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

 

[dik]

https://www.ctvnews.ca/world/indonesian-plane-feared-to-have-crashed-with-62-aboard-1.5260216

“The nearly 27-year-old Boeing 737-500 was much older than Boeing's problem-plagued 737 MAX model, one of which crashed off Jakarta in late 2018, killing all 189 people aboard the Lion Air flight. Older 737 models are widely flown and do not have the system implicated in the MAX safety crisis.”

It wasn’t a max... this time.

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

-Dik

 

[Eufalconimorph]

https://www.ctvnews.ca/world/indonesian-plane-feared-to-have-crashed-with-62-aboard-1.5260216

When the FAA said that the MAX has to handle like the older 737s to keep the same type certificate that's how they wanted it accomplished!

 

[dik]

The older ones, it seems, appear to be handling like the Max...

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

-Dik

 

[davefitz]

3 things you can no longer believe: test pilots, the FAA, and the media. 3 other things that almost never lie: a young kid, a total drunk, and yoga pants.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick

 

[IRstuff]

Young kids learn to lie by around age 4. "Who broke the vase?" "I dunno"

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

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

http://www.eng-tips.com/forumlist.cfm

 

[dik]

Catch the related thread...

thread2-477827

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

-Dik

 

[Sparweb]

Wow. I don't know exactly what to make of this, yet:

https://www.easa.europa.eu/newsroom-and-events/press-releases/easa-lays-out-its-proposed-conditions-return-service-boeing-737[/URL]]
"We also pushed the aircraft to its limits during flight tests, assessed the behaviour of the aircraft in failure scenarios, and could confirm that the aircraft is stable and has no tendency to pitch-up even without the MCAS."

https://theaircurrent.com/aircraft-development/mcas-may-not-have-been-needed-on-the-737-max-at-all/[/URL]]
"...confirm that the aircraft is stable and has no tendency to pitch-up even without the MCAS"

Then the back-pedaling begins:

https://www.theregister.com/2020/11/25/easa_737_max_return/[/URL]]
"In the case of the MAX: the MCAS is necessary to meet the safety regulation and obtain the necessary safety margins. However, when it is lost (failed and inoperative), an averagely skilled and trained crew is still able to safely fly and land the airplane,"

https://theaircurrent.com/aircraft-development/mcas-may-not-have-been-needed-on-the-737-max-at-all/[/URL]]
"...determined during a 20-month review that MCAS was a necessary part of the flight control system..."

http://www.sparweb.ca

 

[waross]

Dare I suspect...Politics?

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

 

[dik]

#3, if it happens, will be a bit more difficult to explain...

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

-Dik

 

[waross]

From what I have read so far, is this a reasonable scenario?
Imagine a Max that has had some system failures.
The failure are serious and either directly or indirectly lead to MCAS being turned off or inoperative.
The plane is hard to control but still flyable.
There are lights flashing and alarms ringing.
For some reason a landing is missed and the pilot executes a climbing turn on his go-around.
Without MCAS active, the flight characteristics change.
With the information overload and the "fog of war" the distracted pilot keeps steady pressure on the controls.
Even though the plane has no tendency to pitch up, it has a real ability to mislead the pilot into "pitching up" the aircraft.

I understand that Boeing identified this possibility very early on and MACS was their attempted solution.
So, if MCAS fails, it does not make the plane unstable but does introduce a change in the response to the controls that may easily lead the pilot to inadvertently pitch up and stall the plane.
Given the months of testing and the thousands of pages of reports, it may not have been hard to lose sight of the original issue.

"We also pushed the aircraft to its limits during flight tests, assessed the behaviour of the aircraft in failure scenarios, and could confirm that the aircraft is stable and has no tendency to pitch-up even without the MCAS."

Is this ill advised as a public statement?
It seems to imply that MCAS doesn't matter and the pilots may ignore any MCAS failure.

If it's Boeing..........

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

 

[moon161]

That's careful wording. It is stable, always had been. It had been sold as 'close enough to all the other 737s that it flys the same and minimal conversion training is needed' and also sold as 'meeting regulatory specs on control feel and stability derivatives' which it didn't without MCAS.

 

[dik]

I betcha they didn't run it past their legal department... seems like a really dumb statement.

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

-Dik

 

[LittleInch]

My understanding is that it doesn't "pitch up" in the sense that you need to apply negative force to stop it going vertical, but that the control forces reduce as you hit high AoA.

This is contrary to the regulations.

The reason is to prevent stall in a panic mode of operation when the pilot keeps pulling back on the controls.

Good pilots in relatively easy conditions cab catch this. But some may not. Hence MCAS was born.

Happy to be corrected.

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

 

[Sparweb]

It helps to return to the graph that was posted back in about April 2019.
While I can't find the original source or how the details on it were created, it matches the problem as described by Boeing and repeated oh-so-often in the media as it relates to pitch forces. There are popular graphics still being republished in the press that get this wrong.

The chart plots "Cm,CG" which in the flight-control context means "Coefficient of pitching moment about the Center of Gravity".
It relates to the proportional force on the control column that you need to push/pull to maintain a steady angle of attack, with all other conditions in equilibrium (enough power to maintain constant altitude and speed at the same time).
The desired response of the aircraft is to resist an input force on the control column with a reaction that returns the aircraft to the stable condition, which is called "trim" in aviation. When the reaction is the opposite of the action, the "feedback is negative" so to speak, and that makes the result of the force balance negative. That's why the line passes into the negative region of the curve.

What the regulatory officials said - in these terms - is that "the Cm curve does not cross over the X-axis". If it did, that would be unstable, which means a pull backward on the controls would lead to the nose constantly going up without self-correcting. So what Patrick Ky and Steve Dickson said was that it doesn't go run away from you. They are probably aware that the popular press has exaggerated this often. The regulators probably felt that once they had experienced the control responses for themselves, they could make a statement to counteract the mistakes made by the press.

The spokespeople later had to clarify, because it seemed to undermine the need for MCAS at all. In closer detail, there isn't an instability being addressed by MCAS. However there is the tendency of the restoring force to level off at high angle of attack. That's the solid blue line in the chart below. For the forces to seem relatively constant over a range of angle of attack, even though they are pushing against you as you pull up, they aren't giving you the "feel" of nose pitch up because the force doesn't increase much. The restoring force should increase as you pull more and more away from the trimmed condition. This takes us down to a finer level of subtlety. I would like to know if it is really perceptible by a pilot when MCAS is off, or just measurable with flight test instruments.

http://www.sparweb.ca

 

[LittleInch]

A much better technical explanation. Really is only that last little uptick that looks odd.

So both statements are true but not many will actually understand the difference.

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

 

[waross]

From that uptick took up at the at the horizontal line.
That is the stall point.

Do I read that graph correctly Spar?
One cycle of MCAS will bring handling characteristics into regulatory compliance out to the stall point?
From that graph, it would seem as if one cycle of MCAS would be all that was ever needed to counteract the uplift.
From the graph it would seem as if any further increase in AoA after one action of MCAS would be due to reasons other than nacelle uplift.

Are there other factors that are not apparent from the graph?

Suppose that these few lines of code had been added to the program.

Is angel of attack past the set point;
If no, no action.
If yes, engage MCAS.
Repeat;
ADD THIS CODE
Is angel of attack past the set point;
If no, no action;
If MCAS flag is set, No action;
If yes, engage MCAS;
Set MCAS flag;
Repeat;

Failure modes:
MCAS fails to activate when it is required.

"We also pushed the aircraft to its limits during flight tests, assessed the behaviour of the aircraft in failure scenarios, and could confirm that the aircraft is stable and has no tendency to pitch-up even without the MCAS."

MCAS activates when it is not required.
Won't happen.

This could also be implemented with a hardware solution, so that the trim is adjusted by other means than turning the jack screw.
A hardware solution would avoid the possibility of a MCAS malfunction leading to a trim runaway.


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

 

[dik]

We're heeerrrrreeeeee:
Canada said on Monday it will lift a near two-year flight ban on Boeing's 737 Max on Jan. 20, joining other countries like the United States that have brought the aircraft back following two fatal crashes involving the model.

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

-Dik

 

[Sparweb]

So both statements are true but not many will actually understand the difference.

Sadly so, and even I've fallen victim to the verbal short-hand of mistaking a measured force gradient for the force itself in various conversations. It's like hearing the same lie enough times to believe it. In this case, not a lie, but a mistake misspoken enough times that it confuses the real details. Heck, I should also repeat that I still haven't found the origin of the chart I'm using as an illustration of my argument. As such, there could be other errors or oversimplifications in it that I'm not noticing because I'm not paying attention to anything but the trend on it. So Waross's follow-up questions are no surprise. Actually, the point where the chart passes the point of zero Cm, over 5 degrees, is a bit high. The plane would not have that much nose-up attitude unless it's at max gross weight. Although, on second thought, max GW is a common point of reference for aerodynamic parameters, so maybe not so strange. I think the stuff on that chart is square with reality, I just can't prove it.

http://www.sparweb.ca

 

[moon161]

This could also be implemented with a hardware solution, so that the trim is adjusted by other means than turning the jack screw.
A hardware solution would avoid the possibility of a MCAS malfunction leading to a trim runaway.

Something close to this was the previous design and why there are two switches to turn off the trim in the cockpit. One motor drive was the STS (Speed Trim System ) and the other was the pilot controlled trim. [edit] If it was still this way then yes, MCAS and STS could share a trim input to the stabilizer and importantly, bet cut off and the pilots would still have powered control of the stabilizer.[/edit] I forget why there was no way to use the old design in the new application.

 

[dik]

more news...

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

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

-Dik