between the motor and jackscrew. Demonstrated by being able to halt auto-trim by grabbing and holding the heel.

If it is possible to halt auto-trim by hand grabbing and holding a trim wheel, it means that a clutch would be between the electrical trim motor and the jackscrew, possibly within the gearbox.

There would be not clutch between the trim wheels and the jackscrew. Boeing 737 Aircraft Maintenance Manual (AMM) is not showing any clutch in this part of the system either.

 

[Kleinstein]

From the video (Yesterday at 01:42:52 am ) explaining preliminary report explain why manual trim did not work: With high speed at low altitude there is too much aerodynamic pressure to do a manual trim. It even looks like the electric trim has more power. Beside the trim problem the indication is that the plain was too fast - just too much power for an about leveled flight.

My (not a pilot) conclusion from the report is that the crew initially did things according to the changed instruction from Boing and turned off electric trim after the second AND event.  After they where not able to do a manual trim, they turn on the electric trim (against the Boing instructions) an did a successful correction via the switches on the yoke  - however not enough (maybe they where to cautious) and they forgot to turn off the electric trim after this.

Do the pilots see the raw information from the AoA sensors ? If so it should have been obvious that the left AoA sensor was bad. Even the computer should have know that the value is obviously wrong - maybe the 2nd change Boing needs to do.

The changed engine position and thus more coupling of engine power to trim caused an additional problem, not just the danger of getting too high an AoA where the MACS system was made for. In the situation the pilots possible did not reduce the engine power to reduce the speed as this would have cause an additional force towards nose down. To get a comparable feeling to the old 737 they might need assistance for the opposite direction too (e.g. trim nose up when the engine power is reduced).

A point that sounds odd to me is that there was not only the left side AoA sensor not working, but other sensors on the left side were also giving poor readings - maybe there was a common cause, effecting more than one sensor.