Boeing 737 Max8 Aircraft Crashes and Investigations [Part 8] 24

[Sparweb]

This post is the continuation from this series of previous threads:

thread815-445840
thread815-450258
thread815-452000
thread815-454283
thread815-457125
thread815-461989
thread815-466401

This topic is broken into multiple threads due to the length to be scrolled, and images to load, creating long load times for some users and devices.
If you are NEW to this discussion, please read the above threads prior to posting, to avoid rehashing old discussions.

Thank you everyone for your interest! I have learned a lot from the discussion, too.

Some key references:
Ethiopian CAA preliminary report (Link from Ethiopia is now broken. See link from NTSB Investigations below)

Indonesian National Transportation Safety Committee preliminary report

NTSB Investigations

NTSB Safety Recommendation Report: Assumptions Used in the Safety Assessment Process and the
Effects of Multiple Alerts and Indications on Pilot Performance

A Boeing 737 Technical Site

Washington Post: When Will Boeing 737 Max Fly Again and More Questions

BBC: Boeing to temporarily halt 737 Max production in January

Pulitzer Prize, For groundbreaking stories that exposed design flaws in the Boeing 737 MAX that led to two deadly crashes and revealed failures in government oversight.

http://www.sparweb.ca

 

[waross]

Hi moon161.
By hardware solution I visualized a part of the linkage that could be extended or retracted by an actuator.
If you need MCAS, the linkage is extended.
If you don't need MACS the linkage is retracted.
Any failure would be limited to either one MCAS operation or no MCAS operation.
The graph shows that one operation restores the control pressure to the stall point.
You should never need more than one action to counteract upthrust.
There are many ways that this could be implemented somewhere in the stab trim linkage.


Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[thebard3]

You should never need more than one action to counteract upthrust.

I don't know. Maybe it would be proportional to the amount the throttles are moved? Regardless, if I interpreted everything right, then the decision to give MCAS full authority over the elevator (or is it stabilizer?)control was obviously ill-conceived. I think in the end it was really functioning as an anti-stall device, not really associated with movement of the throttle.

Brad Waybright

The more you know, the more you know you don't know.

 

[thebard3]

more news...

https://www.bbc.com/news/business-55751150

From the article--
His report draws on material from the official investigations. It claims that both of the crashed aircraft suffered from - what he believes - were production defects, almost from the moment they entered service.

That's an interesting article but if something has a production defect, wouldn't it be there on day one?

Brad Waybright

The more you know, the more you know you don't know.

 

[LionelHutz]

I don't see how some kind of linkage fix would make it any safer compared to limiting MCAS authority. To me, that actually sounds like a bad idea.

 

[moon161]

Hi moon161.
By hardware solution I visualized a part of the linkage that could be extended or retracted by an actuator.
If you need MCAS, the linkage is extended.
If you don't need MACS the linkage is retracted.
Any failure would be limited to either one MCAS operation or no MCAS operation.
The graph shows that one operation restores the control pressure to the stall point.
You should never need more than one action to counteract upthrust.
There are many ways that this could be implemented somewhere in the stab trim linkage.

Oh hey I like that idea, a series arrangement instead of parallel. This could have safety designed in by limiting the MCAS design stroke to ± something small and the manual trim to the full range of motion required for flying trim.

And mom just called me Andy

 

[waross]

The KISS principle, Lionel.
How much software and hardware has it taken to make MCAS safer?
There are a number of possible solutions to an adjustable link that will only move between two positions, and will be reliably restrained between those positions in the event of an actuator failure.
Unfortunately Boeing management never got out of the rabbit hole of a "cheap" software solution.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[LionelHutz]

A mechanism added into the stabilizer screw still sounds like a very bad idea and besides it would most likely break any possibility of the variant ever being certified to fly.

 

[LittleInch]

They could have told the pilots about it and given them a button / switch to disarm it if it ever went wrong.

Now that sounds like it was a good plan that never happened. Possibly linked to this non SIM training rubbish which should have been laughed out of anyones thought pattern. You only have to look at the two cockpits to realise you were looking at something very different never mind this MCAS issue. If there is a lesson learnt anywhere then that really should be it - these really were two different airplanes and trying to pretend it wasn't for the sake of $1M per plane to SWA has proved to be a very false economy.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[waross]

I may be wrong but I am under the impression that the 737 at one time had two drive mechanisms on the stab-trim jack screw.
A second drive on the bottom of the jack screw could easily have its movement restricted enough to just counteract the lessening control force.
Just one of a number of possible solutions.
Don't fight the problem.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Eufalconimorph]

The other thing I don't get about MCAS's design is why the "single activation" performance is OK. Why not tune a PID to more closely approximate the desired performance curve (linear up to stall)? That's totally separate from the safety issues, just it seems odd to me to have it working in large discrete steps when an analog circuit with appropriate feedback could do a potentially better job. I expect the answer is that new active circuitry would change the type rating, they needed something that could run on the existing computers with the existing inputs.

 

[LionelHutz]

Bill, you do understand that MCAS just moves the stabilizer once when it activates? It doesn't move it back later. The processing capabilities of the controls sound like they are too limited to have any such extra smarts added to them. I haven't seen any proof there was once 2 drive motors on the jack screw, but even if you put 2 motors on the screw both would have to be able to rotate with the screw through the full range of travel so the 2nd one could not be travel limited. After MCAS moved the screw, the other motor would need the ability to move it back.

 

[waross]

Bill, you do understand that MCAS just moves the stabilizer once when it activates? It doesn't move it back later.

That sounds like another engineering failure.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[LittleInch]

The idea was that once your AoA went below the limit for activation, it promptly de-activated and moved the stab the other way back to where it was before.

And don't forget the 737s have a Speed trim system which kicks in at different speeds etc to trim the aircraft automatically as thrust and AoA change. Again a set of limits apply as to when and how much it trims the stab. The A/P does the same thing. So it wasn't a surprise to the pilots when the trim wheel keeps spinning around all on its own.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[LittleInch]

If I recall correctly, on the original 737s I think there were two motors on the jack screw - one commanded by the A/P etc and one by the pilot via the thumb switches. Hence you could lose or disable one set but use the other. But it was a lot of extra weight and the NG reduced this to one motor but two switches in the cockpit to disable the A/P / STS input. Then the Max just made it two switches in series. So all or nothing / manual. In the mean time they had also reduced the size of the manual trim wheel because the bigger one on the earlier version got in the way of something. And made the stab bigger so it became harder to trim it manually at high speeds.

SO as usual what seems a small individual change which on its own is Ok finally all came together in a bad way. That's the problem with keeping the same type certification over time. It all starts to add up and sometimes that number goes negative.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[dik]

More news...

The 737 Max will fly again in Europe, after regulators gave Boeing’s BA, -2.82% bestselling aircraft the all-clear for takeoff following nearly two years on the tarmac.

The plane was grounded by the European Union Aviation Safety Agency, or EASA, in March 2019 after two deadly crashes claimed 346 lives.

Approval from the EASA came with the mandate of a package of software upgrades, electrical work, maintenance checks, operations manual updates, and crew training.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Eufalconimorph]

That's the problem with keeping the same type certification over time. It all starts to add up and sometimes that number goes negative.

That wouldn't be a problem if the rule were that the total set of changes has to be evaluated against the original model, and that any newly approved versions must satisfy all current requirements at the time of their approval (rather than the requirements from the time of the original model's approval). If it changes too much to behave like the original, it shouldn't be part of the same type certificate. If the rules are stricter than they originally were, it should comply with the new rules (and keep the original behavior as closely as possible). If it can't do both, it should get a new type certificate.

Of course that would be expensive, so it won't happen.

 

[dik]

That's likely the way it should happen if it weren't mired in politics...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[IRstuff]

I would caution against stepping onto that slippery slope. Had the MAX required a new type cert, it would have likely doubled or tripled its certification process time. That would make it less likely for companies to create new planes and possible cause stagnation and less innovation. This would result in older fleets and fewer secondary market planes for startup airlines to buy, and possibly, what they could buy would be too old to be practical, so possibly less over competition in the airliner industry.

California's remodeling industry has some parallels; the California remodel pegs tax assessments against the ability to claim that a house has not been so substantially rebuilt that it should assessed as a new house. So, the law is written such that a "California remodel" is considered to be the original house, and tax base, if even a single wall of the original house remains; I was on the verge of making an offer on a house that was originally a 1200-sf ranch-style, that was remodeled into a 5700-sf mansion. There were eventually too many other negatives to move on the deal, so we bought something else. So that's one extreme, but the other extreme would be if someone owned a house for 30 yr and decided to add, say, a single room, and the house required compliance to ADA, full tax reassessment etc., which would require double or triple the cost of single-room addition, in addition to increasing property tax by 7x. That would stifle remodeling of any kind, to avoid that shock to owners that possibly couldn't afford to sustain a 7x property tax payment in perpetuity.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

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[LionelHutz]

any newly approved versions must satisfy all current requirements at the time of their approval

I'm thinking that would kill the ability to grandfather any changes on any plane designed more than a few years ago.

 

[dik]

...maybe not a bad idea, Lionel.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik