MAX’s Return Delayed by FAA Reevaluation of 737 Safety Procedures

[.Scott]

Here's another news article on the elevator crank used on the 737 NG and MAX:
Wall Street Journal Article (https://www.wsj.com/articles/physical-strength-of-pilots-emerges-as-issue-in-returning-737-max-to-flight-11560937879)
... and let's tolerate the technically incorrect descriptions, WSJ is trying to communicate to the lay folk.

Turning the crank moves a horizontal panel on the tail, which can help change the angle of the plane’s nose. Under certain conditions, including at unusually high speeds with the panel already at a steep angle, it can take a lot of force to move the crank in certain emergencies. Among other things, the people familiar with the details said, regulators are concerned about whether female aviators—who typically tend to have less upper-body strength than their male counterparts—may find it difficult to turn the crank in an emergency.

I have never flown a 737 and hopefully none of us have had the opportunity to work this crank under the most adverse conditions.
So is a printed sheet of paper describing this difficulty good enough training?
From what I gather, existing 737 simulators do not reproduce this problem. But they should!

[DaveReidUK]

Here's another news article on the elevator crank used on the 737 NG and MAX:
Wall Street Journal Article (https://www.wsj.com/articles/physical-strength-of-pilots-emerges-as-issue-in-returning-737-max-to-flight-11560937879)
... and let's tolerate the technically incorrect descriptions, WSJ is trying to communicate to the lay folk.

Behind the WSJ's paywall, so we'll have to take your word that "elevator crank" is their term (Google doesn't seem to think so). :O

[Alchad]

Re the elevator crank and difficulty in turning, I've posted this link before but worth reposting

Shutdown caused Boeing crash. - Page 4 - International Skeptics Forum (https://www.pprune.org/left=http:/www.internationalskeptics.com/forums/showthread.php?t=335236&page=4)

Quote..

"I agree it's a flawed design. And I used to work there. I'm glad I don't now.

Regarding the trim wheels: When the NG was being introduced, I happened to be the Lead Engineer in charge of them and a whole lot of other stuff. There were some issues. The new display system created a pinch point between the dash and the wheel. We had to make the wheel smaller. And the new trim motor resulted in the wheel, which is directly connected to the stabilizer by a long cable, springing back when electric trim was used. It was an undamped mass on the end of a spring. We had to add a damper. Result: Depending on the flight conditions, the force to manually trim can be extremely high. We set up a test rig and a very fit female pilot could barely move it.
As I said, I'm glad I'm no longer there."

Alchad

[Loose rivets]

We had to add a damper.

I can see the logic in damping such an inadvertently sprung system, but I'm mindful that I am unaware of this damper despite reading everything I can lay my hands on since November. Is it me that's just being forgetful?

[Smythe]

OUCH....Sully and Carey testimony before Congress...

Retired pilot Chesley "Sully" Sullenberger told a congressional panel Wednesday that pilots should practice the failure of Boeing flight-control software on simulators, not planes full of passengers.

"Reading about it on an iPad is not even close to sufficient," he told the House aviation subcommittee.

The president of the pilots' union at American Airlines says Boeing made mistakes in its design of the 737 Max and not telling pilots about new flight-control software on the plane.

Daniel Carey says Boeing's zeal to minimize pilot-training costs for airlines that would buy its 737 Max jet contributed to errors that led to two deadly crashes and left a "crisis of trust" around aviation safety.
In his testimony Wednesday, Carey drew attention to MCAS, which was designed to make the Max feel like previous 737 models to pilots despite engines that were larger and placed more forward on the wings and changed the plane's aerodynamics.

"This was a fatal design flaw built into the aircraft at the factory," Carey said in an earlier interview

Carey is concerned that pilot training on the updated MCAS system may not be comprehensive enough. He also upbraided those who believe the crashes could not have happened in the United States, calling that notion presumptuous and disrespectful to foreign pilots.

https://www.yahoo.com/finance/news/lawmakers-hear-pilots-criticized-boeing-040539837.html

[futurama]

Yet even Carey in the same testimony says that sim training is not necessary for the MAX to fly again, and "watching videos" is enough training for MAX pilots. So at the end of the day, it's all talk. :rolleyes:

[Fly Aiprt]

Yet even Carey in the same testimony says that sim training is not necessary for the MAX to fly again, and "watching videos" is enough training for MAX pilots. So at the end of the day, it's all talk. :rolleyes:

Considering Sullenberger's aura and his advice that sim training be mandatory, it seems doubtful that the flying public will accept to board a flight with a pilot having just "watched a video".

"This is your Captain speaking. Thank you for choosing XX America Airlines. This is to let you know my First Officer and I just watched the MCAS video.
Please turn off your electronic devices..."

[futurama]

It's not that simple. In the testimony Sully acknowledged that requiring sim training for everyone would be a logistical problem, although he wants pilots to get the sim session as soon as possible.

It looks like the pilot unions, the airlines, and the FAA FSB will recommend that CBT (including the video) will be enough for return to service. Then pilots will get full MCAS / trim runaway training during their next scheduled recurrent training, which could be up to 9 - 12 months away depending on the airline and individual circumstances.

There's a notion that maybe pilots specifically flying the MAX could get the training first, but assuming the software fix is implemented, the probability of a trim runaway will be practically similar between the MAX and NG.

[joe_bloggs]

What has happened to the investigation of the trigger for these events- the AoA vane/s?
Wasn’t the Lion Air “faulty” one sent to the manufacturer for analysis/testing?

[tdracer]

What has happened to the investigation of the trigger for these events- the AoA vane/s?
Wasn’t the Lion Air “faulty” one sent to the manufacturer for analysis/testing?


You'll have to wait for the Indonesians to say what was found - by procedure no one else is permitted to talk publicly about it.
I read somewhere that the Ethiopian AOA was damaged by a bird strike, but I don't think even that's official from the investigation team.

[groundbum]

What has happened to the investigation of the trigger for these events- the AoA vane/s?
Wasn’t the Lion Air “faulty” one sent to the manufacturer for analysis/testing?

I'm not sure finding what happened to the vane is on the critical path. Yes it failed and everything else resulted from this failure, but this a part that is known to fail infrequently. As such a failure should not lead to 360 families losing loved ones, it's a failure that should create a small degradation that can be handled by the pilots and automatics. Things fail all the time on aircraft, and multiple safety systems make sure nothing catastrophic happens.

G

[Fly Aiprt]

It looks like the pilot unions, the airlines, and the FAA FSB will recommend that CBT (including the video) will be enough for return to service. Then pilots will get full MCAS / trim runaway training during their next scheduled recurrent training, which could be up to 9 - 12 months away depending on the airline and individual circumstances.
.

"This is your Captain speaking. Our training on this MAX is scheduled in 6 months from now.
In the meanwhile, will depart in 5 minutes, please fasten seat belts..."

[krismiler]

This also opens up the issue of what constitutes sufficient training when moving amongst aircraft of the same type but different series. Would an pilot flying an old A319 be happy getting straight into an A321 NEO ? Types have now evolved over a few decades and more advanced systems have been gradually incorporated. Possibly a type rating needs to be more narrowly defined with more training given between variants ?

There is a thread running concerning the differences in rotation between the current and NEO airbus aircraft, nothing major but the pitch rate is noticeably different.

[Bend alot]

It is laughable that a regulator will state that simulator training is a requirement, but then allow companies to operate for any period without having done that training.

If any accident should happen in that window before simulator training - the regulator/s will be left carrying the can, and I just can not see more than one regulator putting up their hand to carry someone else's poor past decisions.

[Loose rivets]

I'm not sure finding what happened to the vane is on the critical path. Yes it failed and everything else resulted from this failure, but this a part that is known to fail infrequently. As such a failure should not lead to 360 families losing loved ones . . .

This is a point I've raised more than once. Technically, the AoA measuring system failures were almost certainly unrelated. Given the timescale, it is the bizarre coincidence that is so hard to get to grips with. More time and the LionAir accident investigation would hopefully have spread more solid information around the world.

[Smythe]

I did notice that the IAG order called them the 737-8 and 737-10...no MAX mention.

[averow]

Nice summation krismiller.

[averow]

Nice, succinct summation Krismiler.

[Aso]

Interesting to see the 2014 report on the 787 again... The 737 isnt broken... Boeing is :(

https://www.youtube.com/watch?v=rvkEpstd9os

[Turbine Overheat]

I did notice that the IAG order called them the 737-8 and 737-10...no MAX mention.

Both of those are the Max models.
The NG models are labelled-700, -800, -900 etc
A 737 labelled 737-8,-9, -10 are all Max models

[Maninthebar]

Both of those are the Max models.
The NG models are labelled-700, -800, -900 etc
A 737 labelled 737-8,-9, -10 are all Max models

Any attempt by Boeing to obscure the 'build' of the aircraft will only deepen the loss of trust in the overall 'brand' in my view.

[Kerosene Kraut]

MAX always was a pure marketing name. Certification, sims and such always had 737-8 written in or painted on. No surprise that marketing won't use MAX anymore.

[Smythe]

SWA goes rogue...

Count Jon Weaks (https://www.bizjournals.com/chicago/search/results?q=Jon%20Weaks), the powerful and tell-it-like-it-is president of Southwest Airlines’ (https://www.bizjournals.com/profile/company/org_ch_5b9ef60278d289fa027f565d5e9e27c9) Pilots Association (SWAPA), as pretty fed up with Boeing Co. and the aircraft manufacturer’s handling — so far — of the far-from-resolved 737 MAX crisis.

That was made abundantly clear in a new memo distributed on Wednesday to the more than 8,500 pilots who fly for Southwest Airlines. The Chicago Business Journal obtained a copy of the lengthy memo.

Southwest is the world’s largest operator of the MAX aircraft with 34 in its fleet and hundreds more on order. The MAX was abruptly grounded in the United States on March 13, after two fatal crashes of the MAX within a five-month period that killed nearly 350 people. A Lion Air MAX crashed last October, and an Ethiopian Airlines MAX crashed in February.

Weaks’ new memo to SWAPA pilots makes abundantly clear his mounting frustration with the conflicting information coming out about the MAX and its possible return to service.

Right at the top of his new memo Weaks notes: “With the confusing information coming from the Federal Aviation Administration (https://www.bizjournals.com/profile/company/org_ch_d9230460b953005a89488c8b30b2e2ba) (FAA), national and geopolitics and Chicago-based Boeing’s (NYSE: BA) continued missteps, there is no accurate estimate of when the MAX will return to service. Present projections range from September to December.”

But Weaks was particularly critical of the MAX manufacturer in his missive: “Boeing seems to receive more bad news with every passing week, and still needs to learn how to rebuild trust as well as the airplane. Boeing failed to disclose MCAS initially, failed to build in redundancy, and failed to notify the FAA of issues related to MCAS in a timely manner.” MCAS is the software built into the MAX that is believed to have been a key factor in both fatal crashes of the aircraft.

Though Boeing CEO Dennis Muilenberg publicly apologized for the crashes in a recent interview with CBS News, Weaks still doesn’t like some of the airplane manufacturer’s tactics behind the scenes.

In his memo Weaks said: “In March they (Boeing) expectedly, but reprehensibly, asked to have the venue switched from the United States District Court for the Northern District of Illinois to Indonesia in order to settle the Lion Air Flight 610 accident for minimal amounts. If Boeing’s tactic succeeds, the cases for the families become nearly worthless and a similar strategy might be attempted for the Ethiopian accident as well."

As Weaks surveys where things stand at this juncture in the MAX crisis, he told his SWAPA pilots on Monday he doesn’t think Boeing is anywhere near free and clear yet.

“While there are still questions and issues with each respective airline’s management, training and flight crews, Boeing still has substantial responsibility and liability and will undoubtedly face many legal issues, civilly and perhaps criminally in the United States federal court system," Weaks said. "A requested change of venue (for lawsuits) only exacerbates and continues to harm Boeing’s image and trustworthiness in the eyes of the public and Congress.”

[futurama]

SWA goes rogue...
SWAPA, not SWA.

SWAPA is maneuvering to get reimbursements from Boeing, mostly for legal expenses the union has to pay to outside attorneys in support of the various MAX probes, hence there's incentive to make a lot of noise right now. SWAPA is also talking about getting compensation for SWA pilots for lost hours, but that's likely a dead end.

SWA, on the other hand, have incentive to negotiate privately, while publicly supporting Boeing. In fact, their CEO reaffirmed SWA's commitment to buy "hundreds more" 737 MAX planes.

[Smythe]

Of course, and therefore, there is nothing of substance in what he states?

[WHBM]

Now we may remember Boeing taking the C-Series/Delta deal to law to try and stop it, on the basis that it was too cheap for international trade and thus Dumping. Is Airbus going to do the same in reprisal against the Boeing Max deal with IAG ? The circumstances do look similar. And an anti-Dumping action will require the price negotiated to be revealed in court.

https://www.reuters.com/article/us-france-airshow-boeing/airbus-demands-chance-to-bid-for-iags-surprise-boeing-narrow-body-order-idUSKCN1TL0S1

[futurama]

Of course, and therefore, there is nothing of substance in what he states?

​​​​​​​Look, his real point and complaint is about where the LionAir lawsuits will be consolidated -- in US Federal Court or in Indonesia. You know, the part you yourself bolded.

Was Southwest Airlines or SWAPA involved in the LionAir crashed in any possible way? No. Then why is he complaining so much about it?

It's not because anyone gives a **** about the LionAir victim families. There have been many crashes in Indonesia over the years and SWAPA never before gave a damn where the lawsuits were resolved.

He's complaining now because having the lawsuit in the US is better for SWAPA as they try to seek compensation from Boeing.

And personally I find that self serving and rather distasteful.

[yoko1]

He's complaining now because having the lawsuit in the US is better for SWAPA as they try to seek compensation from Boeing.



Any legal basis SWA or its union has for damages against Boeing for the grounding of the MAX is entirely independent of the jurisdiction for those seeking damages related to the accidents. Airlines will have recourse based on the purchasing agreements they signed. It is highly unlikely affected employees will have any legal standing to sue for lost wages.

[futurama]

Now we may remember Boeing taking the C-Series/Delta deal to law to try and stop it, on the basis that it was too cheap for international trade. Is Airbus going to do the same in reprisal against the Boeing Max deal with IAG ? The circumstances do look similar.

https://www.reuters.com/article/us-france-airshow-boeing/airbus-demands-chance-to-bid-for-iags-surprise-boeing-narrow-body-order-idUSKCN1TL0S1
The big difference is we don't know how much IAG will pay for each MAX, to determine whether the amount will be below market value.

In the CSeries case, it was clear cut. Boeing was able to "reverse engineer" how much Delta paid for each CS300 from Bombardier's own financial statements. Not to mention the fact that Bombardier had to take a big accounting write-off to support the Delta deal. I mean, who does that.

Hence, Bombardier had to change tactics and pledge to build the CS300 in Alabama. But that was pretty much the end of the program for Bombardier. Given the order book there was no way for them to assemble the CSeries from two factories in two countries and still be profitable.

I have seen no allegation from Airbus or anyone else that Boeing is selling the MAX below cost.

[armchairpilot94116]

Interesting to see the 2014 report on the 787 again... The 737 isnt broken... Boeing is :(

https://www.youtube.com/watch?v=rvkEpstd9os

First Boeing said there was nothing wrong with the Max
Then they suggested improper pilot operation.
Then they had to admit maybe something wasn’t the best and suggested a little tweeking of the software code was all that was needed making a safe plane safer they said.
Then they admitted a small oops with the design but again nothing good piloting couldn’t fix plus a little tweak of the software and maybe a small pilot updating.
Then they basically kept their mouth shut and are actively trying to mitigate damage workdwide.
Such as moving out of US courts.

The Max has a major flaw that is clear.
The FAA is not looking pretty on the Max, that is clear.

Boeing is trying to limit monetary damage right and left that is clear.
What is not clear is the promised 100pxt fix.

Where is the fix? Does it work?

Will it turn the Max back to a race horse from the mad bull that it is now a bull that kills.

[Fly Aiprt]

What is not clear is the promised 100pxt fix.

Where is the fix? Does it work?


There is no video showing how controlling the fixed MAX is a breeze in the sim.
There is no video showing how manually trimming a fixed MAX in real flight is easy.
There is no video showing Boeing top executives on board a flight test simulating an AOA vane failure.

Maybe there is no fix ?

Or maybe there is more to it than just the MCAS ?

[Smythe]

Wasnt part of the Ryanair deal, that they would always get the lowest cost?

[MemberBerry]

MAX always was a pure marketing name. Certification, sims and such always had 737-8 written in or painted on. No surprise that marketing won't use MAX anymore.

That is factually incorrect. The flight simulators were clearly branded "737 MAX":

And that's not surprising at all since the 737-8 is only one of the MAX variants.

https://news.files.bbci.co.uk/include/extra/shorthand/assets/news/sd9LGK2S9m/assets/kW9A3vDiJX/img_2638-1000x1000.jpeg

[Bend alot]

Why is the reason that Boeing rescinded it's invitation to the Allied Pilots Association's MAX simulator session for the 5th June?

Hardly a good way to gain trust or show transparency.

[yoko1]

Why is the reason that Boeing rescinded it's invitation to the Allied Pilots Association's MAX simulator session for the 5th June?

Hardly a good way to gain trust or show transparency.

It's probably because APA secretly recorded a private meeting with Boeing officials a few months back and then gave excerpts to a local newspaper.

[Water pilot]

​​​​​​​Look, his real point and complaint is about where the LionAir lawsuits will be consolidated -- in US Federal Court or in Indonesia. You know, the part you yourself bolded.

Was Southwest Airlines or SWAPA involved in the LionAir crashed in any possible way? No. Then why is he complaining so much about it?

It's not because anyone gives a **** about the LionAir victim families. There have been many crashes in Indonesia over the years and SWAPA never before gave a damn where the lawsuits were resolved.

He's complaining now because having the lawsuit in the US is better for SWAPA as they try to seek compensation from Boeing.

And personally I find that self serving and rather distasteful.
How many of those crashes involved design flaws in Boeing aircraft? If somebody in Canada crashes a Nissan pickup because they were drunk then I don't give an *** about it, but if somebody in Canada is driving a Nissan pickup and the wheels fall off due to a design flaw, I do since I drive one.

[Smythe]

Branded Max trainer MAX and 8...

https://cimg4.ibsrv.net/gimg/pprune.org-vbulletin/2000x1504/img_0222_ed2a6a365161e5480a9fd8c2fa5cfb8fc8eb9b49.jpg

Boeing Campus in Miami/Singapore (not branded) 737-8

https://cimg4.ibsrv.net/gimg/pprune.org-vbulletin/1200x800/33027_o_52ed863a347ca959b637dabb53d89c248c39d574.jpg

[jantar99]

Branded Max trainer MAX and 8...
Boeing Campus in Miami/Singapore (not branded) 737-8

This "not branded" on the last pic clearly says MAX on its display in big friendly letters :)

https://cimg4.ibsrv.net/gimg/pprune.org-vbulletin/507x695/clipboard01_21ce5909a519d684227880c2454d7ab1f92ec08e.jpg

[DaveReidUK]

This "not branded" on the last pic clearly says MAX on its display in big friendly letters :)

I'm not sure what we're trying to prove here.

Nobody is disputing that "MAX" is a (now dumped) brand name, so up to now it's been reasonable to find it in all the places you would expect to see branding. It has never carried any more weight than that, and it will be increasingly hard to find traces of it in future.

[Bend alot]

It's probably because APA secretly recorded a private meeting with Boeing officials a few months back and then gave excerpts to a local newspaper.
The excerpts that imply Boeing is not exactly honest/trustworthy?

APA is not a small group and a very good chance they would be gifted as much time as they want in one of the very few 737 MAX simulators in the world.

There are rumours that the simulators on the NG & MAX under-simulate in elevator trim area, so better work with your enemy than start a war when your wounded.

It would be reasonable to assume that the recorded private meeting with Boeing was also presented to the FAA - question is when was it presented to the FAA?

Before or after subpoena. With the number of copy's and transcripts of that meeting now in circulation, I expect it will be pretty public soon.

[edmundronald]

The excerpts that imply Boeing is not exactly honest/trustworthy?

APA is not a small group and a very good chance they would be gifted as much time as they want in one of the very few 737 MAX simulators in the world.

There are rumours that the simulators on the NG & MAX under-simulate in elevator trim area, so better work with your enemy than start a war when your wounded.

It would be reasonable to assume that the recorded private meeting with Boeing was also presented to the FAA - question is when was it presented to the FAA?

Before or after subpoena. With the number of copy's and transcripts of that meeting now in circulation, I expect it will be pretty public soon.


As of two days ago the recording and transcript had been handed to the congressional committee, so I guess it is now in the public record.

Edmund

[Bend alot]

As of two days ago the recording and transcript had been handed to the congressional committee, so I guess it is now in the public record.

Edmund

Mr Mitchell I am sure will allow everyone to hear the recording, and read the transcript - unless it is damaging! (around the 1 hour mark of the hearing).

He should also look up the ICAO training and experience requirements, and remember the USA is a signatory to ICAO. At least one congressman learnt from the last hearing, but he was clearly outclassed by the two captains that spoke well and understandable to general public and congress.

It seems Boeing and FAA have not provided anything to the hearing as of two days ago, certainly building that trust issue is a high priority or is it not?

[Smythe]

Branded meaning the sim itself is specifically in airline paint like the Air Canada or Ethiopian Airline versions shown.

Unfortunately like 'drones' has stuck for unmanned platforms, it will be years before the MAX is gone...

[MemberBerry]

Unfortunately like 'drones' has stuck for unmanned platforms, it will be years before the MAX is gone...

Yes, but first Boeing would have to choose a new name for it. I propose they call it the 737 FU (Final Update).

Now, joking aside, they said they have considered changing the name, and they are not ruling that out yet, but currently they don't have a plan to change it:

Boeing is open to changing the name of the 737 Max - FOX 40 WICZ TV - News, Sports, Weather, Contests & More (http://www.wicz.com/story/40666519/boeing-is-open-to-changing-the-name-of-the-737-max):

Boeing cautioned that it has no plans at this time to change the name, and that it is focused on the safe return of the aircraft to service.

So not sure where the information that the MAX brand name was dumped came from.

Of course, most airlines will avoid using MAX as the name publicly for a while, fearing the reaction of the PAX. But it's debatable if Boeing needs to drop it as well and change it to something else. After all Boeing sells aircraft to airlines, not to PAX, and I doubt the current name would affect buying decisions.

[Bend alot]

Given the back log of orders for the MAX 2024-25 for a delivery and no firm orders at Paris for any 737 MAX - seems airlines have issues with the name.

Most public opinion on the MAX will be well forgotten by 2024, or are airlines concerned it will still be grounded by then?

[MemberBerry]

Well, at the Paris air show the balance was 200 B737 MAX vs 234 A320Neo, so just 17% more for Airbus.

I think Boeing losing trust as a company could be a more important factor than the MAX name. After the recent revelations about Boeing hiding issues with their aircraft for years, from both the airlines and the FAA, I wouldn't blame airlines for thinking "What else is Boeing hiding from us? Is it worth the risk to keep buying from them when there is an alternative?"

Later edit: I just noticed you mentioned no firm orders. Indeed there were no firm MAX orders, but orders that were not firm represented 85% of the total number of aircraft ordered at Paris. So, with 4 MAX orders of 50 aircraft each, statistically there was about an 50-50 chance that one of those orders would be firm (0.85 to the power of 4 is about 0.52, so the probability that none of the 4 orders are firm would be around 52%). So it could be just a fluke.

[Kerosene Kraut]

200 was some LoI only no firm order yet. However they mentioned delivery dates. Wonder how long they will wait to sign?

[Zeffy]

https://www.seattletimes.com/seattle-news/times-watchdog/the-inside-story-of-mcas-how-boeings-737-max-system-gained-power-and-lost-safeguards/

The inside story of MCAS: How Boeing’s 737 MAX system gained power and lost safeguards
June 22, 2019 at 2:00 pm Updated June 22, 2019 at 2:43 pm
By Dominic Gates and Mike Baker
Seattle Times staff reporters

Early in the development of the 737 MAX, engineers gathered at Boeing’s transonic wind tunnel in Seattle to test the jet’s aerodynamics using a scale model with a wingspan comparable to that of an eagle.

The testing in 2012, with air flow approaching the speed of sound, allowed engineers to analyze how the airplane’s aerodynamics would handle a range of extreme maneuvers. When the data came back, according to an engineer involved in the testing, it was clear there was an issue to address.

Engineers observed a tendency for the plane’s nose to pitch upward during a specific extreme maneuver. After other efforts to fix the problem failed, the solution they arrived at was a piece of software — the Maneuvering Characteristics Augmentation System (MCAS) — that would move a powerful control surface at the tail to push the airplane’s nose down.

This is the story, including previously unreported details, of how Boeing developed MCAS, which played a critical role in two airliners nose-diving out of the sky, killing 346 people in Ethiopia and off the coast of Indonesia.

Extensive interviews with people involved with the program, and a review of proprietary documents, show how Boeing originally designed MCAS as a simple solution with a narrow scope, then altered it late in the plane’s development to expand its power and purpose. Still, a safety-analysis led by Boeing concluded there would be little risk in the event of an MCAS failure — in part because of an FAA-approved assumption that pilots would respond to an unexpected activation in a mere three seconds.

The revised design allowed MCAS to trigger on the inputs of a single sensor, instead of two factors considered in the original plan. Boeing engineers considered that lack of redundancy acceptable, according to proprietary information reviewed by The Seattle Times, because they calculated the probability of a “hazardous” MCAS malfunction to be virtually inconceivable.

As Boeing and the FAA advanced the 737 MAX toward production, they limited the scrutiny and testing of the MCAS design. Then they agreed not to inform pilots about MCAS in manuals, even though Boeing’s safety analysis expected pilots to be the primary backstop in the event the system went haywire.

In the wake of the two crashes, despite an outcry from the public and from some pilot and airline industry officials, Boeing has defended the processes behind its MCAS design decisions and refused to accept blame.

The grounding of the MAX has entered its 15th week. Safety officials around the world are scrutinizing the changes to MCAS that Boeing has proposed to ensure such accidents won’t happen again. And they are assessing what training pilots may need on the new system.

“Safety is our top priority,” Boeing said in a statement. “Through the work we are doing now in partnership with our customers and regulators to certify and implement the software update, the 737 MAX will be one of the safest airplanes ever to fly.”

This investigation examines what’s known about the origins and operation of MCAS ahead of the final official accident-investigation reports, expected late this year for Lion Air Flight 610 and next year for Ethiopian Airlines Flight 302.

Wind-tunnel and simulator tests
Though Boeing was locked into a plan to revamp its popular 737 model, the Seattle wind-tunnel tests in 2012 revealed a problem.

During flight tests to certify an airplane, pilots must safely fly an extreme maneuver, a banked spiral called a wind-up turn that brings the plane through a stall. While passengers would likely never experience the maneuver on a normal commercial flight, it could occur if pilots for some reason needed to execute a steep banking turn.

Engineers determined that on the MAX, the force the pilots feel in the control column as they execute this maneuver would not smoothly and continuously increase. Pilots who pull back forcefully on the column — sometimes called the stick — might suddenly feel a slackening of resistance. An FAA rule requires that the plane handle with smoothly changing stick forces.

The lack of smooth feel was caused by the jet’s tendency to pitch up, influenced by shock waves that form over the wing at high speeds and the extra lift surface provided by the pods around the MAX’s engines, which are bigger and farther forward on the wing than on previous 737s.

This was verified in early simulator modeling, with planes tested in scenarios at about 20,000 feet of altitude, according to one of the workers involved.

While the problem was narrow in scope, it proved difficult to cope with. The engineers first tried tweaking the plane’s aerodynamic shape, according to two workers familiar with the testing. They placed vortex generators — small metal vanes on the wings — to help modify the flow of air, trying them in different locations, in different quantities and at different angles. They also explored altering the shape of the wing.

Two people familiar with the discussions said 737 MAX chief test pilot Ray Craig preferred such a physical solution to solve the plane’s aerodynamics. Philosophically, Boeing had long opposed efforts to create automated actions such as a stick-pusher — a device used on some aircraft that without pilot action pushes the control column forward to lower the jet’s nose — that would seize control of a situation from the pilot, according to one of the people.

But the aerodynamic solutions didn’t produce enough effect, the two people said, and so the engineers turned to MCAS.

It was simple in concept but powerful in effect, quickly solving the issue.

In the midst of a wind-up turn, the software would automatically swivel up the leading edge of the plane’s entire horizontal tail, known as the horizontal stabilizer, so that the air flow would push the tail up and correspondingly push the nose down.

As the pilot pulled on the control column, this uncommanded movement in the background would counter the jet’s tendency to pitch up and smooth out the feel of the column throughout the maneuver.

An engineer recalled Craig testing MCAS for the first time in the simulator.

“Yeah! This is great,” Craig gushed after seeing how MCAS responded, according to the engineer. (Craig left Boeing before the operation of MCAS was revised.)

This original version of MCAS, according to two people familiar with the details, was activated only if two distinct sensors indicated such an extreme maneuver: a high angle of attack and a high G-force.

Angle of attack is the angle between the wing and the oncoming air flow. G-force is the plane’s acceleration in the vertical direction.

How much MCAS moved the tail when activated was a function of the angle of attack and the jet’s speed, said one of the people familiar with the MCAS design who, like many of the sources in this story, asked for anonymity because of the sensitivity of ongoing investigations.

The fix didn’t stir much controversy.

Another Boeing plane, the KC-46 Air Force tanker, has a software-driven system that similarly moves the stabilizer in a wind-up turn and even has the same MCAS name, though the design is very different.

Boeing’s failure analysis
When Boeing was ready to certify the 737 MAX, it laid out its plan for MCAS in documents for the FAA.

Under the proposal, MCAS would trigger in narrow circumstances. It was designed “to address potentially unacceptable nose-up pitching moment at high angles of attack at high airspeeds,” Boeing told the FAA in a proprietary System Safety Assessment reviewed by The Times.

In a separate presentation made for foreign safety regulators that was reviewed by The Times, Boeing described MCAS as providing “a nose down command to oppose the pitch up. Command is limited to 0.6 degrees from trimmed position.”

Two people involved in the initial design plans for MCAS said the goal was to limit the system’s effect, giving it as little authority as possible. That 0.6-degree limit was embedded in the company’s system safety review for the FAA.

The Boeing submission also included an analysis that calculated the effect of possible MCAS failures, with each scenario characterized as either a minor, a major or a hazardous failure — increasingly severe categories that determine how much redundancy must be built in to prevent the event.

Virtually all equipment on any commercial airplane, including the various sensors, is reliable enough to meet the “major failure” requirement, which is that the probability of a failure must be less than 1 in 100,000.

A “major failure” is not expected to produce any serious injuries and is defined more as something that would increase the cockpit crew’s workload. Such systems are therefore typically allowed to rely on a single input sensor.

Boeing analyzed what would happen if, in normal flight mode, MCAS triggered inadvertently up to its maximum authority and moved the horizontal stabilizer the maximum 0.6 degrees.

It also calculated what would happen on a normal flight if somehow the system kept running for three seconds at its standard rate of 0.27 degrees per second, producing 0.81 degrees of movement, thus exceeding the supposed maximum authority.

Why three seconds? That’s the period of time that FAA guidance says it should take a pilot to recognize what’s happening and begin to counter it.

Boeing assessed both of these failure modes as “major.” Finally, the analysis looked at the inadvertent operation of MCAS during a wind-up turn, which was assessed as “hazardous,” defined in a cold actuarial analysis as an event causing serious or fatal injuries to a small number of people, but short of losing the plane (that’s called “catastrophic”).

Hazardous events typically demand more than one sensor — except when they are outside normal flight conditions and unlikely to be encountered, such as a wind-up turn.

According to a document reviewed by The Seattle Times, Boeing’s safety analysis calculated this hazardous MCAS failure to be almost inconceivable: Given the improbability of an airliner experiencing a wind-up turn, compounded by the unlikelihood of MCAS failing while it happened, Boeing came up with a probability for this failure of about once every 223 trillion hours of flight. In its first year in service, the MAX fleet logged 118,000 flight hours.

So even though this original version of MCAS required two factors — angle of attack and G-force — to activate, Boeing’s analysis indicated that just one sensor would be acceptable in all circumstances.

In flight test, MCAS changes
About a third of the way through flight testing in 2016, as first reported by The Seattle Times in March, Boeing made substantial changes to MCAS.

The flight-test pilots had found another problem: The same lack of smooth stick forces was also occurring in certain low-speed flight conditions. To cover that issue too, engineers decided to expand the scope and power of MCAS.

Because at low speed a control surface must be deflected more to have the same effect, engineers increased the power of the system at low speed from 0.6 degrees of stabilizer nose-down deflection to 2.5 degrees each time it was activated.

On the stabilizer, maximum nose down is about 4.7 degrees away from level flight. So with the new increased authority to move the stabilizer, just a couple of iterations of the system could push it to that maximum.

Because there are no excessive G-forces at low speed, the engineers removed the G-force factor as a trigger. But that meant MCAS was now activated by a single angle of attack sensor.

One of the people familiar with MCAS’s evolution said the system designers didn’t see any need to add an additional sensor or redundancy because the hazard assessment had determined that an MCAS failure in normal flight would only qualify in the “major” category for which the single sensor is the norm.

“It wasn’t like it was there to cover some safety or certification requirement,” the person said. “The trigger isn’t a safeguard. It tells (the system) when to operate.”

While the changes were dramatic, Boeing did not submit documentation of the revised system safety assessment to the FAA.

An FAA spokesman said the safety agency did not require a new system safety analysis because it wasn’t deemed to be critical.

“The change to MCAS didn’t trigger an additional safety assessment because it did not affect the most critical phase of flight, considered to be higher cruise speeds,” he said.

The person familiar with the details of MCAS’ evolution said Boeing did the extra analysis of the new low-speed, higher-authority changes. He said the effect of the potential failures at low speed was less, and so didn’t add any risk to the prior analysis. So the documents sent to the FAA with the failure analysis were not revised.

“You turn in the answer,” he said. “You don’t have to document all your work.”

MCAS as it was actually implemented differed in another way from what was described in the safety analysis turned in to the FAA.

The failure analysis didn’t appear to consider the possibility that MCAS could trigger repeatedly, as it did on both accident flights. Moving multiple times in 0.6 or 2.5 increments depending on the speed, it effectively had unlimited authority if pilots did not intervene.

Discussions around this new MCAS design appear to have been limited during flight testing.

Two former Boeing test pilots described a culture of pressure inside the company to limit flight testing, which can delay projects at a time when orders are stacking up, costing the company money.

Matt Menza, a different pilot who did test flights on the MAX, recalled times when test pilots at Boeing would have the chance to thoroughly examine systems in what he called a “system-safety murder board” to explore all the potential failures. But he reported that the general corps of test pilots didn’t have a lot of technical details about the MCAS design, such as the single-sensor input.

Boeing never flight-tested a scenario in which a broken angle-of-attack sensor triggered MCAS on its own, instead relying on simulator analysis, according to a person familiar with the process. One of the former test pilots expressed bewilderment that the angle-of-attack failure was never explored in the air.

A variety of employees have described internal pressures to advance the MAX to completion, as Boeing hurried to catch up with the hot-selling A320 from rival Airbus.

Mark Rabin, an engineer who did flight-testing work unrelated to the flight controls, said there was always talk about how delays of even one day can cost substantial amounts. Meanwhile, staff were expected to stay in line, Rabin said.

“It was all about loyalty,” Rabin said. “I had a manager tell me, ‘Don’t rock the boat. You don’t want to be upsetting executives.’”

Do pilots need more training?
Boeing’s system safety analysis of MCAS, in working out the failure probabilities, assumes that the pilots will take steps in response to anything that arises, and will do so quickly.

The pilots’ struggles to control their planes before both MAX crashes suggest that the FAA’s three-second guidance for expected pilot response time, upon which part of Boeing’s system safety analysis was based, needs to be carefully reassessed.

“If the three seconds is not an appropriate amount of time to be able to catch a runaway stabilizer, and it actually takes seven seconds, then … we need to understand that,” said the person familiar with the details of MCAS.

When MCAS is activated in the cockpit and moves the horizontal stabilizer, a large wheel beside each pilot that’s mechanically connected to the stabilizer begins to spin. This is the manual trim wheel. As a last resort to stop a stabilizer moving uncommanded, a pilot can grab and hold the wheel.

In a 737 MAX cockpit console, the black wheels on each side are connected to the horizontal tail and will spin if the stabilizer swivels. The instruments next to them have green indicators showing the angle of the stabilizer trim, with 0 being maximum nose-down. The two switches at bottom right, labeled “STAB TRIM,” are the cutoff switches that will end automated movement of the horizontal tail.
(Dimas Ardian / Bloomberg)

In a 737 MAX cockpit console, the black wheels on each side are connected to the horizontal tail and will spin if the stabilizer swivels. The instruments... (Dimas Ardian / Bloomberg) More
The person familiar with MCAS said the wheel will spin noisily and fast, 30 or 40 times, for each activation. Meanwhile the stabilizer movement will increase the force needed to hold the control column, by about 40 to 50 pounds for a 2.5 degree movement. Such uncommanded movement that won’t stop is referred to as a “runaway stabilizer.”

Boeing has said that to deal with this, pilots need first to have basic hand-flying skills — pull the nose up to where you want it, then use the thumb switches on the yoke that connect electrically to the stabilizer to neutralize the forces — and then shut off MCAS with a pilot checklist procedure on how to handle a “runaway stabilizer.”

However on both accident flights, the angle-of-attack sensor failure set off multiple alerts causing distraction and confusion from the moment of takeoff, even before MCAS kicked in.

On the Ethiopian Airlines flight, for example, a “stick shaker” noisily vibrated the pilot’s control column throughout the flight, warning the plane was in danger of a stall, which it wasn’t; a computerized voice repeating a loud “Don’t sink!” warned that the jet was too close to the ground; a “clacker” making a very loud clicking sound signaled the jet was going too fast; and multiple warning lights told the crew that the speed, altitude and other readings on their instruments were unreliable.

Exactly what pilot training for MCAS is appropriate has become a big issue that threatens to prolong the grounding of the MAX.

While the FAA and U.S. airlines seem ready to clear the plane to fly with just iPad training for American pilots on the MCAS fixes, some foreign regulators want more intensive simulator training for all pilots on how to handle a runaway stabilizer.

Early in the process of selling the MAX, according to two people familiar with the discussions, Boeing promised to give Southwest Airlines a substantial rebate for every plane if the MAX required simulator training.

One former MAX worker, Rick Ludtke, said the rebate reported to him by managers was $1 million per plane, a figure another Boeing employee indicated is roughly accurate.

A Southwest spokesperson said, “We do not discuss publicly the specific details of our contractual agreements,” but added that “the purchase of an aircraft is a significant investment, and guarantees for various items … are incorporated into every 737 contract.”

Ludtke and two other former workers described internal pressures during the MAX certification to avoid any changes to the design of the plane that might cause the FAA to lean toward a simulator mandate.

It became a significant point of attention for Michael Teal, the 737 MAX program manager, and Keith Leverkuhn, vice president and general manager of the 737 MAX program, according to a person involved in the discussions. They felt confident based on past experience that the MAX would be approved without simulator training, but they were wary, according to the worker.

Meanwhile, Boeing’s chief technical pilot on the MAX, Mark Forkner, was also facing pressure, according to another person involved in the project. The person recalled Forkner as frequently anxious about the deadlines and pressures faced in the program, going to some of his peers in the piloting world for help.

As first reported by The New York Times, Forkner suggested to the FAA that MCAS not be included in the pilot manual, according to a person familiar with the discussions.

“Mark never dreamed anything like this could happen,” said Forkner’s attorney, David Gerger. “He put safety first – at this job and in the Air Force.”

U.S. pilot unions have expressed concern at the omission of MCAS from the manual. One reason is that when MCAS activates, it changes somewhat the response of the airplane.

For example, there is a cutout switch in the control column so that when a pilot pulls or pushes in the opposite direction to a runaway stabilizer, it cuts electric power to the stabilizer. When MCAS is active, this cutout switch doesn’t work, which could surprise a pilot who didn’t know about the system.

Boeing ultimately won the FAA’s approval to give pilots just an hour of training through an iPad about the differences between the MAX and the previous 737 generation. MCAS was not mentioned.

The FAA, after internal deliberations, also agreed to keep MCAS out of the manual, reasoning that MCAS was a software code that operates in the background as part of the flight-control system, according to an official familiar with the discussions.

Two angle of attack sensors that influence the MCAS system are located on each side and below other instruments on the 737 MAX. Boeing chose to use only one angle of attack sensor at a time while the plane is flying. (Mike Siegel / The Seattle Times)

A single sensor
Boeing has avoided accepting direct blame in public, saying MCAS was only one link in a chain of events. Its leaders have also said MCAS was designed according to the standard procedures it has used for years.

“The 737 MAX was certified in accordance with the identical FAA requirements and processes that have governed certification of previous new airplanes and derivatives. The FAA considered the final configuration and operating parameters of MCAS during MAX certification, and concluded that it met all certification and regulatory requirements,” Boeing said in a statement.

The most controversial detail of the MCAS design has been the reliance on a single angle-of-attack sensor. On both of the deadly flights, everything started with a faulty sensor. In the second crash in Ethiopia, the data trace strongly suggests that the sensor was destroyed in an instant, likely by a bird strike.

There are two such sensors, one on either side of the fuselage. Why didn’t Boeing, especially after discarding the G-force as a trigger, use both angle-of-attack sensors?

The thinking was that requiring input from two angle-of-attack sensors would mean that if either one failed the system would not function.

That has implications not only for safety but for airline costs. If the system is down, a pilot might fly into a situation where it’s needed and find it unavailable. Or the airline might have to take the plane out of service and lose money.

Both factors point toward a principle of not adding complexity: Keep a system as simple as possible.

“You don’t want to disrupt your customer’s operations,” said the person familiar with the MCAS details. And you don’t want to “increase the risk that the system fails when you need it.”

In this case, as simple as possible meant as minimal as the safety regulations allow. Since Boeing’s system safety analysis concluded that one sensor was acceptable, that’s what it went with.

But that’s not the logic followed for a system on the KC-46 Air Force tanker, also called MCAS.

Boeing says the MCAS systems on the MAX and on the tanker share only a name and a similar function, and have completely different avionics.

But they both move the horizontal stabilizer to smooth the pilot stick forces in a wind-up turn. Their basic design architecture can be compared to some extent.

Air Force spokeswoman Ann Stefanek says “MCAS on the KC-46 has two sensors and the system compares the two readings.”

Boeing’s proposed update to MCAS for the MAX will have the same.

Last Sunday at the Paris Air Show, Boeing CEO Dennis Muilenburg reiterated the company’s position that while the original MCAS was properly designed, “we know we can improve it.”

The fixes include relying on two sensors rather than one, limiting MCAS to one rather than multiple activations, and revising the software.

“We are confident that they will result in a safe airplane, one of the safest airplanes ever to fly, and that MCAS will not contribute to a future accident,” he said.

Dominic Gates: 206-464-2963 or [email protected]; on Twitter: @dominicgates.
Mike Baker: 206-464-2729 or [email protected]; on Twitter: @ByMikeBaker.

[joe_bloggs]

Australian ABC reporting -

Boeing sued by more than 400 pilots in class action over 737 MAX's 'unprecedented cover-up'

https://www.abc.net.au/news/2019-06-23/over-400-pilots-join-lawsuit-against-boeing-over-737-max/11238282

[gums]

Salute!

Best damn summary to date.
Note MCAS not described as “stall prevention”

Gums sends....

[Loose rivets]

Hazardous events typically demand more than one sensor — except when they are outside normal flight conditions and unlikely to be encountered, such as a wind-up turn.


"Hazardous", as most know is a specific category.

In this detailed summary there were perhaps four significant things I was unaware of. The above is one of them. It seems a small factor, but when you think what the term's qualifiers were, it becomes a major factor - inasmuch that if that qualifier wasn't there this thread might never have existed.

The article also gives several hints of how lives within the corporation must have been affected. Profoundly so.

[KRUSTY 34]

Hubris, Technical expertise subjugated by corporate culture, a disfunctional Regulator,

It’s nothing short of a Greek Tragedy.

[billybone]

Salute!

Best damn summary to date.
Note MCAS not described as “stall prevention”

Gums sends....

pull up tiday ( 22 June ) seattle times article for more facts and data by those who were involved !

[Bend alot]

Well, at the Paris air show the balance was 200 B737 MAX vs 234 A320Neo, so just 17% more for Airbus.

I think Boeing losing trust as a company could be a more important factor than the MAX name. After the recent revelations about Boeing hiding issues with their aircraft for years, from both the airlines and the FAA, I wouldn't blame airlines for thinking "What else is Boeing hiding from us? Is it worth the risk to keep buying from them when there is an alternative?"

Later edit: I just noticed you mentioned no firm orders. Indeed there were no firm MAX orders, but orders that were not firm represented 85% of the total number of aircraft ordered at Paris. So, with 4 MAX orders of 50 aircraft each, statistically there was about an 50-50 chance that one of those orders would be firm (0.85 to the power of 4 is about 0.52, so the probability that none of the 4 orders are firm would be around 52%). So it could be just a fluke.
All 200 were announced by the parent of the 4 listed entities (50 each), so the single LoI is for 200 aircraft - a single deal not 4 deals.

That changes the stats somewhat.

Airbus is the current type they use and Airbus did not get a chance to bit on this "order" - they still hope to. No doubt the MAX LoI will be attractive and Airbus will know this if negotiations happen for a competing bid for the 200 aircraft. It is similar to SWA position, do we mix the fleet or stay standardised.

* Krusty - yes very careful not to use the words "Stall Prevention"!
Risk assessment seems to have been a little off by the manufacturer - an area externals should be used, so shades of grey are not called white every time. It seems very sloppy and too many commercial pressures applied from elevated levels.

[edmundronald]

Who knew all about MCAS?

Boeing's chief test pilot of the time knows all the plane's weaknesses. In the interest of public safety he should be given immunity and extensively debriefed about the plane and the certification process so that both the airframe and the FAA can be fixed.

Edmund

[Bend alot]

Edmund, the trend seems to be that - things may have been "hidden" from lots of people and departments, that could include the Chief Test Pilot.

No do not do that we have simulated it and it is fine - fly this series of tests.

Hurry up we need to meet the deadline on this.

[568]

Edmund, the trend seems to be that - things may have been "hidden" from lots of people and departments, that could include the Chief Test Pilot.

No do not do that we have simulated it and it is fine - fly this series of tests.

Hurry up we need to meet the deadline on this.Agree totally, deadlines were a part of this whole debacle!The MAX was required to complete stall tests, just like other aircraft in the "experimental" stage of it's life.The pitch up tendency during MAX stall tests were documented, along with control control column force, by the test pilots at the earliest time, but MCAS wasn't a part of the original design until the test and wind tunnel data were analyzed and confirmed. What happened after the introduction of MCAS in the MAX and it's path through the "Boeing and FAA system" will be a part of the continuing investigations.

[fizz57]

" Then they agreed not to inform pilots about MCAS in manuals, even though Boeing’s safety analysis expected pilots to be the primary backstop in the event the system went haywire."

Pathetic.

[Kerosene Kraut]

Let's assume the MAX would need a different type certificate in the future. What would this mean in real life for return into service? How fast would new flight simulators be available? How long would it take about altogether?

[Bend alot]

Let's assume the MAX would need a different type certificate in the future. What would this mean in real life for return into service? How fast would new flight simulators be available? How long would it take about altogether?
Not sure how long it would take - but I lost a family of friends in a aircraft crash many years ago - tears still flow today.

Question is why are there only 4 simulators for the MAX at 2 customer locations? = another bad decision based on very bad certification (allegedly) or many simulators would be available - that actually simulate the MAX and possible failures.

The issue is that Boeing seem to still be hiding very relevant information - on information they had but decided not to supply to others, including the FAA. Once that is sorted and trusted a time line maybe possible to predict.

Till that happens the MAX might fly in the USA soon due to commercial pressures - that would be a very foolish long term position for America.

* Is there any reason a NG simulator can not be programmed to be a MAX? There is very little cosmetic differences in the cockpit.

[BDAttitude]

seems to be their major line of defence, passing on the monkey to FAA for the 3s requirement. After all the input gathered here and elswhere in the community it's gonna be a massacre in court.

[GordonR_Cape]

I forsee a Pulitzer Prize nomination for Dominic Gates.

Having read all of the MAX/MCAS threads carefully, I have a few quibbles about minor details in the article. However, that does not detract from the overall clarity it brings to a complex subject.

[bill fly]

This is probably the most informative and best researched information to appear in this sorry matter.

Apart from one small confusing paragraph about the AoA on the instrument panel it is a valuable contribution - possibly more so than the accident reports will prove to be.

Perhaps the most significant lesson to be taken from the article is the assumption that within three seconds, pilots will recognise and deal with a trim runaway. That may be the case taken as a single failure but in the context of the cockpit environment in this case of MCAS activation, where the crew was already trying to deal with a rogue stall warning, in an after takeoff phase etc., it was just one more item on a loaded list and three seconds was not enough for recognition and action. Meantime the controls get heavier and MCAS repeats...

The cause of all this chaos was the same - the AoA failure. It didn’t just set off MCAS - it set off multiple spurious warnings. It did this on three known occasions and only when a third pilot was available could the situation be partially defused. That took more than three seconds too...

So when designing on probabilities of failure, it isn’t enough to examine a single system or to presume that everything else in the cockpit is normal. Some basic design rules need looking at here.

As for the company culture, others are already commenting on that aspect.

[safetypee]

bill fly :ok:

The certification reaction times have to consider each element - awareness and action, not necessarily in isolation, but in context.
There is no ‘trim runaway’ alert, thus the failure has to be deduced from other cues.
If autopilot engaged then the focus of attention could be on the unexpected autopilot disconnect, and flight path management.
Thereafter, as for manual flight, the main cue to abnormality might be stick force, inability to trim. This has to be related to the stage of flight, the flight path, and causation. In these context is critical; is the abnormality associated with configuration change, wind gusts, icing, speed, manoeuvre, all of which diverts mental attention from the main problem.

How long to read the above; more time than was assumed in certification?

[CurtainTwitcher]

So when designing on probabilities of failure, it isn’t enough to examine a single system or to presume that everything else in the cockpit is normal. Some basic design rules need looking at here.
Bill, there is much suggestion this wasn't a mistaken analysis of the single system failure. On the contrary, it was a deliberate decision, for purely financial motivations to go against all established engineering philosophy and deliberately make it a single sensor system to avoid customer airline training costs. This process was enabled by the system of Authorised Representatives (AR) - Boeing employees who certify on behalf of the FAA that the design complies with the rules. Can you imagine the immense pressure on the Boeing AR to certify the MCAS in this configuration? Upton Sinclair prophetically once wrote "It is difficult to get a man to understand something, when his salary depends upon his not understanding it!".

Nothing in this MCAS certification process was accidental or missed, the only exception was the gross underestimation of the chance of the active AoA vane failure or damage. Boeing senior management appears to have come to believe the MCAS would never be seen in the wild, therefore they could get away with it. A certification hack, for a certification problem. That assumption came with tragic loss of life, and in time, enormous costs.

[GordonR_Cape]

So when designing on probabilities of failure, it isn’t enough to examine a single system or to presume that everything else in the cockpit is normal. Some basic design rules need looking at here.

This raises an important question of 'grandfather' certification with a new model variant. When one change is made, does every other system on the aircraft need to be considered from scratch?

In retrospect this should have been done, and the facts are now beyond doubt. The regulatory details may not be clear, or subject to interpretation.

[oggers]

it was a deliberate decision, for purely financial motivations to go against all established engineering philosophy and deliberately make it a single sensor system to avoid customer airline training costs..

Okay, go ahead and explain how they saved on training costs by desigining the software such that it only used one of two alpha vanes already fitted....

[groundbum]

lets not forget B filed false paperwork with the FAA when they said trim authority was only 0.5 and thus got MCAS in under the radar as a minor software, then built it with 2.5 units of authority which turns it into a big beast! I would wonder if the 0.5 was a deliberate underestimation just to get it approved, and there was a fair suspicion from the very beginning more than 0.5 would be needed. It's like a child telling a small white lie to a parent to get what they want. Cute in a 5 year old, deadly in an engineering organisation trusted with people's lives. And if they've done it once, there's a high likelyhood imho there's other whoppers out there waiting to be found.

(apologies if the 0.5/2.5 isn't quite accurate, the numbers are buried in this thread someplace..)

G

[Bend alot]

Okay, go ahead and explain how they saved on training costs by desigining the software such that it only used one of two alpha vanes already fitted....
Using two - was deemed a higher level of certification would be required and more info needed to be given both to FAA and customers and may require more than the training wanted by Boeing.

That is just one leaked report - might be a disgruntled ex employee - seems to be a few of them.

[Bend alot]

lets not forget B filed false paperwork with the FAA when they said trim authority was only 0.5 and thus got MCAS in under the radar as a minor software, then built it with 2.5 units of authority which turns it into a big beast! I would wonder if the 0.5 was a deliberate underestimation just to get it approved, and there was a fair suspicion from the very beginning more than 0.5 would be needed. It's like a child telling a small white lie to a parent to get what they want. Cute in a 5 year old, deadly in an engineering organisation trusted with people's lives. And if they've done it once, there's a high likelyhood imho there's other whoppers out there waiting to be found.

(apologies if the 0.5/2.5 isn't quite accurate, the numbers are buried in this thread someplace..)

G
Think you will find it was 0.6 max at normal speed as "original"

It morphed to 2.5 at a double speed and repetitive.

or close to that still secret stuff.

[KRUSTY 34]

Who knew all about MCAS?

Boeing's chief test pilot of the time knows all the plane's weaknesses. In the interest of public safety he should be given immunity and extensively debriefed about the plane and the certification process so that both the airframe and the FAA can be fixed.

Edmund

Agree, but how would his legal team choose to spin it?

Our client cannot be held responsible because he could not have predicted the possible outcome. Or...,

Our client cannot be held responsible because his technical and Human Factors expertise had been overwhelmed by corporate expectations.

Mmm? Incompetence, or Ethical blindness. Tough, but obvious choice.

[fdr]

This is probably the most informative and best researched information to appear in this sorry matter.

Apart from one small confusing paragraph about the AoA on the instrument panel it is a valuable contribution - possibly more so than the accident reports will prove to be.

Perhaps the most significant lesson to be taken from the article is the assumption that within three seconds, pilots will recognise and deal with a trim runaway. That may be the case taken as a single failure but in the context of the cockpit environment in this case of MCAS activation, where the crew was already trying to deal with a rogue stall warning, in an after takeoff phase etc., it was just one more item on a loaded list and three seconds was not enough for recognition and action. Meantime the controls get heavier and MCAS repeats...

The cause of all this chaos was the same - the AoA failure. It didn’t just set off MCAS - it set off multiple spurious warnings. It did this on three known occasions and only when a third pilot was available could the situation be partially defused. That took more than three seconds too...

So when designing on probabilities of failure, it isn’t enough to examine a single system or to presume that everything else in the cockpit is normal. Some basic design rules need looking at here.

As for the company culture, others are already commenting on that aspect.

The ST story is a road map to what happened, and is the starting point for the rectification of the system that existed at that time. The 3 seconds is that noted in 25.255(a), irrespective of the opinion of that regulation being directly or indirectly related to the problem, it is the only part of the rules that gives guidance on compliance for an out of trim case, whether taht cause is by the pilots thumb as zzuf has maintained it pertains to.... The further concern on the lack of robustness of the regulatory requirement is that 25.255 requires a demonstration using a mis trim for the time, 3 seconds, at the rate applicable to the airspeed that the aircraft is at, and MCAS drilled a big hole in the middle of that assumption, it had a greater rate and authority than was envisaged with the drafting of 25.255, which did indeed arise from early jet upsets from the pilot incorrect input as much as runaway trim systems. 25.255 needs to be reinforced, and that then raises the matter of all manual trim systems being inherently compromised without special training, the matter recognised in this forum at least from the old timers experience, lost in the march of time.

3 seconds

OK, for a reject we expect the crews to react promptly, however, all up there is considerable delay for recognition, cognition and physical initiation of the response. Crews often do react promptly to the canned, pre-primed decision making and process that is routinely briefed in full before each takeoff, including a run through of the exact actions that the crew will undertake. Even so, as often as not, the crew do not respond as briefed, and the decisions occur over a longer period of time than was envisaged, and, in testament to the ingenuity of humans, the response is quite often not as briefed and trained, it is a solution determined on the day, under the stress of the moment. The DER of runways all around the world are littered with evidence of the occasions where human variability comes into play (as well as how often the performance information and analysis is flawed).

Crew response within 3 seconds for a runaway trim is inadequate. The crews in the cruise are not set up poised to hit the cutouts; crew activity routinely takes half of the crew out of the direct loop dealing with other matters beyond the flying of the aircraft, and the remaining pilot whether 200 hours or 20,000 hours is not sitting on a hair trigger response to intervene on the runaway.

Crew response to a fire warning takes an inordinate amount of time, and oddly, it takes longer in the real world events than it does in the simulator. In the real world, on the number of debriefs post such events, looking at what the crew actually did vs the QRH procedure, the crew often rationalise that the warning is false, and that they have taken a process that is different to the QRH to ensure that they do not respond to an false warning. That is admirable in saving on fire bottle squibs, refills and possible engine shop visits, but it is not what the procedure is designed to be followed. This variation is observed in punitive corporate cultures, where one would expect that compliance with the procedure would be iron clad, yet it is not, the crews act to ensure their heads do not rise above managements parapet. Relevance to runaway trim, MCAS etc?, well, 25.255 intimates a 3 second response time, and the probability is that you will routinely exceed that by a large margin. You will also see crew response by other means, such as elevator input, which is often the immediate response to an autopilot attitude error, contrary to the standard procedure in most systems. Variability of crew response needs to be considered to a greater extent, unless the training is explicit in demonstrating the need for timely response, and that the response is simple, and reliable. At present, the MMEL permits the despatch of a Max with one yoke trim switch inoperative, any operator should consider the impact of the other sides pilot being in the galley or head and what the impact would be to the response time... or simply amend the MEL to preclude despatch with any yoke trim switch inoperative.

The Max can be flown safely today with awareness; the crew training however is necessary to make that reliably the outcome, and the architecture of the system needs to come into compliance with 25.671 and 672 as far as unmistakeable warning of a SAS system failure. Respectfully, all B737 operators should consider reviewing the out of trim case disregarding MCAS, and ascertain whether training to ensure crew can recover manual trim operation can occur in the event of a severe out of trim case. Regulators need to be involved in ascertaining whether the inherent weakness that exists in that area is acceptable or not; personally, I think that it needs to be incorporated into training matrices promptly for the peace of mind of the public, the confidence in the system and in and of the pilots who operate these aircraft every day. The training overhead to accomplish the defence from this issue is not onerous, and it is at least as rational a use of FFS time as flying warm and fuzzy CRM vignettes which can be done as or more effectively in a cardboard bomber, FBS, FTD, or PC with a generic sim program. FFS, notwithstanding the limitations of the slip between QTG accepted aero-model dynamics and the real world, and the issues pertaining to the edge of envelope are worthwhile assets to the industry that are squandered by the lack of common sense in training requirements to achieve the training objectives.

It is quite possible that the industry will be better for the painful experience that it has just gone through. Faith in the OEM however is further eroded by their continued litany of lapses of judgement and questionable ethics that appear to be systemic; every few years a new saga erupts onto the public stage, and the OEM vows to cure their ethical lapses, it is time to get serious about doing so, and perhaps they can start by reflecting on their response to their QA staff that they acted so disgracefully against on the NG ring frame saga. Show some spine, and make your company what it needs to be, not what it has devolved into.

[edmundronald]

Agree, but how would his legal team choose to spin it?

Our client cannot be held responsible because he could not have predicted the possible outcome. Or...,

Our client cannot be held responsible because his technical and Human Factors expertise had been overwhelmed by corporate expectations.

Mmm? Incompetence, or Ethical blindness. Tough, but obvious choice.





























An offer of immunity to the chief test pilot is in the public interest. Nobody asks the lawyers to spin it, he gets immunity so all the weaknesses in the plane get located and fixed.

There are undoubtedly other booby traps hidden in that plane. They need to be found and fixed, this guy knows where they are.

I've sat on standards bodies as an engineer. I believe if something is a real issue, there are always "nudge nudge wink wink" engineer to engineer methods to get tech issues fixed across the industry, one of the nastiest of which is just talking to a friend who works for a competitor, who has a friend in the specialist press. My feelings is that if he wanted to, the Chief Test Pilot at Boeing had discrete ways to get the FAA to have a hard look at MCAS and get Boeing to make it robust or add training materials without making too many waves.

Boeing certainly had the ability, resources and even time to solve a few more "minor bugs" before launching the plane. This is what engineering firms do, they polish the design until it is good. the Chief Test Pilot's job is to tell them if it's good, in this case he can tell us for safety's sake what was left undone. .

Edmund

[dufc]

Continuing talk about pilots having to interpret matters within a very short period of time. Is there not a case for an alert in the cockpit that MCAS (MCAS 2.0?) is active?

[safetypee]

Is there not a case for an alert in the cockpit that MCAS (MCAS 2.0?) is active?
Not for MCAS, providing the modifications provide sufficient protection; MCAS becomes a background function similar to STS. No action required no alert required; MCAS ‘off’, pilot awareness.

However, for trim runway there is a strong case for an alert, particularly in the difference between the 737 and the less severe systems, and also those aircraft fully protected. Urgent action required, clear unambiguous alert required, but how.

The difficulty is with how to detect a trim runaway which is essentially a single strand system; the human is a superior detector, but unreliable, requires mental attention and time.
Adding more safe guards must not generate more failure paths.

[moosepileit]

* Is there any reason a NG simulator can not be programmed to be a MAX? There is very little cosmetic differences in the cockpit.

I know for the jet I fly that we "fly" it as a P&W powered model. If the maintainers of the plane train on it as a GE powered version, the sim needs a full reboort, which takes at least 20 minutes.

Might be just part of the reasons.

I've had to wait on that

[yoko1]

According to a document reviewed by The Seattle Times, Boeing’s safety analysis calculated this hazardous MCAS failure to be almost inconceivable



Inigo Montoya: "You keep using that word. I do not think it means what you think it means."

https://www.youtube.com/watch?v=Z3sLhnDJJn0

[billybone]

An offer of immunity to the chief test pilot is in the public interest. Nobody asks the lawyers to spin it, he gets immunity so all the weaknesses in the plane get located and fixed. Just a guess- all those named in the Seattle Times article have already provided testimony to the authorities or their names would not be revealed.
You can be absolutely sure that Boeing is carefully monitoring all employees possibly involved not only on company computers and just barely within legal limits.

Months ago, the Seattle Times revealed a grand jury investigation had started.

[Smythe]

Nose pitches up....High AoA and High G stall...

"The lack of smooth feel was caused by the jet’s tendency to pitch up, influenced by shock waves that form over the wing at high speeds and the extra lift surface provided by the pods around the MAX’s engines, which are bigger and farther forward on the wing than on previous 737s."

Low speed stall

"The flight-test pilots had found another problem: The same lack of smooth stick forces was also occurring in certain low-speed flight conditions. To cover that issue too, engineers decided to expand the scope and power of MCAS.

Because at low speed a control surface must be deflected more to have the same effect, engineers increased the power of the system at low speed from 0.6 degrees of stabilizer nose-down deflection to 2.5 degrees each time it was activated."

So now, we are getting a clearer picture of how it went from 0.6 to 2.5 degrees. According to the article, they first attempted to fix the problems by adding vortex generators and changing the wing, that did NOT work. They chose software.
Changing the wing design? This does not sound like an aerodynamically stable aircraft to me...

The class action suit by the 400 pilots is from AUS?

Spokespeople for the pilots' legal team — Queensland's International Aerospace Law and Policy Group (IALPG) and Chicago's PMJ PLLC

[yoko1]

This does not sound like an aerodynamically stable aircraft to me...



You are conflating the concepts of aerodynamic stability and linear feel response. MCAS was created to address the second, not the first, issue.

[Smythe]

You are conflating the concepts of aerodynamic stability and linear feel response. MCAS was created to address the second, not the first, issue.

No, the aircraft pitches up in high AoA and high G manuvers. The aircraft tends to stall at low speed. You dont try to modify the wings for stick feel.
MCAS pitches the nose down 2.5 degrees in low speed stall, that is not to make the stick feel the same...it is anti-stall.

stick feel is the result, not the cause.

[yoko1]

No, the aircraft pitches up in high AoA and high G manuvers.

Incorrect. The aircraft does not pitch up all by itself. It pitches up because the pilot is pulling back on the yoke. However, the pull forces required were not linear.

The aircraft tends to stall at low speed.

All aircraft tend to stall at low speed. They also stall at high speed if you pull enough g's. No surprises here.

You dont try to modify the wings for stick feel.

Not enough an aero guy to comment except to say that the first response was apparently to add some vortex generators. Does that count as modifying the wing?

MCAS pitches the nose down 2.5 degrees in low speed stall, that is not to make the stick feel the same...it is anti-stall. stick feel is the result, not the cause.

Sorry, but you are misreading all the information that has been published to date. Read the article again. Absolutely no one who is a competent authority has said that MCAS is an anti-stall system, though a number of journalists have mistakenly made this assertion. The issue is the lightening of stick forces as the aircraft approaches the high AOA environment. Quoting from the article:Engineers determined that on the MAX, the force the pilots feel in the control column as they execute this maneuver would not smoothly and continuously increase. Pilots who pull back forcefully on the column — sometimes called the stick — might suddenly feel a slackening of resistance. An FAA rule requires that the plane handle with smoothly changing stick forces.



Later in same article in reference to low-speed testing:The flight-test pilots had found another problem: The same lack of smooth stick forces was also occurring in certain low-speed flight conditions.

​​​​​​It is reasonable to debate if MCAS was the appropriate solution to this problem, but the problem had nothing at all to do with aerodynamic stability. Aerodynamic stability has a very specific meaning in aviation. There are many online sites that discuss the topic including here. (https://www.boldmethod.com/learn-to-fly/aerodynamics/3-types-of-static-and-dynamic-stability-in-aircraft/)

[fizz57]

​​​​​​It is reasonable to debate if MCAS was the appropriate solution to this problem, but the problem had nothing at all to do with aerodynamic stability. Aerodynamic stability has a very specific meaning in aviation. There are many online sites that discuss the topic including here. (https://www.boldmethod.com/learn-to-fly/aerodynamics/3-types-of-static-and-dynamic-stability-in-aircraft/)


It's not as simple as that. The classical stability analysis you quote assumes that the control surfaces are fixed. Having movable surfaces complicates matters, for example as discussed here (http://www.dept.aoe.vt.edu/~lutze/AOE3134/Stickfreecharacteristics.pdf). TL,DR: the last line of that paper states that the stick force gradient is proportional to the stick free static margin. In other words, the stick force gradient specified in the regulations is a measure of the static stability of the aircraft.

The MAX may not be unstable in that corner of the envelope where MCAS is needed - we don't really know since the data is not available in the public domain -, but we do know that it is not stable enough.

[Smythe]

Originally Posted by Smythe https://www.pprune.org/images/buttons/viewpost.gif (https://www.pprune.org/rumours-news/621879-max-s-return-delayed-faa-reevaluation-737-safety-procedures-30.html#post10501288)
No, the aircraft pitches up in high AoA and high G manuvers.
Incorrect. The aircraft does not pitch up all by itself. It pitches up because the pilot is pulling back on the yoke. However, the pull forces required were not linear.

Did you read the article in the Seattle Times?

Why do you think it gets light on the stick? It is pitching up, all by itself.
The lack of smooth feel was caused by the jet’s tendency to pitch up, influenced by shock waves that form over the wing at high speeds and the extra lift surface provided by the pods around the MAX’s engines, which are bigger and farther forward on the wing than on previous 737s.

The aircraft tends to stall at low speed.
All aircraft tend to stall at low speed. They also stall at high speed if you pull enough g's. No surprises here.

Yes, they do, but not as a function of an aerodynamic design flaw. Really, all by themselves? They had to automatically push the nose down 2.5 degrees! That is far outside the envelope. What about the AP trim, it couldnt handle the stall without MCAS...


While the problem was narrow in scope, it proved difficult to cope with. The engineers first tried tweaking the plane’s aerodynamic shape, according to two workers familiar with the testing. They placed vortex generators — small metal vanes on the wings — to help modify the flow of air, trying them in different locations, in different quantities and at different angles. They also explored altering the shape of the wing.

Why would they need to alter the aerodynamic shape, attempt to modify the flow of air......even to consider modifying the wing, if it was aerodynamically stable??


really, what are you thinking?

The classical stability analysis you quote assumes that the control surfaces are fixed.

Exactly, fixed and linearized about a chosen flight condition. Extrapolating that to fixed control surfaces per condition.
When they noted that there is shock waves forming over the wings and extra lift from the nacelles, that is not a linear function, nor an anticipated condition. Shock waves meaning that over certain surfaces, the airflow is going supersonic. (this is why they tried vortex generators to redirect the airflow.) The supersonic airflow can mean a whole bunch of things to the wing, flutter being the most common, but in this case, the loss of laminar airflow over the wing was not able to be controlled using their typical solution, the vortex tabs. (I mean, the engine nacelle is higher than the wing portion behind it)

What does loss of laminar airflow (ie separated airflow or turbulence) mean over the wing? Stall.

[Kerosene Kraut]

The nacelles themselves start to create aerodynamic lift and move the nose up at high alpha.

[yoko1]

The nacelles themselves start to create aerodynamic lift and move the nose up at high alpha.

I understand the nacelles contributed to a nose up moment, which is in part why the stick forces decreased. But nothing in the literature suggests that the nose would have kept pitching up absent those same stick forces. You could say that the positive static stability trended toward neutral static stability, but there is no evidence of negative static stability.

[yoko1]

Did you read the article in the Seattle Times?

Yes, and many similar articles since the first aircraft accident.

Why do you think it gets light on the stick? It is pitching up, all by itself.

If the pilot released the controls and the aircraft continued to pitch up "all by itself", then I would agree with you. That would have been an example of negative static stability. The fact that there were still "pull" forces present means that if the stick was released, then the pitch would, at a minimum, stop increasing (neutral stability) and likely decrease (positive stability). There is nothing, anywhere, in any data released to date that states that the 737 demonstrated negative static stability in the area in question.

[Smythe]

You are stuck on stick forces, and how it feels to the pilot....what about AP? Does MCAS operate with AP on?

The AP cannot handle the stall, or that would have been added to the algorithms in the AP, right?

You did not address the issue of the discontinuity or disruption of the airflow over the wing which causes stall. From what is written, it obviously introduces stall at a much lower AoA than previous variants, and much lower AoA then was designed for (hence a software patch)

Pitch the nose down 2.5 degrees on final???? to make the stick feel the same???...that is lemming talk.

[yoko1]

You are stuck on stick forces, and how it feels to the pilot....what about AP? Does MCAS operate with AP on?

I am not "stuck on" stick forces. Stick forces are simply what MCAS was designed to modify, so that is where the discussion should be focused. You are making the (incorrect) leap that the demonstrated nonlinear stick forces equate to negative static stability. Those are two distinct issues. Show me any data which indicates that the pitch angle would have kept increasing if the controls were released, and then perhaps you would have evidence to support your position. Until then, throwing out assertions that the design was "unstable" are not supportable.

The AP cannot handle the stall, or that would have been added to the algorithms in the AP, right?

I'm not aware of any autopilot that can handle a stall. Maybe one exists, but I've never seen one. Generally the automation is set up to keep far away from the stall environment, but there are ways to induce them into flying an aircraft into a stall. In those case, the autopilot will disconnect.


You did not address the issue of the discontinuity or disruption of the airflow over the wing which causes stall. From what is written, it obviously introduces stall at a much lower AoA than previous variants, and much lower AoA then was designed for (hence a software patch)


The fact that one can stall a 737 is not in dispute. I don't know if it stalls at a lower AOA or not, but that has nothing to do with stability.

[ARealTimTuffy]

Sorry Smthye but you are incorrect.

Unless we have been lied to, the Max does not have an absolute pitch up tendency at high AOA. The RATE OF CHANGE of force of back pressure must be linear within the flight envelop and slightly beyond.

MCAS was created because the RATE OF CHANGE did not meet the requirements. You still need additional back pressure to pull into a higher AOA but the increase in force is not linear. It is the same as the difference between velocity and acceleration. You can be going faster and faster velocity but your rate of acceleration can be slowing down. Rate of change.

The aircraft does not continue to pitch up if relaxing aft control input.

It is not applicable to the autopilot because the autopilot will do what it does regardless of the “feel” on the stick.

[Loose rivets]

Weeks ago a poster put forward a series of comments which ended with (roughly) "So MCAS is not an anti-stall device . . . but it is, sort of."

Well, that about sums up the fuzziness of the public perception. The ST article substantiated my understanding, though the arguments above are thought provoking. I can well imagine the silent moments as test pilots and designers sat around a table staring at each other and taking on board just how that aircraft handled. I wonder what the very first suggested fix was. I expect the last suggestion was, "I know, we can rob MCAS from the military offering. No need for it to work the same, or bother with dual inputs . . ."

***

3 seconds. Hmm . . . in a quiet cockpit that didn't have STS clanking around, just maybe, but not in the real world.

***

In the ST, there was a (outline) suggestion that an extreme split-@rsed turn etc., etc., was so unlikely that it could be discounted. Just wouldn't happen in the real world. I was in my early twenties when I tore the controls out of my captain's hands, opened up the four Darts to firewall and started such a turn. The captain had finally conceded we had turned the wrong way in the Innsbruck valley but was making a series of jerky movements saying, 'we'll stall if we try to turn at this height'. We had been briefed on the impossibility of climbing out if we turned the wrong way and what I had in front of me was a layer of stratus with pointy granite islands. During a turn that exceeded 60 degrees, all I could thing was, 'I hope to God I'm right.' I was right, but right or wrong, just occasionally bewilderingly bizarre things happen. The thought of the controls coming light and mush as I pulled fills me with a dread that make me realise, this new Boeing must retain at least most of the characteristics of a real aircraft. It just can not be a botch with hidden computer corrections.

[yoko1]

Weeks ago a poster put forward a series of comments which ended with (roughly) "So MCAS is not an anti-stall device . . . but it is, sort of."

Well, that about sums up the fuzziness of the public perception.

I think a big part of the problem is that words employed in common usage may have more specific meanings in the world of aircraft certification. In a sense, any system that helps keep an aircraft away from the stall environment (for example, autothrottle alpha floor protection, stick shaker, visual and/or audio alerts, etc) could be generically called an "anti-stall" device. However, strictly from a certification standpoint I'm not sure there is even a definition for an "anti-stall" system, much less a requirement. On the other hand, I know there are requirements that commercial aircraft have stall warning systems and demonstrate certain handling characteristics approaching and recovering from a stall.

MCAS exists because of requirement for a linear control feel response through the high AOA environment. To the extent that someone wants to call this an "anti-stall" system and there is no conflicting definition in the FAR's, then I guess there is no harm.

However, in some cases words really do mean something, hence my interest when someone claims the 737 is "unstable." There are very specific definitions of what stability means, and there is no evidence that the 737 demonstrates anything but positive static stability through its flight envelope. I would accept the premise that the this positive stability may trend toward neutral stability approaching high AOA values, but it never goes negative.

[mattler]

yoko1: I get what you're saying, and it's absolutely correct. I just don't think that it's the whole picture. And unfortunately, we don't have the information required to assemble the whole picture.

It seems like people are using the term "anti stall device" to imply that MCAS has a similar function to alpha prot/floor from the Airbus. So is MCAS that kind of a system? According to Boeing, no. But according to two Boeing engineers that I have been speaking with with, possibly yes. They assert that the data that they saw strongly suggested instability at high AOA and poor stall characteristics, leading us to believe that 737 MAX has stall issues. Nader asserts this too, and regardless of what one thinks about him, we can bet that he's got better info than most of us. He has been speaking with numerous whistleblowers.

Here is a direct quote from a Boeing engineer whom I trust, and who shall remain nameless despite recently retiring. His last sentence is chilling:

"I recall seeing the Cl and Cm Alpha for the MAX from the transonic test and it didn't look good. I retired shortly after that so don't know how it went from there [...] Boeing can claim it was just trying to standardize stick feel but the bottom line is that it did not meet FAA requirements for stability. I would hope that the lift and moment curves would be made public along with the results of the certification flight test. That's the only way I would feel comfortable flying on a MAX."

It seems like the issue between you, and those who disagree with you, is that we don't actually have all of the information yet. We desperately need to see the relevant data before we can assess what MCAS is, or is not. And we need to have this settled before the aircraft is once again cleared for flight.

[fdr]

Weeks ago a poster put forward a series of comments which ended with (roughly) "So MCAS is not an anti-stall device . . . but it is, sort of."

Well, that about sums up the fuzziness of the public perception. The ST article substantiated my understanding, though the arguments above are thought provoking.

3 seconds. Hmm . . . in a quiet cockpit that didn't have STS clanking around, just maybe, but not in the real world.

The terminology used in the media and this forum is frequently incorrect from a compliance perspective.

MCAS's original intent was for a part of the operating envelope where it was obvious that the issue was a longitudinal stability compliance matter. The later disclosed issue about low speed brought along the terms of anti-stall, however that is most likely the application of a incorrect term to the real issue, at least I hope so for the OEM's sake. The approach to stall requires particular longitudinal responses which the engine design would act against at increasing AOA's, so a SAS system to normalise those characteristics is reasonable even if the design was unfortunate in practice. The risks related to an unstable longitudinal stability condition would need a high reliability system with warnings, in a similar manner to say, stall warning, stick pushers etc. It is unlikely that the oversight of incorporating a SAS system without warning would have extended to the point of installing an analogue of a stick pusher system without warning of system failure. Within the rules, such systems as pushers are not specifically noted for compliance, they are mechanisms to ensure that the respective paragraph is complied with. Where that outcome has a risk that is high, then the reliability of the architecture becomes more important, as does warning of a failure of that system. The despatch relief of the MMEL would also reflect the severity of the outcome following a failure. Overlooking the fact that the design had a SAS component that doesn't appear to meet the criteria of the regs, related to static stability is one thing, a system that is introduced to meet an unstable static stability approaching or in a stall without warning or system architecture providing reliability is a completely different level of error, and one that I doubt that an OEM such as Boeing would have fallen into. THe first is bad enough, the second is unimaginable for a company that has an obligation to provide compliant products.

Expect the aircraft has a reduced static stability in parts of the envelope, but at all times it would be stable, just not compliant. If it was truly unstable at any time, then the fur would fly.

[billybone]

In the seattle Times article The inside story of MCAS: How Boeing’s 737 MAX system gained power and lost safeguards --
The following quote does not seem to have raised any comments here

For example, there is a cutout switch in the control column so that when a pilot pulls or pushes in the opposite direction to a runaway stabilizer, it cuts electric power to the stabilizer. When MCAS is active, this cutout switch doesn’t work, which could surprise a pilot who didn’t know about the system.



Seems to me thats a major gotcha - leaving HAL in full command and the pilot wondering WTF for more than 3 seconds the first time- and then HAL repeats it again and again - How on earth did anyone allow that little change in wiring ( software ? ) get by anyone ??

[BDAttitude]

Yes, and many similar articles since the first aircraft accident.



If the pilot released the controls and the aircraft continued to pitch up "all by itself", then I would agree with you. That would have been an example of negative static stability. The fact that there were still "pull" forces present means that if the stick was released, then the pitch would, at a minimum, stop increasing (neutral stability) and likely decrease (positive stability). There is nothing, anywhere, in any data released to date that states that the 737 demonstrated negative static stability in the area in question.

Trimm the craft to neutral during the manoeuvre, which you are free to do, and you would be exactly in that situation. Therfore it's a minimum gradient requirement. You can disregard the (push or pull) offset.

MCAS exists because of requirement for a linear control feel

See above. No linearity requested but a minimum gradient. Nothing to do with linearity but with monotonicity.

[BDAttitude]

... has updated his findings about the incidence of runaway trims and stabilizer jams. (https://www.satcom.guru/2019/05/737-pitch-trim-incidents.html)

Note that there has been at least one incident where excessive force had to be used to break the clutch for manual mechanic trim loose. And another one where trim was only movable after pulling certain CBs.

... has pulled out a percedence where Lear was required to use dual channel inputs for a stick pusher by the FAA in the 60ies

https://twitter.com/Satcom_Guru/status/1142818977111928833

[petit plateau]

Trimm the craft to neutral during the manoeuvre, which you are free to do, and you would be exactly in that situation. Therfore it's a minimum gradient requirement. You can disregard the (push or pull) offset.

See above. No linearity requested but a minimum gradient. Nothing to do with linearity but with monotonicity.


Thank you BD :)

[edmundronald]

Is this a pilots' forum or a lawyer's forum? As an engineer and an SLF, I believe civilian airplanes are supposed to be flyable by normal humans, not Chuck Yeager or even Sully. And now we have all this talk about 3 second response times and neutral stability etc. It's a joke.

No employee of the FAA or Boeing would allow a family member to get into an airplane with a 200 hour first officer who is PF after clean up and now and has 3 seconds to respond to HAL's runaway trim while half a dozen of alarms and alerts are going off. Nor even a 55 year old captain with 20K+ hours but 200 on type in a brand new airplane which he got trained on for 1 hour on an iPad, and who may bring lots of experience but zero knowledge of MCAS to the table.

As for "neutral stability", in my book it sounds suspiciously like the edge of a coin.

This is lawyer talk. Engineer talk is "this system is solid, and we have built in such a fat margin of error that not even the most idiotic rest-deprived pilot should ever have an issue with the controls, and also no force is required, every pilot should be able to move the trim wheel with no great effort with one hand."

Pilots should tell their companies, and Boeing, that they want an airframe that is clearly engineered to help them stay in the air. Which in fact is something which the superb engineers at Boeing used to be very happy to deliver.

Or else maybe the pilots who want to discuss semantics in the case of the 737's averred faults should retrain and embrace a more lucrative career in corporate law.

Edmund


I think a big part of the problem is that words employed in common usage may have more specific meanings in the world of aircraft certification. In a sense, any system that helps keep an aircraft away from the stall environment (for example, autothrottle alpha floor protection, stick shaker, visual and/or audio alerts, etc) could be generically called an "anti-stall" device. However, strictly from a certification standpoint I'm not sure there is even a definition for an "anti-stall" system, much less a requirement. On the other hand, I know there are requirements that commercial aircraft have stall warning systems and demonstrate certain handling characteristics approaching and recovering from a stall.

MCAS exists because of requirement for a linear control feel response through the high AOA environment. To the extent that someone wants to call this an "anti-stall" system and there is no conflicting definition in the FAR's, then I guess there is no harm.

However, in some cases words really do mean something, hence my interest when someone claims the 737 is "unstable." There are very specific definitions of what stability means, and there is no evidence that the 737 demonstrates anything but positive static stability through its flight envelope. I would accept the premise that the this positive stability may trend toward neutral stability approaching high AOA values, but it never goes negative.

[ARealTimTuffy]

In the seattle Times article The inside story of MCAS: How Boeing’s 737 MAX system gained power and lost safeguards --
The following quote does not seem to have raised any comments here



Seems to me thats a major gotcha - leaving HAL in full command and the pilot wondering WTF for more than 3 seconds the first time- and then HAL repeats it again and again - How on earth did anyone allow that little change in wiring ( software ? ) get by anyone ??

There is nothing sneaking through there. The cutout switch inside the column is there to prevent a trim runaway. However the MCAS as designed must function despite back pressure. If it cutout MCAS wouldn’t do its job.

[gums]

Salute!

Lemme see if I understand this from a “pilot”?
There is nothing sneaking through there. The cutout switch inside the column is there to prevent a trim runaway. However the MCAS as designed must function despite back pressure. If it cutout MCAS wouldn’t do its job.

So the pilots expect that pulling/pushing will override the AP or whatever by cutting out the electric trim commanded by HAL?

But we lowly pilots are not told this or even aware that a new system might be at work and what we have expected for decades is not true anymore. BEAM ME UP!

There is no reason to have an unknown system on my plane that operates in a “bang bang” mode versus a proven curve relating AoA to pitch moment and/or stick force. Oh yeah, you will encounter the black cat while the stick is shaking and you are getting warning lights about speed and such. GASP! You trim up for a second and then handle the shaker problem IAW the procedures, but the cat is still at work. A few seconds later she strikes again.

The whole thing flat out sucks. Poorly mechanized fix for an aerodynamic problem, then not come clean with FAA and then not tell the folks operating the plane.

Gotta calm down, man....

Gums rants....

[krismiler]

Boeing keeping quiet about the MCAS is the equivalent of concealing information from your doctor and still expecting him to provide you with the correct treatment.

[ARealTimTuffy]

I’m simply responding to the previous post. Forgetting about failure modes for a second, it wouldn’t make sense to have the control column cutout the trim if the pilot continues to pull back while mcas trims AND. That is the entire reason it is there. I’m talking about the MCAS functioning when it should.

Having said that, the whole thing is a goat show and there is plenty wrong. Just that isn’t it.

[Smythe]

I am looking at what was stated, and provide the aerodynamic reasoning behind it. Forget about stick feel.

If, at high angle of attack, or low speeds, if, due to certain areas of the wing the airflow is going supersonic, the resulting lack of laminar airflow over the wing (as stated by the Boeing wind tunnel tests) induces a stall...
The aerodynamics of the engine and the wing are creating stall conditions at lower AoA and other conditions than the system is programmed for.

Look at what they tried to do, adding vortex tabs, changing the wing design....

Changing the wing design?? Do you change the wing design to make the stick pressure the same, or to prevent stall?
The differential pressure on the yoke is a RESULT of the stall.
Stick pressure? I feel that is a half baked response by Boeing to mask the problems with the aerodynamics of the wing/engine design, and simply does not make sense. Maybe that is how is was presented to the FAA, but I dont think that is reality. Boeing will never admit that the aircraft was not aerodynamically stable.

Is MCAS operational in AP? While I keep hearing the mantra, it only operation with AP off, it appears it is operational according to several reports that show turning off the AP resolves the problem. Wasnt it the case with the last crash, that when they turned AP back on, MCAS engaged again?

In one incident, an airline pilot reported that immediately after engaging the Max 8’s autopilot, the co-pilot shouted “DESCENDING,” followed by an audio cockpit warning, “DON’T SINK! DON’T SINK!”

“I immediately disconnected AP (Autopilot) (it WAS engaged as we got full horn etc.) and resumed climb,” the pilot writes in the report, which is available in a database compiled by NASA (https://asrs.arc.nasa.gov/search/database.html). “Now, I would generally assume it was my automation error, i.e., aircraft was trying to acquire a miss-commanded speed/no autothrottles, crossing restriction etc., but frankly neither of us could find an inappropriate setup error (not to say there wasn’t one).

In reality, MCAS is anti-stall.

[bill fly]

Smythe,

You make a lot of work for people sometimes - correcting your misconceptions.

Just for now - No, MCAS is not on with Autopilot on. Only if:

AP Off
Flap Retracted
AoA high

Suggest before carrying out long winded arguments to check back through all the threads, where much is explained.

Cherrs, B

[Smythe]

Really, how certain are you that it functions that way? Because Boeing stated so? How many pilot reports show that turning OFF AP resolves the situation? Lets see, to disconnect MCAS, you disconnect AP trim?

AoA high? really, how high does the AoA have to be for the wing to stall? MCAS pushes the nose down 2.5 degrees at low speeds to prevent stall? That is a hell of a different AoA on final...

Aside from that issue, what about the rest of the argument by myself and others?

Been grounded about 100 days now...just for stick feel...

[yoko1]

Really, how certain are you that it functions that way? Because Boeing stated so? How many pilot reports show that turning OFF AP resolves the situation?

AoA high? really, how high does the AoA have to be for the wing to stall? MCAS pushes the nose down 2.5 degrees at low speeds to prevent stall? That is a hell of a different AoA on final...

Aside from that issue, what about the rest of the argument by myself and others?



Yes, I am sure MCAS works that way.

The nose down upon autopilot engagement you referred to was an autopilot problem, not an MCAS problem. I've had the same thing happen on the 737NG. It's not really a big deal - just disconnect the autopilot and try the other one.

The 2.5 degree nose down trim input is actually less abrupt than the stick pusher devices installed in numerous other commercial aircraft. I assume you don't have a problem with those? The FAA and EASA certainly don't.

The 737MAX is aerodynamically stable throughout its flight envelope. Just because you want to believe otherwise doesn't make it so.

Yoko out.

[BDAttitude]

The 737MAX is aerodynamically stable throughout its flight envelope. Just because you want to believe otherwise doesn't make it so.

Yoko out.
Except where MCAS kicks in by design - not by error.

[Fly Aiprt]

Just for now - No, MCAS is not on with Autopilot on. Only if:

AP Off
Flap Retracted
AoA high


That is, if what Boeing said is accurate.
So many hidden traits have surfaced that there might be other things about MCAS, that Boeing didn't tell us yet...

[ARealTimTuffy]

Yes, I am sure MCAS works that way.

The nose down upon autopilot engagement you referred to was an autopilot problem, not an MCAS problem. I've had the same thing happen on the 737NG. It's not really a big deal - just disconnect the autopilot and try the other one.

The 2.5 degree nose down trim input is actually less abrupt than the stick pusher devices installed in numerous other commercial aircraft. I assume you don't have a problem with those? The FAA and EASA certainly don't.

The 737MAX is aerodynamically stable throughout its flight envelope. Just because you want to believe otherwise doesn't make it so.

Yoko out.

To Add for Smthye.

in the congressional testimony on May 15th of Dan Elwell they bring up these other autopilot on reports in the safety reporting system. He states that these were an unrelated malfunction as yoko says. As far as everyone is aware there have been 3 cases of false MCAS. The 2 accident flights and the lion air flight before the accident. You may believe there is a cover up but that is speculation as there have not been any other validated reports

Secondly, the trim doesn’t input a reduction of 2.5 degrees to the AOA when the MCAS operates. It puts in 2.5 units of AND trim. Trim will adjust the control force requirements as we all know. Which was the goal all along.

The aircraft aren’t grounded due to stick force, they are grounded because the system designed to deal with it had major flaws in the way it was thought up and tested that led to numerous lives being lost. This is a time for reflection from all of us in the industry on how can we do better.

The autopilot does not require a consistency of feel to aid in how it will fly the airplane so the system need not function with the autopilot on. Again there has been information conveniently left out along the way these last number of months but to say it is a cover up, again is pure speculation at this point.

[yoko1]

Except where MCAS kicks in by design - not by error.

Apples and oranges. In both accidents, MCAS activated due to an false AOA input, not because it was supposed to. An erroneous flight control input is an erroneous flight control input. This has nothing to do with aerodynamic stability.

[yoko1]

So many hidden traits have surfaced that there might be other things about MCAS, that Boeing didn't tell us yet...

If our criteria is now that we can spin whatever narrative we want based on "hidden" information that no one else knows about, then this whole discussion can go very, very far out into the weeds. Personally, I'll stick to the facts as they are currently understood.

[Fly Aiprt]

The autopilot does not require a consistency of feel to aid in how it will fly the airplane so the system need not function with the autopilot on. Again there has been information conveniently left out along the way these last number of months but to say it is a cover up, again is pure speculation at this point.

There are very peculiar things in this MCAS crisis.
For instance, in his introduction to the last hearing, Chairman Larsen complained that the FAA is very slow in providing documents required by the Subcommittee.
So not exactly cover up if you like, but reluctance to disclose info on the MAX certification process, lack of tranparency, dissimulation, your pick...
Maybe there's more to this MCAS business than meets PPRuNe's eyes...

[yoko1]

Maybe there's more to this MCAS business than meets PPRuNe's eyes...

I suspect a lot of the delay has to do with the re-discovery of issues related to runaway stab trim and the quirks of the manual trim system. Much of this knowledge has been "lost" over the years, and some folks are probably learning about this for the first time.

[Fly Aiprt]

Personally, I'll stick to the facts as they are currently understood.

What facts are your referring to ?
Nobody outside Boeing and possibly the FAA actually knows why exactly MCAS was there in the first place, what exactly it was supposed to do and under what circumstances.
Apart from some unsupported declarations by non-engineer PR people, that the MCAS was supposed to do this or that, auto pilot off etc. we don't have much to understand its operation.
All we have are parts of DFDR recordings from the accidents.
Maybe talking of hard facts when speculating on what this MCAS is able to do and when exactly is a bit hard to swallow.

[yoko1]

Nobody outside Boeing and possibly the FAA actually knows why exactly MCAS was there in the first place, what exactly it was supposed to do and under what circumstances.
Apart from some unsupported declarations by non-engineer PR people, that the MCAS was supposed to do this or that, auto pilot off etc. we don't have much to understand its operation.


Please tell me you aren't being serious here. If you really feel we are that bereft of information, there really isn't any point in continuing this discussion.

[Fly Aiprt]

I suspect a lot of the delay has to do with the re-discovery of issues related to runaway stab trim and the quirks of the manual trim system. Much of this knowledge has been "lost" over the years, and some folks are probably learning about this for the first time.

I'm with you on that.
We might be speculating a bit, though ;-)

[billybone]

In reading the many threads and posts, this SLF (engineer type) has what may be a yet to be discussed issue . with MCAS and the following rare scenario -

Cruise at altitude on autopilot
TCAS warning equivalent to " pull up turn left or right - now " - Does MCAS care ? Does MCAS engage ? how much change in AOA allowed before MCAS pushes nose down ?

[mattler]

The 737MAX is aerodynamically stable throughout its flight envelope. Just because you want to believe otherwise doesn't make it so.

And just because you want to believe otherwise, doesn't make it so. That line goes both ways.

You're either very bright, or posing as someone who is; it's not always obvious. Regardless, I'm scratching my head as to how you can be so sure about yourself, given that there are Boeing engineers from the MAX program who would vociferously beg to differ with what I can only assume is your opinion. So if you're going to speak with that kind of authority, you're going to need to give us something more than Boeing's official stance, as they've got a bit of a credibility problem atm.

In the mean time, the following was sent to me by one of those Boeing engineers. The last sentence says it all (emphasis added):
"I recall seeing the Cl and Cm Alpha for the MAX from the transonic test and it didn't look good. I retired shortly after that so don't know how it went from there [...] Boeing can claim it was just trying to standardize stick feel but the bottom line is that it did not meet FAA requirements for stability. I would hope that the lift and moment curves would be made public along with the results of the certification flight test. That's the only way I would feel comfortable flying on a MAX."

Let's hope that the data cited above made it to the FAA so that it can be accessed by way of a FOIA request (should it fail to be disclosed voluntarily).

The actual fact appears to be that we don't know, one way or another.

[568]

In reading the many threads and posts, this SLF (engineer type) has what may be a yet to be discussed issue . with MCAS and the following rare scenario -

Cruise at altitude on autopilot
TCAS warning equivalent to " pull up turn left or right - now " - Does MCAS care ? Does MCAS engage ? how much change in AOA allowed before MCAS pushes nose down ?If you fly an "RA" TCAS event, then the autopilot and Auto throttle should be disengaged. TCAS "TA" is to alert crew of other "traffic" but is not deemed to be a threat.TCAS "RA" maneuvers are generally flown with "smooth" control inputs for pitch up or down depending on the course of action. These events are not subject to high or low pitch angles in general, so MCAS probably wouldn't be a factor.

[bill fly]

Really, how certain are you that it functions that way? Because Boeing stated so? How many pilot reports show that turning OFF AP resolves the situation? Lets see, to disconnect MCAS, you disconnect AP trim?

AoA high? really, how high does the AoA have to be for the wing to stall? MCAS pushes the nose down 2.5 degrees at low speeds to prevent stall? That is a hell of a different AoA on final...

Aside from that issue, what about the rest of the argument by myself and others?

Been grounded about 100 days now...just for stick feel...

Words fail... That whole post once again disqualifies you from commenting on aviation matters.

Are you a troll?

[Fly Aiprt]

Please tell me you aren't being serious here. If you really feel we are that bereft of information, there really isn't any point in continuing this discussion.

Indeed.
Lots of speculation here - one way or the other - but accurate technical info is scarce.
As usual, when things are sorted out and information is made public, some opinions here will appear to be right, and others to be wrong, but in the mean time, who actually knows ?

[WHBM]

American Airlines CEO Douglas Parker says the Max grounding is now down to politics

https://www.cnbc.com/2019/06/23/american-airlines-ceo-politics-a-factor-in-return-of-boeings-737-max.html

Extraordinary ...

[yoko1]

And just because you want to believe otherwise, doesn't make it so. That line goes both ways....


.

The thing is, the word "stability" has a very particular meaning when it comes to aviation. It is a definition that anyone can look up and see if the data supports one's position. Part of the problem is that some posters like to use the word "unstable." That in itself is not correct usage. There are two types of stability - static and dynamic. Within these category, there are further qualifications of positive, neutral, and negative. When someone says the 737 is "unstable," then that implies that it demonstrates negative static or dynamic stability. There is no evidence in the record that supports this position.

That being said, "positive" stability is not a fixed value. An aircraft can demonstrate more or less positive stability according to how quickly it returns to its initial state after a perturbation. The Part 25 of the FAR's specify the minimum required stability performance for transport aircraft. The lightening of the control forces in the pitch axis at high AOA is an indication that the degree of positive static stability decreases, but that doesn't mean it goes negative. MCAS does increase the positive stability characteristics of the 737, but that is a very different thing from saying that the aircraft is "unstable" without it.

[Less Hair]

What possible legal consequences concerning certification would it have to call MCAS some "anti stall" device? It feels like all that spin doctoring concerning the fine print wording might somehow be legally motivated?

[mattler]

yoko1. Fair enough, but it would seem to me that if an aircraft requires less and less back pressure as its angle of attack continues to increase, then you're looking at some kind of instability in that particular part of the envelope. The statement made by the Boeing engineer that I quoted above also implies this much.

We definitely ought to be using correct terminology here though, so I'm 100% with you there. This subject matter is a big beyond the scope of the avg aviator, myself included.

[etudiant]

American Airlines CEO Douglas Parker says the Max grounding is now down to politics

https://www.cnbc.com/2019/06/23/american-airlines-ceo-politics-a-factor-in-return-of-boeings-737-max.html

Extraordinary ...
Seems quite reasonable comment, that the FAA does not want to be alone in releasing the MAX.
Given that the initial FAA certification left issues that caused other regulators to ground the aircraft, there is a rebuilding trust issue for the FAA which imho is most important for the agency. The MAX schedule is not their primary concern.

[zzuf]

The thing is, the word "stability" has a very particular meaning when it comes to aviation. It is a definition that anyone can look up and see if the data supports one's position. Part of the problem is that some posters like to use the word "unstable." That in itself is not correct usage. There are two types of stability - static and dynamic. Within these category, there are further qualifications of positive, neutral, and negative. When someone says the 737 is "unstable," then that implies that it demonstrates negative static or dynamic stability. There is no evidence in the record that supports this position.

That being said, "positive" stability is not a fixed value. An aircraft can demonstrate more or less positive stability according to how quickly it returns to its initial state after a perturbation. The Part 25 of the FAR's specify the minimum required stability performance for transport aircraft. The lightening of the control forces in the pitch axis at high AOA is an indication that the degree of positive static stability decreases, but that doesn't mean it goes negative. MCAS does increase the positive stability characteristics of the 737, but that is a very different thing from saying that the aircraft is "unstable" without it.

How do you know that it is not related to manoeuvre stability?
After all MCAS = Manoeuvre Characteristics Augmentation system.

[yoko1]

What possible legal consequences concerning certification would it have to call MCAS some "anti stall" device? It feels like all that spin doctoring concerning the fine print wording might somehow be legally motivated?

The phrase "anti-stall" falls into the much the same category as "unstable." It is not a defined term for the purposes of the aircraft certification. While we can use it generically for any system or device that helps in the prevention of or recovery from stalls, it does not correspond to any particular certification requirement. There are requirements for stall warning devices and handling characteristics approaching and during a stall that are spelled out in detail in FAR Part 25, and it is those requirements that Boeing will point to when they explain why MCAS was needed.

In particular, FAR Part 25 make frequent references for the requirement that "the stick force curve must have a stable slope" in various flight regimes and that approaching a stall the "longitudinal control force must be positive up to and throughout the stall." The requirement for a "stable slope" in particular is what drove the need for MCAS.

[yoko1]

How do you know that it is not related to manoeuvre stability?


Define "manoeuvre stability" as it applies to Part 25 certification requirements and I might better be able to address your question.

[DaveReidUK]

American Airlines CEO Douglas Parker says the Max grounding is now down to politics

https://www.cnbc.com/2019/06/23/american-airlines-ceo-politics-a-factor-in-return-of-boeings-737-max.html

Extraordinary ...

In the sense that the FAA will almost certainly feel the need to get EASA on side before lifting the grounding, then yes, it's down to politics.

[billybone]

If you fly an "RA" TCAS event, then the autopilot and Auto throttle should be disengaged. TCAS "TA" is to alert crew of other "traffic" but is not deemed to be a threat.TCAS "RA" maneuvers are generally flown with "smooth" control inputs for pitch up or down depending on the course of action. These events are not subject to high or low pitch angles in general, so MCAS probably wouldn't be a factor.

Thank you ! I realize that normally TCAS only provides up/down altitude change alerts supposedly at a range/time sufficient to avoid collision. As I understand it- MCAS only works/engages if/when AP is off, flaps up, etc. And that normally, the AOA sensor provides input to AP. But the disconnect/removal of the yoke/stick switch which has been standard for decades such that a pull or push in opposition to stab/trim input under a TCAS maneuver which would normally cut electric power to stabilizer might be a surprise to many. Seems to me if HAL( MCAS) is/was G limited by design - but no longer is - then a conflict between pilot and HAL is probable ??

[WHBM]

In the sense that the FAA will almost certainly feel the need to get EASA on side before lifting the grounding, then yes, it's down to politics.
That is indeed correct and appropriate. However there's a subtext there that some political manoeuvring will occur between foreign certification authorities and that of the US.

[SLF3]

Have Boeing formally submitted ‘the fix’ to the FAA for approval?

[armchairpilot94116]

The Max is turning out to be the worst debacle that Boeing has ever faced. I can't think of a worst one.

I'm rooting for Boeing to get it all sorted out !

https://www.sfgate.com/technology/businessinsider/article/Boeing-is-crowding-its-employee-parking-lot-with-14039499.php

[bnt]

Boeing are definitely working on the planes: according to a footage shot by the King 5 News channel in Seattle, Boeing is even using parts of the employee parking lots to house them.

46InmJexzYg

[bill fly]

Boeing are definitely working on the planes: according to a footage shot by the King 5 News channel in Seattle, Boeing is even using parts of the employee parking lots to house them.

46InmJexzYg

Interesting pic - and the lifting influence of the Leap engines can be better appreciated from that angle.

Could be though, that they have been letting the employees use the odd Max at weekends...

[yoko1]

Thank you ! I realize that normally TCAS only provides up/down altitude change alerts supposedly at a range/time sufficient to avoid collision. As I understand it- MCAS only works/engages if/when AP is off, flaps up, etc. And that normally, the AOA sensor provides input to AP. But the disconnect/removal of the yoke/stick switch which has been standard for decades such that a pull or push in opposition to stab/trim input under a TCAS maneuver which would normally cut electric power to stabilizer might be a surprise to many. Seems to me if HAL( MCAS) is/was G limited by design - but no longer is - then a conflict between pilot and HAL is probable ??

I'm having a hard time trying to make out what you are asking here, however let me throw out a few things out there that might help clarify.

MCAS only works with A/P off.

AOA provides an input into several different systems. That has not changed. However, I don't believe there is a direct input from the AOA sensor to the autopilot, but rather feeds indirectly through the Air Data Inertial Reference Unit (ADIRU) and the Stall Management Yaw Damper (SMYD).

When you refer to the "removal of the yoke/stick switch," I think you are talking about the control column cutout switches that prevent trimming in the opposite direction that the control column is displaced (that is, stops nose down trim when pulling back, stops nose up trim when pushing forward). This feature has not been removed and still functions as designed except that these switches are bypassed for MCAS activation (and only for MCAS activation).​​​​​​

Any maneuvers flown in response to a TCAS alert would be hand flown, but how the plane reacts would really be no different from any other hand flown maneuver.

Keep in mind that the MCAS software has already been changed to prevent the type of erroneous activation that occurred in the accident aircraft. The only time then that one would see MCAS activate during a TCAS event would be if the pilot mishandled the procedure and brought the aircraft close to a stall. In that case, you want MCAS to activate!

Let's please remember that last point. When the 737MAX approaches a stall, we want the MCAS to activate just like we want a stick pusher to activate on aircraft that are so equipped. We just don't want it to activate when it is not needed.

[BDAttitude]

The phrase "anti-stall" falls into the much the same category as "unstable." It is not a defined term for the purposes of the aircraft certification. While we can use it generically for any system or device that helps in the prevention of or recovery from stalls, it does not correspond to any particular certification requirement. There are requirements for stall warning devices and handling characteristics approaching and during a stall that are spelled out in detail in FAR Part 25, and it is those requirements that Boeing will point to when they explain why MCAS was needed.

In particular, FAR Part 25 make frequent references for the requirement that "the stick force curve must have a stable slope" in various flight regimes and that approaching a stall the "longitudinal control force must be positive up to and throughout the stall." The requirement for a "stable slope" in particular is what drove the need for MCAS.
From BA (https://www.boeing.com/commercial/aeromagazine/aero_02/textonly/fo01txt.html) - I assume you are either directly or indirectly on their payroll:
As airspeed varies from a trimmed condition, the column force required to maintain a new speed (without re-trimming) is a measure of static longitudinal stability. For any conventional airplane, the location of the CG has the strongest influence on static longitudinal stability. For a statically stable airplane the required column force, as speed varies from the trimmed condition, is less at an aft CG than it is at a forward CG. The minimum average gradient allowed by U.S. Federal Aviation Administration FAR Part 25 is one pound for each six knots. As the CG moves aft (BDAttitude: or center of lift moves forward), it reaches a point where the stick force per knot drops to zero, then reverses. This location is called the neutral point.The difference between the actual CG location and the neutral point is called the static margin. With a CG forward of the neutral point, an airplane has a positive static margin and positive static longitudinal stability. At a CG aft of the neutral point, an airplane has a negative static margin, is statically unstable, and requires some form of augmentation to be flown with an acceptable workload.

So I will not call it "unstable" but "smaller than required or negative static margin hence not stable enough to be certifyable".

Never the less it is a stability problem not a handling beauty contest as you would like to spin it.

[yoko1]

So I will not call it "unstable" but "smaller than required or negative static margin hence not stable enough to be certifyable".

Thanks for the link, a lot of good information there. However, this document does not specifically address the 737. It does reference flight control augmentation on the MD-11 and 777, but I believe those aircraft actively manage c.g. for cruise performance and hence reduces the longitudinal stability.

The text you need to focus on is this:

The minimum average gradient allowed by U.S. Federal Aviation Administration FAR Part 25 is one pound for each six knots. As the CG moves aft, it reaches a point where the stick force per knot drops to zero, then reverses.

The reported performance of the 737MAX approaching a stall is one of stick forces decreasing, but never stick forces reversing. Thus we can say that the longitudinal stability decreases at high AOA, but it does not go negative. MCAS does improve the longitudinal stability at high AOA, but the 737MAX still demonstrates positive static stability in this area of the envelope.

​​​​​​There's probably another important point worth mentioning here. No one is claiming that the MAX doesn't need MCAS to meet the stability requirements of the FAR's. Well guess what? The 737NG doesn't meet the stability requirement without its Speed Trim System. I also suspect that just about every swept wing commercial transport doesn't meet the stability requirements without a yaw damper. Having a stability augmentation system (SAS) is nothing new in commercial aircraft - they've literally been in use for decades. It is worth noting, however, that the FAR's also place requirements on these same aircraft in case of failure of a SAS, and those requirements would basically preclude negative stability characteristics in absence of the SAS.

[BDAttitude]

Keep in mind that the MCAS software has already been changed to prevent the type of erroneous activation that occurred in the accident aircraft. The only time then that one would see MCAS activate during a TCAS event would be if the pilot mishandled the procedure and brought the aircraft close to a stall. In that case, you want MCAS to activate!


But not yet certified. And compared to solutions from competitors with three channels and two computational units with two independent processors still not state of the art.

[yoko1]

And compared to solutions from competitors with three channels and two computational units with two independent processors still not state of the art.

I never said MCAS was the best solution to resolve the MAX's high-AOA handling characteristics. Heck, I'm not sure anyone would claim it was the "best" solution. It was, however, the solution Boeing went with. And yes, it was poorly designed in as much that it activated at a time that it wasn't needed and created a hazardous flight situation. However, I haven't seen anyone claim that it would not actually do what it was designed to do in an actual stall situation.

[billybone]

yoko1 said in answer to my awkward query When you refer to the "removal of the yoke/stick switch," I think you are talking about the control column cutout switches that prevent trimming in the opposite direction that the control column is displaced (that is, stops nose down trim when pulling back, stops nose up trim when pushing forward). This feature has not been removed and still functions as designed except that these switches are bypassed for MCAS activation (and only for MCAS activation).​​​​​​

It was my understanding that the installation/addition of MCAS software AND the wiring change shown in peter lemmes blog resulted in the disconnect of the control column cutout switch-. Thus the change from NG wherein at anytime the pilot pushed or pulled in opposition to the stabilizer trim, the electric motor stabilizer trim was shutoff leaving only elevator? and manual trim wheel and ? .

Apologies if I've totally botched this up ;((

[HighWind]

Keep in mind that the MCAS software has already been changed to prevent the type of erroneous activation that occurred in the accident aircraft. CS 25.671
C: The aeroplane must be shown by analysis, test, or both, to be capable of continued safe flight and landing after any of the following failures or jamming in the flight control system and surfaces (including trim, lift, drag, and feel systems) within the normal flight envelope, without requiring exceptional piloting skill or strength.
C3: A runaway of a flight control to an adverse position and jam must be accounted for if such runaway and subsequent jamming is not extremely improbable. (Probability of 1 x 10-9 or less per flying hour )Electronics with probability of failure less than 1 x 10-9 per flying hour, must be designed according to DAL Level A. In this forum DAL Level C have been mentioned, this is 10000 times less reliable.
I don’t understand why fixing the MCAS algorithm helps, when THS is controlled by a DAL C system that is expected to fail at less than 1 x 10-5 pr. Hour.

[MemberBerry]

If you fly an "RA" TCAS event, then the autopilot and Auto throttle should be disengaged. TCAS "TA" is to alert crew of other "traffic" but is not deemed to be a threat.TCAS "RA" maneuvers are generally flown with "smooth" control inputs for pitch up or down depending on the course of action. These events are not subject to high or low pitch angles in general, so MCAS probably wouldn't be a factor.

I'm not a pilot, but what about: "Terrain! Terrain! Pull up!" ?

From what I have heard in that situation you may need to pull up until the stick shaker activates, and then keep around that AoA until you clear the terrain, with the stick shaker activating intermittently, as demonstrated in this simulator session (the relevant part of the video starts at 13:30):

https://youtu.be/4qsFI9l0bJk?t=13m30s

Could MCAS interfere with the terrain escape maneuver? I think that could be the worst moment for MCAS to activate.

[MemberBerry]

It already has...twice.

Indeed. But what I meant was if MCAS could interfere with the terrain escape maneuver when the AoA vanes are working properly.

More precisely I think my question would be if MCAS activates before the stick shaker, simultaneously with the stick shaker, or after the stick shaker.

[yoko1]

I'm not a pilot, but what about: "Terrain! Terrain! Pull up!" ?

From what I have heard in that situation you may need to pull up until the stick shaker activates, and then keep around that AoA until you clear the terrain, with the stick shaker activating intermittently, as demonstrated in this simulator session (the relevant part of the video starts at 13:30):

https://youtu.be/4qsFI9l0bJk?t=13m30s

Could MCAS interfere with the terrain escape maneuver? I think that could be the worst moment for MCAS to activate.

Actually, it would be more of a problem if MCAS did not activate. Keep in mind the purpose of MCAS - to augment the control feel at high AOA. If you have a high AOA for any reason - including windshear or GPWS maneuvers - then you are in the flight regime for which MCAS was designed. The original problem occurred when MCAS activated when it wasn’t needed.

[Fly Aiprt]

Could MCAS interfere with the terrain escape maneuver? I think that could be the worst moment for MCAS to activate.
Interesting question;
According to what is generally posted on this forum :
Depending of what the stickshaker and MCAS set point are, this potentially could happen.
With MCAS 1.0 it was unrecoverable.
With MCAS 2.0, you still have the authority for 1.2 g maneuver. Or so they say.

Actually, no one outside Boeing actually knows much about this MCAS operation.
Not sure even people at Boeing really knew what they were doing...

[Fly Aiprt]

More precisely I think my question would be if MCAS activates before the stick shaker, simultaneously with the stick shaker, or after the stick shaker.

Another good question.
Not sure anyone here can answer.

[Fly Aiprt]

The original problem occurred when MCAS activated when it wasn’t needed.




Another question comes to mind.
What would have happened in a real terrain avoidance maneuver with the stickshaker active ? Wouldn't it have run for 9-10 seconds, adding 2.5 units of nose down trim ? And so overwhelming the elevator authority ?
What's the pilot to do in that case ? Counter trim, with a risk of negating the MCAS raison-d'etre ?
Flip the trim cutout switches on the pedestal ?

If you have to counter/cut off the MCAS at the very moment it is supposed to be useful, why not have the freedom to counter it just by pulling on the yoke ?

[MemberBerry]

Actually, it would be more of a problem if MCAS did not activate. Keep in mind the purpose of MCAS - to augment the control feel at high AOA. If you have a high AOA for any reason - including windshear or GPWS maneuvers - then you are in the flight regime for which MCAS was designed. The original problem occurred when MCAS activated when it wasn’t needed.


If indeed that's what MCAS only did, augment feel, I would agree. But based on everything I read that's not exactly what it does, at least not the old version. As far as I know it doesn't take into account stick force or deflection at all as an input. Which is a strange behavior for a system which officially is designed to augment stick feel, but I guess in theory it could work as designed even with this limitation.

So, from what I understand, MCAS activates when it detects an angle of attack "it doesn't like", and it keeps adjusting the stabilizer until the angle of attack returns to a value "it likes", or until it hits its hard limit of 2.5 stabilizer trim units (0.6 units at higher speeds).

We don't know exactly what the trigger point is, so I'm just guessing here, but let's assume MCAS was designed to activate before the stick shaker, and you are in a terrain escape maneuver when MCAS activates.

The result would be that the angle of attack reduces with the same stick deflection, and the pilot would have to apply more and more deflection (and force) to reach the stick shaker limit. At some point MCAS would reach its 2.5 units of trim limit and stop adjusting the stabilizer. Then, assuming the elevator has enough authority to counter those 2.5 units of stabilizer trim, the pilot will finally be able to reach the stick shaker limit.

Now, at this point, with the old version, if the pilot blipped the trim switches, 5 seconds later MCAS would reactivate, it would again not like the AoA, and it would feel free to adjust the trim another 2.5 units. Now, would anyone want up to 5 units of additional nose down trim during a terrain escape maneuver?

What I'm trying to say is that, depending on where its trigger AoA point was configured in the original version, MCAS might have been a problem even when the AoA sensors worked properly. Of course, a lot of what I said above is speculation, but MCAS doesn't smell right to me, at least not when described as a "stick force augmentation system".

[billybone]

Another question comes to mind.
What would have happened in a real terrain avoidance maneuver with the stickshaker active ? Wouldn't it have run for 9-10 seconds, adding 2.5 units of nose down trim ? And so overwhelming the elevator authority ?
What's the pilot to do in that case ? Counter trim, with a risk of negating the MCAS raison-d'etre ?
Flip the trim cutout switches on the pedestal ?

If you have to counter/cut off the MCAS at the very moment it is supposed to be useful, why not have the freedom to counter it just by pulling on the yoke ?

Yikes- consider that most terrain avoidance will be at less than 15,000 feet, and by definition within a few thousand feet AGL depending- (as in flying into Juneau Alaska where a steep turn and descend is normal.) Unless flaps are down, MCAS will no doubt be active. IF- big IF the correction to a bird strike or similar does not disable MCAS, the bit about flipping a switch to revert to manual a few thousand feet or less above ground is no place to start cranking trim wheel the few dozen turns needed to reduce the nose down . .

Of course I'm sure the whiz bang kids in their safety analysis will have covered such a case ??? Trust but verify comes to mind !!

[568]

Thank you ! I realize that normally TCAS only provides up/down altitude change alerts supposedly at a range/time sufficient to avoid collision. As I understand it- MCAS only works/engages if/when AP is off, flaps up, etc. And that normally, the AOA sensor provides input to AP. But the disconnect/removal of the yoke/stick switch which has been standard for decades such that a pull or push in opposition to stab/trim input under a TCAS maneuver which would normally cut electric power to stabilizer might be a surprise to many. Seems to me if HAL( MCAS) is/was G limited by design - but no longer is - then a conflict between pilot and HAL is probable ??Just to be sure.There are many inputs to the A/P (AFDS in Boeing terminology, autopilot flight director system) through respective FCC's (Flight control computers) ADC's (Air Data Computers) and a whole lot more,and of course MCAS.

[zzuf]

Define "manoeuvre stability" as it applies to Part 25 certification requirements and I might better be able to address your question.

No, your answer was perfect, you don't quite understand manoeuvre stability and how it may differ from longitudinal static stability.
But, I am at a total loss at how a situation where an aircraft is noted, by the pilot, to have stick force lightening, while it is approaching the stall, with changing speed, alpha, pitch rate, elevator angle, etc, etc and determine that is a static stability problem. The aircraft is not static, it is manoeuvering.
The problem with FAR 25 and the AC's is that the information that you are seeking may well be elsewhere.
But you could try FAR 25.143, and review the guidance material of AC 25.7d for 25.143 and 25.255.

[yoko1]

It was my understanding that the installation/addition of MCAS software AND the wiring change shown in peter lemmes blog resulted in the disconnect of the control column cutout switch-.


Yes, but only for the purposes of MCAS activation. The control column cutout switches work as normal for all other applications.

[Icarus2001]

Could MCAS interfere with the terrain escape maneuver? I think that could be the worst moment for MCAS to activate. I read that you said you are not a pilot. You need to understand that Airbus, Boeing and Embraer all use FBW technology that would prevent excessive pitch attitudes in an EGPWS recovery. The stick shaker can be expected to activate but there may be no stick pusher fitted as the software will not allow pitch up to an excessive AoA. JUST LIKE MCAS. Non pilots seem to be getting indignant about a piece of software "taking over" control of the elevator. Well we already have that in all the FBW aircraft. The big difference is that the B737 is not FBW and Boeing allowed input from one AoA vane. The concept of software limiting pitch is sound. Just like anti tail strike parts of the software.

[MemberBerry]

Icarus It's fine with me if MCAS is a system designed to prevent excessive AoA, which could result in stalls, just like I don't have a problem with the Airbus alpha floor protection. If MCAS activates to prevent reaching an AoA higher than the safe limit, it sounds like a nice feature to have. Of course, assuming MCAS was designed properly with the adequate redundancy in its inputs.

My main problem is the insistence of some people that the purpose of MCAS is just to adjust the stick feel, and it isn't related at all to stall prevention. That may be the case, but Boeing seems to have nevertheless implemented a system that focuses on limiting AoA rather than just adjusting stick feel. So, whatever their original intent was, it seems they actually implemented a stall prevention system.

[bill fly]

...The concept of software limiting pitch is sound...

Trouble is MCAS doesn’t directly limit pitch as FBW can but kicks in a bunch of down stab at high AoA, which is a crude way to affect up feel - and in failure mode can overpower a pilot.

[Icarus2001]

which is a crude way to affect up feel - and in failure mode can overpower a pilot. Not if the pilot uses their thumb on the trim switch on the control column, that will stop MCAS. Or if a pilot holds the centre console trim wheel to stop it moving.

[bill fly]

Not if the pilot uses their thumb on the trim switch on the control column, that will stop MCAS. Or if a pilot holds the centre console trim wheel to stop it moving.

Oh good. There’s nothing to worry about then. The Max should be back on line pretty soon.

[fdr]

1. I am looking at what was stated, and provide the aerodynamic reasoning behind it. Forget about stick feel.

2. If, at high angle of attack, or low speeds, if, due to certain areas of the wing the airflow is going supersonic, the resulting lack of laminar airflow over the wing (as stated by the Boeing wind tunnel tests) induces a stall...

3. The aerodynamics of the engine and the wing are creating stall conditions at lower AoA and other conditions than the system is programmed for.

4. Look at what they tried to do, adding vortex tabs, changing the wing design....

5. Changing the wing design?? Do you change the wing design to make the stick pressure the same, or to prevent stall?
The differential pressure on the yoke is a RESULT of the stall.

6. Stick pressure? I feel that is a half baked response by Boeing to mask the problems with the aerodynamics of the wing/engine design, and simply does not make sense. Maybe that is how is was presented to the FAA, but I dont think that is reality. Boeing will never admit that the aircraft was not aerodynamically stable.

7. Is MCAS operational in AP? While I keep hearing the mantra, it only operation with AP off, it appears it is operational according to several reports that show turning off the AP resolves the problem. Wasnt it the case with the last crash, that when they turned AP back on, MCAS engaged again?

8. In one incident, an airline pilot reported that immediately after engaging the Max 8’s autopilot, the co-pilot shouted “DESCENDING,” followed by an audio cockpit warning, “DON’T SINK! DON’T SINK!”

9. “I immediately disconnected AP (Autopilot) (it WAS engaged as we got full horn etc.) and resumed climb,” the pilot writes in the report, which is available in a database compiled by NASA (https://asrs.arc.nasa.gov/search/database.html). “Now, I would generally assume it was my automation error, i.e., aircraft was trying to acquire a miss-commanded speed/no autothrottles, crossing restriction etc., but frankly neither of us could find an inappropriate setup error (not to say there wasn’t one).

10. In reality, MCAS is anti-stall.



1. "I am looking at what was stated, and provide the aerodynamic reasoning behind it. Forget about stick feel."

stick force is a consequence of aerodynamics, control system architecture and modifiers.

2. (A) "...going supersonic..."

High AOA is inconsistent with high mach numbers in general, wings tend to fall off Par 25 planes when pulling high AOA at high mach, available AOA is constrained by buffet in such cases, so the terminology used is vague... At low speeds, there is no transonic flow on a BAC447-450 type section at any AOA. The BAC airfoil section may have gone supersonic in the Silk air bingle, and possibly Adam Air splash, but otherwise it is rather unlikely to get to a point where the section is supersonic, e.g., has an oblique shock formation. It will usually have a normal shock at around 0.5c-0.6c in cruise, at say M0.78 and above, at 2.3 AOA.

(B) "...lack of laminar flow...";

irrespective of what is being smoked, and how much was spent on the design, there is no laminar flow worth noting on any RPT jet transport. If you want laminar flow, go and look at a standard sail plane. the slat TE destroys laminar flow, as does the first rivet head, screw head etc that exists on the slat. For the first 1% of the chord, which more or less is in the radius of the LE, around the Kutta point, there may be laminar flow dependent on when the plane was last washed. FYI, the slat TE eats up the boundary layer, it is a discontinuity in the surface, and causes separation in the presence of a shear, and that always gives an initial span-wise vortex structure which is highly unstable as is any flow behind an aft facing step... That vortex structure starts to shed with a Strouhal number that is identifiable as a harmonic of the how frequency vibration that arises from the instability of the normal shock and the associated SBLI foot, which is observable oscillating on the wing in steady flight. Look out the window with the sun aligned down the mid chord span and you will have a Schlieren iamge of the shadow of the densty change in the foot of the shock. In essence, at any time, your Boeing, or Bus doesn't have laminar flow anywhere except in the idealised models of the airfoil in simplified CFD modelling. Sorry. That is not to say it can't be improved...

(C ) "...induces a stall..."

not the way flow works. Sorry. A turbulent BL has one benefit over laminar and that is it can cope with adverse pressure gradient perturbations before becoming messy. Flow behind a shock is separated near the surface, but that is not a stall as such, which is defined in the regs... as a number of specific conditions that occur. separation may be a pain, particularly geographic ones, but it is a normal part of life. Stalls are stalls... per the regs. Taking your comment to an extreme, pulling say 10 AOA at M0.82 will still not end up in a stall, it will take the wings right off the aircraft, but before you get to that point, flow separation will have resulted in severe buffet, and reducing Cl/AOA, which put you back toward the beginning...

3. "...The aerodynamics of the engine and the wing are creating stall conditions at lower AoA and other conditions than the system is programmed for..." what is your evidence for that statement? If that was so, then the aircraft would have rather high V speeds, as they are a function of Vs.

There is an interaction between the nacelle and the wing, and was ever thus. If you look on the side of a B737 Classic, NG A320, 330, B777 B787 etc etc etc, you will find, there is a strake on the side of the nacelle around the location of 10 to 2 o'clock, more often on the inner side of the nacelle, but it can be on both sides. The strakes are agnostic, they are on CF6, RR, GE, CFMI engines etc, and they are there as the standard, pre Max and the GTF designs did indeed have an interaction between the nacelle and the wing abaft of the nacelle that suppressed Cl/AOA at modest to high AOAs, and that caused performance decrements. The strake develops a very powerful vortex filament that passes over the top of the LE HLD's and controls the extent of the interaction. It is a thing of beauty, and pretty to watch on a high humidity day. The Vortex itself is not doing the work, it stops the flow adjacent to the filament from being discombobulated.

The Max issue is entirely different, in fact, it goes in the opposite direction, and it increases lift of the section effectively, or at least in the aggregate results in an reduction in the component Cm of the span-wise region behind the nacelle, which is exhibited as a reduction in stick force per g... If you have a back of an envelope you will see that in order to achieve a reduction in Cm, for the location on the planform there is a high likelyhood that the design actually increased local CL, at the same time as moving the Cp forward on that section, and that gives the Cm outcome. Now, Bill B could cure the deal by reducing or removing the strake that is on the max engine, but that comes with a performance penalty, the Max had better performance than it would have otherwise due to the nacelle design, it just came with a side effect that the regs would need a SAS input to meet longitudinal control reqts. Had that been done well, then the Max would have been acknowledged as a great design. Heck, even now, it can be, if Boeing corporate management grows a spine and get their house in order on ethical matters. The design of the MCAS and its changes did not happen in a vacuum, they occurred as a result of the attitude of the beancounters and the ethics that flow down the sewer that is the board room to the poor sod at the coal face.

4. Look at what they tried to do, adding vortex tabs, changing the wing design....

That is what happens with almost all designs, including Busses etc. The dog tooth on the Bus arises from a surprise in testing... VG's are a good tool for curing issues of separation and shock issues, they don't cook means well, but they have their uses. I would be happier if they did do more work on the wing, there is a lot of room for improvement on all of these designs which are the industrial engineers mass produced product rather than the ASW-21 or other embodiment of elegant design.

5. "...Changing the wing design?? Do you change the wing design to make the stick pressure the same, or to prevent stall? The differential pressure on the yoke is a RESULT of the stall..."

There are a number of manners that the design could be tweaked in testing to attempt to achieve compliance with the stick force gradient requirements. As commented above, removing the strake would get rid of the issue, but comes at a cost, whereas MCAS had no cost had it been implemented competently. There are designs out there that have specific application of VGs to meet the same requirement. Almost every aircraft on the ramp has flow modifiers on them, and they are all specific in their application, what the defect was that was underlying their implementation. Each conventional VG has a drag count penalty, according to NASA research of around 0.0002Cd, so they are used sparingly and only when the gain from their use offsets the Cd increase from their installation. Not sure what a differential pressure on a yoke refers to, I only speak english, but if you are referring to the stick force gradient non linearity, that is not caused by stall. Consider that any premature stall around the nacelle will actually improve handling qualities of any aircraft; it will ensure that the lateral control requirements are met, that the aircraft will pitch down, that buffet on the airframe from impingement of wake on the stab and elevators is more likely to be encountered. These are good things. The GTF nacelle doesn't cause a premature stall, it does the opposite... Now if the wing tips stall, then you get great entertainment, and that is not the problem, nor would it arise from the nacelle.... Think of why simple wings have wash out, and why dog tooths, VG's vortilons etc are used on swept wing aircraft. I don't think I agree with your paragraph on that matter...

6. Stick pressure? I feel that is a half baked response by Boeing to mask the problems with the aerodynamics of the wing/engine design, and simply does not make sense. Maybe that is how is was presented to the FAA, but I dont think that is reality. Boeing will never admit that the aircraft was not aerodynamically stable.

If the aircraft was unstable at any point, which is pretty hard to see how that would be achievable, given the margins that exist between the normal aft envelope and the neutral point, then in any case, no TP would have signed off on the acceptability of the aircraft. It is a very straight forward matter to ascertain if the design is or was unstable, and for the record the data that has been published already is sufficient to show that the aircraft was statically and dynamically stable. Long ago I looked at an RPT aircraft that was on the limit of stability in flight, and it is not hard to detect in the data. THE MAX IS NOT UNSTABLE. PERIOD. Don't take my word for it, look at the public domain data on the control inputs. It had a trim issue, and that is all.

7. Is MCAS operational in AP? While I keep hearing the mantra, it only operation with AP off, it appears it is operational according to several reports that show turning off the AP resolves the problem. Wasnt it the case with the last crash, that when they turned AP back on, MCAS engaged again?

Again, as the aircraft IS NOT UNSTABLE, an autopilot doesnt need MCAS to meet any stick force per g compliance matters. That is the first big clue that the issue is and has always been about the force gradient compliance. MCAS would be redundant, a double negative, nonsense for an autopilot engaged condition.


8. "....In one incident, an airline pilot reported that immediately after engaging the Max 8’s autopilot, the co-pilot shouted “DESCENDING,” followed by an audio cockpit warning, “DON’T SINK! DON’T SINK!”

Autopilots are born of man (or woman etc... )per the bard and fail. The reported issue was a pitch down excursion which is an event that is required to be covered in certification, being an aspect that is considered in the minimum engagement and disengagement heights for autopilots. Had the MCAS not resulted in public misinformation and hysteria, then this particular matter would have been kept in its proper place, that being an APFD anomaly, with no association to the MCAS issue.

9. “I immediately disconnected AP (Autopilot) (it WAS engaged as we got full horn etc.) and resumed climb,” the pilot writes in the report, which is available in a database compiled by NASA (https://asrs.arc.nasa.gov/search/database.html). “Now, I would generally assume it was my automation error, i.e., aircraft was trying to acquire a miss-commanded speed/no autothrottles, crossing restriction etc., but frankly neither of us could find an inappropriate setup error (not to say there wasn’t one).

The crews getting into the Max deserved more than an Ipad briefing. However, the comment on the APFD anomaly has nothing to do with MCAS.

10. In reality, MCAS is anti-stall.

Nope, not even close. Refer preceding.



cheers,

[zzuf]

1. "I am looking at what was stated, and provide the aerodynamic reasoning behind it. Forget about stick feel."

stick force is a consequence of aerodynamics, control system architecture and modifiers.

2. (A) "...going supersonic..."

High AOA is inconsistent with high mach numbers in general, wings tend to fall off Par 25 planes when pulling high AOA at high mach, available AOA is constrained by buffet in such cases, so the terminology used is vague... At low speeds, there is no transonic flow on a BAC447-450 type section at any AOA. The BAC airfoil section may have gone supersonic in the Silk air bingle, and possibly Adam Air splash, but otherwise it is rather unlikely to get to a point where the section is supersonic, e.g., has an oblique shock formation. It will usually have a normal shock at around 0.5c-0.6c in cruise, at say M0.78 and above, at 2.3 AOA.

(B) "...lack of laminar flow...";

irrespective of what is being smoked, and how much was spent on the design, there is no laminar flow worth noting on any RPT jet transport. If you want laminar flow, go and look at a standard sail plane. the slat TE destroys laminar flow, as does the first rivet head, screw head etc that exists on the slat. For the first 1% of the chord, which more or less is in the radius of the LE, around the Kutta point, there may be laminar flow dependent on when the plane was last washed. FYI, the slat TE eats up the boundary layer, it is a discontinuity in the surface, and causes separation in the presence of a shear, and that always gives an initial span-wise vortex structure which is highly unstable as is any flow behind an aft facing step... That vortex structure starts to shed with a Strouhal number that is identifiable as a harmonic of the how frequency vibration that arises from the instability of the normal shock and the associated SBLI foot, which is observable oscillating on the wing in steady flight. Look out the window with the sun aligned down the mid chord span and you will have a Schlieren iamge of the shadow of the densty change in the foot of the shock. In essence, at any time, your Boeing, or Bus doesn't have laminar flow anywhere except in the idealised models of the airfoil in simplified CFD modelling. Sorry. That is not to say it can't be improved...

(C ) "...induces a stall..."

not the way flow works. Sorry. A turbulent BL has one benefit over laminar and that is it can cope with adverse pressure gradient perturbations before becoming messy. Flow behind a shock is separated near the surface, but that is not a stall as such, which is defined in the regs... as a number of specific conditions that occur. separation may be a pain, particularly geographic ones, but it is a normal part of life. Stalls are stalls... per the regs. Taking your comment to an extreme, pulling say 10 AOA at M0.82 will still not end up in a stall, it will take the wings right off the aircraft, but before you get to that point, flow separation will have resulted in severe buffet, and reducing Cl/AOA, which put you back toward the beginning...

3. "...The aerodynamics of the engine and the wing are creating stall conditions at lower AoA and other conditions than the system is programmed for..." what is your evidence for that statement? If that was so, then the aircraft would have rather high V speeds, as they are a function of Vs.

There is an interaction between the nacelle and the wing, and was ever thus. If you look on the side of a B737 Classic, NG A320, 330, B777 B787 etc etc etc, you will find, there is a strake on the side of the nacelle around the location of 10 to 2 o'clock, more often on the inner side of the nacelle, but it can be on both sides. The strakes are agnostic, they are on CF6, RR, GE, CFMI engines etc, and they are there as the standard, pre Max and the GTF designs did indeed have an interaction between the nacelle and the wing abaft of the nacelle that suppressed Cl/AOA at modest to high AOAs, and that caused performance decrements. The strake develops a very powerful vortex filament that passes over the top of the LE HLD's and controls the extent of the interaction. It is a thing of beauty, and pretty to watch on a high humidity day. The Vortex itself is not doing the work, it stops the flow adjacent to the filament from being discombobulated.

The Max issue is entirely different, in fact, it goes in the opposite direction, and it increases lift of the section effectively, or at least in the aggregate results in an reduction in the component Cm of the span-wise region behind the nacelle, which is exhibited as a reduction in stick force per g... If you have a back of an envelope you will see that in order to achieve a reduction in Cm, for the location on the planform there is a high likelyhood that the design actually increased local CL, at the same time as moving the Cp forward on that section, and that gives the Cm outcome. Now, Bill B could cure the deal by reducing or removing the strake that is on the max engine, but that comes with a performance penalty, the Max had better performance than it would have otherwise due to the nacelle design, it just came with a side effect that the regs would need a SAS input to meet longitudinal control reqts. Had that been done well, then the Max would have been acknowledged as a great design. Heck, even now, it can be, if Boeing corporate management grows a spine and get their house in order on ethical matters. The design of the MCAS and its changes did not happen in a vacuum, they occurred as a result of the attitude of the beancounters and the ethics that flow down the sewer that is the board room to the poor sod at the coal face.

4. Look at what they tried to do, adding vortex tabs, changing the wing design....

That is what happens with almost all designs, including Busses etc. The dog tooth on the Bus arises from a surprise in testing... VG's are a good tool for curing issues of separation and shock issues, they don't cook means well, but they have their uses. I would be happier if they did do more work on the wing, there is a lot of room for improvement on all of these designs which are the industrial engineers mass produced product rather than the ASW-21 or other embodiment of elegant design.

5. "...Changing the wing design?? Do you change the wing design to make the stick pressure the same, or to prevent stall? The differential pressure on the yoke is a RESULT of the stall..."

There are a number of manners that the design could be tweaked in testing to attempt to achieve compliance with the stick force gradient requirements. As commented above, removing the strake would get rid of the issue, but comes at a cost, whereas MCAS had no cost had it been implemented competently. There are designs out there that have specific application of VGs to meet the same requirement. Almost every aircraft on the ramp has flow modifiers on them, and they are all specific in their application, what the defect was that was underlying their implementation. Each conventional VG has a drag count penalty, according to NASA research of around 0.0002Cd, so they are used sparingly and only when the gain from their use offsets the Cd increase from their installation. Not sure what a differential pressure on a yoke refers to, I only speak english, but if you are referring to the stick force gradient non linearity, that is not caused by stall. Consider that any premature stall around the nacelle will actually improve handling qualities of any aircraft; it will ensure that the lateral control requirements are met, that the aircraft will pitch down, that buffet on the airframe from impingement of wake on the stab and elevators is more likely to be encountered. These are good things. The GTF nacelle doesn't cause a premature stall, it does the opposite... Now if the wing tips stall, then you get great entertainment, and that is not the problem, nor would it arise from the nacelle.... Think of why simple wings have wash out, and why dog tooths, VG's vortilons etc are used on swept wing aircraft. I don't think I agree with your paragraph on that matter...

6. Stick pressure? I feel that is a half baked response by Boeing to mask the problems with the aerodynamics of the wing/engine design, and simply does not make sense. Maybe that is how is was presented to the FAA, but I dont think that is reality. Boeing will never admit that the aircraft was not aerodynamically stable.

If the aircraft was unstable at any point, which is pretty hard to see how that would be achievable, given the margins that exist between the normal aft envelope and the neutral point, then in any case, no TP would have signed off on the acceptability of the aircraft. It is a very straight forward matter to ascertain if the design is or was unstable, and for the record the data that has been published already is sufficient to show that the aircraft was statically and dynamically stable. Long ago I looked at an RPT aircraft that was on the limit of stability in flight, and it is not hard to detect in the data. THE MAX IS NOT UNSTABLE. PERIOD. Don't take my word for it, look at the public domain data on the control inputs. It had a trim issue, and that is all.

7. Is MCAS operational in AP? While I keep hearing the mantra, it only operation with AP off, it appears it is operational according to several reports that show turning off the AP resolves the problem. Wasnt it the case with the last crash, that when they turned AP back on, MCAS engaged again?

Again, as the aircraft IS NOT UNSTABLE, an autopilot doesnt need MCAS to meet any stick force per g compliance matters. That is the first big clue that the issue is and has always been about the force gradient compliance. MCAS would be redundant, a double negative, nonsense for an autopilot engaged condition.


8. "....In one incident, an airline pilot reported that immediately after engaging the Max 8’s autopilot, the co-pilot shouted “DESCENDING,” followed by an audio cockpit warning, “DON’T SINK! DON’T SINK!”

Autopilots are born of man (or woman etc... )per the bard and fail. The reported issue was a pitch down excursion which is an event that is required to be covered in certification, being an aspect that is considered in the minimum engagement and disengagement heights for autopilots. Had the MCAS not resulted in public misinformation and hysteria, then this particular matter would have been kept in its proper place, that being an APFD anomaly, with no association to the MCAS issue.

9. “I immediately disconnected AP (Autopilot) (it WAS engaged as we got full horn etc.) and resumed climb,” the pilot writes in the report, which is available in a database compiled by NASA (https://asrs.arc.nasa.gov/search/database.html). “Now, I would generally assume it was my automation error, i.e., aircraft was trying to acquire a miss-commanded speed/no autothrottles, crossing restriction etc., but frankly neither of us could find an inappropriate setup error (not to say there wasn’t one).

The crews getting into the Max deserved more than an Ipad briefing. However, the comment on the APFD anomaly has nothing to do with MCAS.

10. In reality, MCAS is anti-stall.

Nope, not even close. Refer preceding.



cheers,

Great post!

[bill fly]

Great post!

Yes but I am afraid it will be wasted on the intended recipient

[safetypee]

fdr,
Thank you for an informative and educational post # 663, representing the very best professional interchange, with benefit for individuals, forum, and industry.
I have learnt from the technicalities, similarly from the style and manner of presentation; a personal credit for you and for everyone in enhancing the credibility of this forum.

[NutLoose]

They're getting desperate for storage space now. I take it the pink spreader pads are because the carpark isn't sufficently strong enough to bear the weight.


https://cimg2.ibsrv.net/gimg/pprune.org-vbulletin/1368x827/boeing_car_park_3_c95c208598cd075e63e0c8727692541bcea007a1.j pg

https://www.independent.co.uk/travel/news-and-advice/boeing-planes-737-max-jet-car-park-storage-aircraft-a8973726.html

[TSR2]

As a matter of fact, there are two rows of three aircraft in that car park. Three for TUI and three for Icelandair.

[yoko1]

Great, detailed post by fdr there. Hopefully, we can put to bed all the claims that the 737 is inherently "unstable."

On the issue of MCAS activation during TCAS, GPWS, and windshear events, a few more thoughts.

First, all of these maneuvers are hand-flown. Not only are they hand-flown, but they are dynamic situations in which there are significant changes in thrust and airspeed. As such, the pilot should be trimming constantly through the maneuver using the yoke trim switch. As we know, the yoke trim switch will always override MCAS. If the pilot doesn't know how to properly fly and trim, then that is a training issue, not a design issue.

Second, properly flown a TCAS maneuver should never enter stick shaker region. Windshear and GPWS maneuvers may be in and out of the stick shaker, but they should not be flown with a continuous stick shaker because you certainly don't want to stall the aircraft during the recovery. MCAS has some threshold values in terms of both AOA and time to trigger activation, so it is unclear whether MCAS will even activate if you are only triggering the stick shaker intermittently.

Third, I think there is too much focus on what happens when MCAS activates at a time when it is not needed as it did with the accident aircraft. Outside of the high AOA regions, the MCAS input will increase control force gradient above what would be normally expected/wanted. Approaching the high AOA region for which it was designed, the MCAS input will attempt to maintain the control force gradient as it offsets the extra lift generated by the engine nacelles. In other words, the controls should feel something close to normal when MCAS works as designed.

Finally, the aircraft is not flown with trim, nor does MCAS override the pilot's ability to trim. In the case of any hand-flown maneuver the pilot should use the primary flight controls to set the aircraft attitude, set the thrust to the desired power setting (full thrust in case of windshear or GPWS warning) and then trim accordingly to relieve control pressures. I say again, the pilot should establish the desired attitude and power setting and then trim accordingly! If the new and improved MCAS was making a trim input that the pilot did not want, that pilot could easily override it. Any consideration into aircraft design will assume that the pilot will employ standard hand-flying procedures.

[yoko1]

You need to understand that Airbus, Boeing and Embraer all use FBW technology that would prevent excessive pitch attitudes in an EGPWS recovery.

Slight correction here. I can't speak for newer Boeing's like the 777 & 787, but the 737MAX is not really a FBW aircraft, and you can demand more pitch than required in these maneuvers. There is some automated control logic inserted into the spoiler and rudder controls, but not for the elevator or ailerons.

[Icarus2001]

If you read my post again you will see that I mention that the 737 is not FBW. We are in agreement.

[Avionista]

Yoko1:

As we know, the yoke trim switch will always override MCAS.

That is a fairly sweeping statement that may well be true, for all I know. However, I'm still wondering about the short NU trim commands visible on the FDR print-outs from both accident aircraft. They appear so similar that I wonder if there is some feature or limitation of the manual electric trim system that has yet to be revealed. Hopefully, the accident reports may provide some explanation.

[yoko1]

However, I'm still wondering about the short NU trim commands visible on the FDR print-outs from both accident aircraft.

A Bjorn’s Corner blog provided a pretty detailed analysis that would explain those “blips.” There is no evidence that the pilot’s yoke trim switch failed to work any time that it was used.

[Speed of Sound]

In the sense that the FAA will almost certainly feel the need to get EASA on side before lifting the grounding, then yes, it's down to politics.

With the FAA under fire for seeing it’s role as lobbying on behalf of Boeing as much as being an impartial regulator, it needs to stay well away from EASA and let Boeing deal with certification outside the US itself.

[Just the fax maam]

"There is no evidence that the pilot’s yoke trim switch failed to work any time that it was used."

Oh contraire, there is more than sufficient evidence to demonstrate that, on the balance of probability, the trim system was unable to overcome (in the ANU moment) the AND aerodynamic forces present at that time.

You are more than welcome to dispute this conclusion of course, but you must be prepared to challenge numerous experts from multiple CAA's around the world in order to do so in any meaningful way. Until that occurs, the aircraft is grounded indefinitely.

[yoko1]

Oh contraire, there is more than sufficient evidence to demonstrate that, on the balance of probability, the trim system was unable to overcome (in the ANU moment) the AND aerodynamic forces present at that time..

I have no idea what you mean by “balance of probability.” The pilot’s trim worked or it didn’t. There is no “probable” about it.

To be clear, we are not talking about the manual trim here. We are referring to the main electric trim controlled by the pilot’s yoke switch.

Please show me any “sufficient evidence” from one of these experts that supports the contention that the main electric trim did not work when used or that it could not override MCAS.

[DaveReidUK]

With the FAA under fire for seeing it’s role as lobbying on behalf of Boeing as much as being an impartial regulator, it needs to stay well away from EASA and let Boeing deal with certification outside the US itself.

Well that's one way of looking at it.

Let's wait and see. I'll be amazed if, in due course, the grounding isn't lifted simultaneously by the FAA and EASA on an agreed date.

If not, which do you think will be first ?

"There is no evidence that the pilot’s yoke trim switch failed to work any time that it was used."

Oh contraire, there is more than sufficient evidence to demonstrate that, on the balance of probability, the trim system was unable to overcome (in the ANU moment) the AND aerodynamic forces present at that time.

Could you quote some of this evidence, please ?

All I've seen are indications that manual (wheel) trim can't provide enough mechanical advantage to move the stab back from extreme AND, no suggestion that electric trim (if enabled) couldn't.

Oh and it's "au contraire", by the way. :O

[PEI_3721]

yoko1, # 676

The FDR printout suggests that the electric trim was operable at that time, however there is no evidence that the tail moved - possibly too short a time scale. Thus probability enters the debate; more accurately we don’t know.

Reconsider the EASA question about lack of electric trim throughout the flight envelope (NG). The Boeing response indicated that this was a design intention to minimise the risk of trim runaway, thus use manual wheel. This has not been confirmed or refuted; again we don’t know.

Thus either the elect switch indication on the FDR remains operable, but by design not the trim motor; or whether the inhibition applies in both tail directions. Alternatively if the trim was operable, then the motor was unable to move the tail due to an accumulation of adverse forces, not identified in certification or in the EASA query due to extreme tail position, e.g. large tail nose-down plus elevator nose-up, which as referenced in Borns blog as hydraulic jack stall; and an indication of the magnitude of forces involved.

A further speculative view is of an adverse interaction between manual trim wheel from one pilot and elect trim from the other; what is the priority re clutching mechanism; does manual input de-clutch the electric motor ?

[Just the fax maam]

I have no idea what you mean by “balance of probability.”

This is likely one source of misunderstanding here, as there are many posters here unable to fully comprehend why the grounding is ongoing or what the long delay is a result of.

In the absence of conclusive evidence that demonstrates with absolute certainty that the main electric trim system was 100% operative throughout all phases of both fatal accidents [for the record, there is none], investigators are left drawing probable conclusions based on the available evidence:

1> It is not very plausible that two separate crews comprised four trained, experienced, pilots all elected not to attempt to trim ANU (more than a sub-second blip) when presented with life-threatening ongoing opposing trim in clear conditions in sight of the ground, whilst simultaneously commanding nose up at forces likely not previously experienced.
2> The "blips" are present in the last seconds of both fatal accidents.
3> The "blips" are consistent with an initial ANU trim command, but each has no resulting actual ANU trim of any significance whatsoever, certainly nothing like that required to improve the situation they were presented with. See 1> above.
4> The "blips" are entirely consistent with a shorted and/or overpowered motor.
5> The "blips" look exactly as we would expect a command to an overpowered motor to record; a power spike then null.
6> The manual trim was also overpowered under the aerodynamic loads experienced at that time.
7> "Trim with me" is not the announcement one would expect to be made by a PF who's trim is functioning correctly.
8> The actions of the MS crew, including attempting to briefly trim AND in such a situation, are consistent with a crew dealing with an inoperable main electric trim in the ANU moment.
9> Both crews were aware that an 'auto' trim was trimming against them but were ultimately unable to prevent their aircraft from trimming them into the ground to their certain death and the demise of all onboard.
10> XAA's all around the world have grounded all aircraft of this type until further notice.

I could go on, however... given just the above, and all the other facts as known at this time, we can safely conclude that ON THE BALANCE OF PROBABILITY the main electric trim was totally inoperable in the ANU moment during the last seconds or minutes of the fatal accidents.

That conclusion inexorably leads to some uncomfortable truths, and it is understandable that many here do not wish to go there. But, just as the Soviets learnt the hard way at Chernobyl, in the end, only by shining a bright light on all of the flaws can real progress be made towards rectification.

This aircraft remains grounded worldwide until that happens, despite protestations otherwise, whether on this forum or elsewhere.

[gums]

Salute!

All the FDR traces we have seen show the control col thumb trim switches worked up until the lasts seconds of flight or when the crew turned off all the trim power using the new procedure. So the tiny trim wheel is the last resort. I love it.

I can't rely on 15,000 hours of time in the plane because a new "feature" has been implemented and I do not know about it or the conditions that activate the feature. Ohhh baby....

Oh yeah, the decades-old switches on the col that could override the AP or STS don't work if MCAS is doing its dirty work. Gotta love it, huh?

There's folks here that claim :MCAS won't work if "flaps are down", "MCAS won't work if AP is engaged:, amd so on. Fer chrissakes, if the damned system was supposed to work as intended ( not finally designed and implemented), the stick shaker onset would have been notice to the crews that MCAS would also be in effect. MCAS? What's that, Gums? Oh yeah, forgot to tell you......... GASP!

The accidents happened when MCAS was not supposed to be a player of the plane when approaching a high AoA but below the stall AoA stick shaker threshhold. Right? So with a maxed out AoA doofer we get airspeed warning lights, stick shaker, and the unknown MCAS thing commanding nose down with out me doing a thing except analyzing the warning llghts and stick shaker and,,,,,,,,

Gums sends...

[bill fly]

And then there's folks here that claim :MCAS won't work if flaps are down", "MCAS won't work if AP is engaged:, amd so on.

Well that’s what we have all been told from the first thread here - Haven’t heard it disputed properly yet.
Or do you mean it could be triggered by a fault even when the conditions above are met?

Greetings

[GordonR_Cape]

And then there's folks here that claim :MCAS won't work if flaps are down", "MCAS won't work if AP is engaged:, amd so on.

Well that’s what we have all been told from the first thread here - Haven’t heard it disputed properly yet.
Or do you mean it could be triggered by a fault even when the conditions above are met?

Greetings






There is no evidence that MCAS has ever been triggered under conditions which it was not designed to (AOA notwithstanding). On the other hand, as flight critical software, the FAA and others can't just rely on what they have been told by Boeing. MCAS V2.0 needs to be written and certified to very high standards, which the previous version clearly was not.

Edit: Hypothetically if the flaps-up sensor failed, it could trigger a fault condition.

[Longtimer]

An update:
[QUOTE][[h1]FAA says Boeing needs to mitigate a ‘potential risk’ in 737 Max before grounding order can be liftedPublished 20 min agoUpdated 10 min agoKey Points

The issue was discovered during a simulator test last week, Reuters reported.
Shares of the aerospace company dropped more than 1% following the news, but closed the day up.

The Federal Aviation Administration said on Wednesday that is has found an issue with the Boeing (https://www.cnbc.com/quotes/?symbol=BA) 737 Max that the manufacturer must address before it lifts the national grounding order.

“The FAA is following a thorough process, not a prescribed timeline, for returning the Boeing 737 Max to passenger service. The FAA will lift the aircraft’s prohibition order when we deem it is safe to do so,” the agency said in a statement. “The FAA’s process is designed to discover and highlight potential risks. The FAA recently found a potential risk that Boeing must mitigate.”The issue was discovered during a simulator test last week, Reuters reported. The 737 Max has been grounded since March after two deadly crashes involving the plane. Regulators around the world have pointed to a software issue as a potential cause of the accidents.

Shares of the aerospace company dropped more than 1% following the news, but closed the day up.

“The safety of our airplanes is Boeing’s highest priority. We are working closely with the FAA to safely return the Max to service,” a company spokesperson said in a statement to CNBC.

—Reuters contributed to this report.
Two people briefed on the matter told Reuters that an FAA test pilot during a simulator test last week was running scenarios seeking to intentionally activate the MCAS stall-prevention system. During one activation it took an extended period to recover the stabilizer trim system that is used to control the aircraft, the people said.

It was not clear if the situation can be addressed with a software update or if it is a microprocessor issue, but Boeing has told the FAA it believes the issue can be addressed with a software upgrade.

A hardware fix could add new delays to the plane’s return to service.
/QUOTE]

[DaveReidUK]

1> It is not very plausible that two separate crews comprised four trained, experienced, pilots all elected not to attempt to trim ANU (more than a sub-second blip) when presented with life-threatening ongoing opposing trim in clear conditions in sight of the ground, whilst simultaneously commanding nose up at forces likely not previously experienced.
2> The "blips" are present in the last seconds of both fatal accidents.
3> The "blips" are consistent with an initial ANU trim command, but each has no resulting actual ANU trim of any significance whatsoever, certainly nothing like that required to improve the situation they were presented with. See 1> above.
4> The "blips" are entirely consistent with a shorted and/or overpowered motor.
5> The "blips" look exactly as we would expect a command to an overpowered motor to record; a power spike then null.

Occam's Razor would suggest other, arguably more likely, explanations for all of the above.

[yoko1]

In the absence of conclusive evidence that demonstrates with absolute certainty that the main electric trim system was 100% operative throughout all phases of both fatal accidents [for the record, there is none], investigators are left drawing probable conclusions based on the available evidence:

1> It is not very plausible that two separate crews comprised four trained, experienced, pilots all elected not to attempt to trim ANU (more than a sub-second blip) when presented with life-threatening ongoing opposing trim in clear conditions in sight of the ground, whilst simultaneously commanding nose up at forces likely not previously experienced.
2> The "blips" are present in the last seconds of both fatal accidents.
3> The "blips" are consistent with an initial ANU trim command, but each has no resulting actual ANU trim of any significance whatsoever, certainly nothing like that required to improve the situation they were presented with. See 1> above.
4> The "blips" are entirely consistent with a shorted and/or overpowered motor.
5> The "blips" look exactly as we would expect a command to an overpowered motor to record; a power spike then null.
6> The manual trim was also overpowered under the aerodynamic loads experienced at that time.
7> "Trim with me" is not the announcement one would expect to be made by a PF who's trim is functioning correctly.
8> The actions of the MS crew, including attempting to briefly trim AND in such a situation, are consistent with a crew dealing with an inoperable main electric trim in the ANU moment.
9> Both crews were aware that an 'auto' trim was trimming against them but were ultimately unable to prevent their aircraft from trimming them into the ground to their certain death and the demise of all onboard.
10> XAA's all around the world have grounded all aircraft of this type until further notice.



Concerns about the main electric trim system was absolutely a valid line of enquiry when the MAX accidents first occurred. However, after much scrutiny, this has turned into a dry hole. There really isn't anything there.

First, as to the anomalies you point to, we could get out our magnifying glasses and try to discern what was happening in the final moments of these flight, but that really isn't necessary. We can dispense with all the "blip" data you point to because they all occurred at speeds in excess of Vmo. From a certification standpoint, nothing is guaranteed outside the certification envelope, so no corrective action is warranted. Within the flight envelope, on the other hand, ever time we see evidence that the yoke trim switch was depressed, the stab moved as expected. Every time the yoke trim switch was used while MCAS was attempting to trim, the MCAS input stopped - just as designed.

Second, it seems that you are trying very hard to read into the data something that isn't there. I know it is hard to contemplate that the flying pilots (Lion Air First Officer, Ethiopian Captain) were not effectively use the main electric trim, but that really is the most straightforward answer to the data. That is a training issue, not a design issue.

Finally, and probably most significantly, it is important to note that the 737MAX and its related systems have been under a great deal of scrutiny for many months now. People and organizations with significantly greater knowledge and resources than us have been pouring over everything about the design, certification, training, and operations of this aircraft. There are two investigative bodies (Ethiopian and Indonesian) who are very motivated to find any exculpatory evidence that removes the crew as a factor in these accidents. There have been numerous informative investigative reports by outlets such as the Seattle Times, Wall Street Journal, Washington Times, and Aviation Week & Space Technology to name just a few. Every major certificating authority in the world is looking at this aircraft. Through all this, we have learned many, many things that shed light on problems with the MCAS design, the certification process, the simulators, and crew training. These authorities have publicly laid out their concerns about the type of things they would like to see addressed before the MAX is allowed to fly again.

Conspicuously absent from any of these discussions is any call by any authority for a redesign of the main electric trim system. I have heard of no one in a position of authority suggesting a problem with or a need to redesign any switches, relays, linkages, or motors related to the main electric trim system. If I missed something along these lines, please point me to the reference. If this were a Sherlock Holmes mystery, we could call this a case of the dog that did not bark. The dog isn't barking because there is nothing there.

There certainly are issues with the MAX that need to be dealt with. The main electric trim does not appear to be one of them.

[Smythe]

June 26, 2019: FAA identifies new potential risk on Boeing 737 MAX.
The risk was discovered during a simulator test last week, which likely will prevent BA from running a certification test flight until at least July 8, according to the report.

[GordonR_Cape]

June 26, 2019: FAA identifies new potential risk on Boeing 737 MAX.
The risk was discovered during a simulator test last week, which likely will prevent BA from running a certification test flight until at least July 8, according to the report.

That was from the same article as a lengthy text post by Longtimer though there was no link to the original source: https://www.reuters.com/article/us-ethiopia-airplane-faa-boeing-exclusiv/exclusive-faa-says-identifies-new-potential-risk-on-737-max-idUSKCN1TR30J

[gums]

Salute!

My point is that a failed flaps switch could allow MCAS operation at an AoA below the stick shaker. And our design data presented here indicates that MCAS activates a few degrees below the shaker, or am I way off about this? A/P status also in the same bin- does AP tell the MCAS that it is offline using morethan one signal?

All three episodes had the shaker from the get go due to a bad sensor. But knowledge of the MCAS was absent on the first three.

Gums.....

[OldnGrounded]

From the previously-linked Reuters story:

[. . .] Two people briefed on the matter told Reuters that an FAA test pilot during a simulator test last week was running scenarios seeking to intentionally activate the MCAS stall-prevention system. During one activation it took an extended period to recover the stabilizer trim system that is used to control the aircraft, the people said.

It was not clear if the situation that resulted in an uncommanded dive can be addressed with a software update or if it is a microprocessor issue that will require a hardware replacement.

In a separate statement, Boeing said addressing the new problem would remove a potential source of uncommanded movement by the plane’s stabilizer.

[billybone]

Yoko 1 post 669 26th jun 05 35 said "..
"In the case of any hand-flown maneuver the pilot should use the primary flight controls to set the aircraft " attitude, set the thrust to the desired power setting (full thrust in case of windshear or GPWS warning) and then trim accordingly "


or 3000 feet. Normally cutting thrust will increase pitch DOWN. And MCAS apparently trimmed down much faster than yoke trim up.

Cutting all power ( both AP and Trim ) left only the awkward trim wheel- which at almost any speed above maybe 200 kts is very awkward and difficult at best. And making 10 to 30 turns or more to get to NEAR trim takes time.

And about the so called 3 second rule to discover and suitably correct all bells and whistles and flashing lights and nose down and pull up seems appropriate for which superhero ? Did somehow a zero get dropped over the years ?

So the FAA now discovers re simulator another problem- taking too long to correct ??

[YYZjim]

Re: Hardware capacity

IIRC, the 737 computers use the old 80286 Intel processor. (Ah, those were the days; you could still access the I/O ports directly.) It is easy to see how adding extra tasks for each new 737 variant could lead to situation where interrupt requests (sensor failure, for example) are coming in more quickly than the processor can execute the Interrupt Service Routines. The growing stack will eventually overwrite the code. This could easily lead to "blips" like those observed in the FDR traces, where the pilot presses the yoke trim switch but nothing happens. Not enough harware capacity.

YYZjim

[physicus]

And the saga goes on. It remains inconceivable to me how they are able to certify a system that may, under the right circumstances, be flight critical and a single point of failure, and not be equipped with the required redundancy for that case. If a sensor disagree occurs and that takes the system offline, then it's not a viable safety critical system, that's an unnecessary add-on. So get that third vane in there and wired up and try again.

[ve3id]

Re: Hardware capacity

IIRC, the 737 computers use the old 80286 Intel processor. (Ah, those were the days; you could still access the I/O ports directly.) It is easy to see how adding extra tasks for each new 737 variant could lead to situation where interrupt requests (sensor failure, for example) are coming in more quickly than the processor can execute the Interrupt Service Routines. The growing stack will eventually overwrite the code. This could easily lead to "blips" like those observed in the FDR traces, where the pilot presses the yoke trim switch but nothing happens. Not enough harware capacity.

YYZjim

'access the I/O ports directly': this needs a bit of explanation! If you are in kernel mode on any fit-for-use operating system, you can always access the I/O ports directly - how else do you do I/O at all? Or are you confusing the inadequacies of MS Windows of the 20286 era with the hardware itself?
I am sure nobody in the aviation world uses MS Windows on control computers - typically fail-safe control strategies involve three computers, all voting for control (to avoid byzantine failures) , and each of different binary compatibility and O/S origins, all running the same well -tested algorithm using different languages.

[yoko1]

. This could easily lead to "blips" like those observed in the FDR traces, where the pilot presses the yoke trim switch but nothing happens. Not enough harware capacity.

YYZjim

Except for the small problem that there aren’t any processors in that particular circuit.

[YYZjim]

Hello ve3id:

You are right about one thing -- nobody in the aviation world uses Microsoft Windows, or any other widely-used operating system, in airborne components. They write their programs directly in "machine code", or perhaps an assembler that is the humanized version of machine code. Nevertheless, the software designers have no choice but to use the "instruction set" that their chosen processor is based on. The 80286 has 200 or so basic codes in its instruction set. That's all she's got. Furthermore, the designer is constrained to use the "architecture" of that processor. The 80286, and its successors, have one kind of architecture (Von Neumann), in which the program, data and stack all occupy the same addressable memory. Not at all like the Harvard architecture found in many microprocessors. This has nothing to do with Windows.

Remember the descent of the Eagle to Tranquility? The 1201 and 1202 codes that scared Armstrong and Aldrin denoted exactly the kind of hardware limit I described. Luckily for them, whatever Interrupt Service Routines were being skipped because of the overload were low priority, so the landing continued.

(Incidentally, I'm va3aol.)

YYZjim

[MemberBerry]

'access the I/O ports directly': this needs a bit of explanation! If you are in kernel mode on any fit-for-use operating system, you can always access the I/O ports directly - how else do you do I/O at all? Or are you confusing the inadequacies of MS Windows of the 20286 era with the hardware itself?
I am sure nobody in the aviation world uses MS Windows on control computers - typically fail-safe control strategies involve three computers, all voting for control (to avoid byzantine failures) , and each of different binary compatibility and O/S origins, all running the same well -tested algorithm using different languages.

With modern CPUs the I/O ports are on the same bus as the memory. The instruction set is backwards compatible so old software can still work the same way as before. However the modern architecture supports a new feature: I/O devices can read and write directly from/to memory, bypassing the CPU, which can be a huge benefit reducing CPU load during large I/O operations.

[ve3id]

With modern CPUs the I/O ports are on the same bus as the memory. The instruction set is backwards compatible so old software can still work the same way as before. However the modern architecture supports a new feature: I/O devices can read and write directly from/to memory, bypassing the CPU, which can be a huge benefit reducing CPU load during large I/O operations.
This is not a new feature, it was available on the PDP11 way back in the eighties when I was an FE on DEC hardware!

[OldnGrounded]

This is not a new feature, it was available on the PDP11 way back in the eighties when I was an FE on DEC hardware!

Yup, up until the PDP-11/45 Unibus and 11/83 Q-bus systems, when the memory circuitry was physically separated.

[YYZjim]

Hello yoko1:

Re: No computers in that circuit

My only knowledge about 737 electrical systems comes from PPRUNE. (So I'm a fully-informed expert, right?) In the early days of this thread, back before Ethiopia, there was some discussion about the encoding of angular data produced by AOA sensors. The topic came up as one way to explain a constant difference between values reported by the port and starboard vanes, which is to say, a stuck bit.

Digitization of the AOA reading(s) is, presumably, a first step toward further computer processing. A number of schematics have appeared here on PPRUNE, but they seem to have focussed on the switching logic only, with little detail about where the airplane's own decision-making about MCAS takes place.

I have a very hard time believing that there is no digital computing anywhere in the trim circuit. I am mindful that the military versions of 737 MCAS do have the supplementary g-input that was originally proposed for the civilian MAX. That would have required a whole new layer of analysis and decision-making, and I would be surprised to find that task being by an analogue device. No, there have to be digital computers somewhere in the trim system, as the mechanism by which the airplane controls the trim when it is supposed to.

Perhaps the better question is this: When a pilot presses his yoke trim switch, does that action immediately energize the trim motor with current? Or, instead, is the pilot's action routed as an input to the trim computer, which actually controls the voltage applied to the trim motor? The difference could be fatal if the computer is too busy with other things to process the pilot's command first.

YYZjim

[MemberBerry]

This is not a new feature, it was available on the PDP11 way back in the eighties when I was an FE on DEC hardware!

I have no experience with that, so I can't comment on it, but at least in the x86 world you needed additional hardware to do that, for example DMA controllers. With PCI that changed, each PCI device could take control of the bus when it needed it, unless another device was using it. Of course it's not really that simple, there is still the chipset managing that and preventing conflicts.

There are a lot of variations on this theme, the most recent being I/O devices that can write directly to the CPU cache memory to improve performance even further. Anyway got too carried away, sorry for going off topic.

[ve3id]

Yup, up until the PDP-11/45 Unibus and 11/83 Q-bus systems, when the memory circuitry was physically separated.
Best use I ever saw of the 11/45 was in the wind tunnel at Uplands National Research Centre near Ottawa. They had full bipolar memory in it - probably the fastest PDP11 that ever existed. Faster machine for many years after that!

[Speed of Sound]

Let's wait and see. I'll be amazed if, in due course, the grounding isn't lifted simultaneously by the FAA and EASA on an agreed date.

If not, which do you think will be first ?



FAA approval will come first because they are working very closely with Boeing and are the ‘home’ regulator. EASA will lift the grounding after a respectable period of time, mainly for political reasons. The FAA realises how important the MAX is to Boeing and will not let anything slide which may cause the huge embarrassment of the FAA lifting the grounding while other regulators say no, hence the latest ‘hardware issues’ delay. Boeing are smart enough to know that trying to bounce the FAA into an early decision will only serve to increase caution and suspicion in other regulators.

The MAX will only fly again when all parties, including Boeing, are convinced that it is completely safe to do so.

[ve3id]

I have no experience with that, so I can't comment on it, but at least in the x86 world you needed additional hardware to do that, for example DMA controllers. With PCI that changed, each PCI device could take control of the bus when it needed it, unless another device was using it. Of course it's not really that simple, there is still the chipset managing that and preventing conflicts.

There are a lot of variations on this theme, the most recent being I/O devices that can write directly to the CPU cache memory to improve performance even further. Anyway got too carried away, sorry for going off topic.
Most engineers that designed embedded control systems in the 80's and afterwards were weaned on the PDP11 or other DEC machines that used memory-mapped I/O and included it in their designs. Motorola used it on the 6800 and Intel were the ones that promoted separate I/O instructions. Nowadays most control systems use memory-mapped I/O. So as soon as machines were developed that had memory addressing, I/O could not be access in user mode. Maybe memory-mapped I/O was the majority of designs for technical reasons, but I think the fact that you had to buy a $10,000 workstation to develop for the 8008.8080, and Motorola gave away kits of parts for engineers to build and write code on a cross-assembler on their department PDP11 went a long way to giving memory-mapped addressing the edge that it now enjoys.

[yoko1]

Perhaps the better question is this: When a pilot presses his yoke trim switch, does that action immediately energize the trim motor with current? Or, instead, is the pilot's action routed as an input to the trim computer, which actually controls the voltage applied to the trim motor? The difference could be fatal if the computer is too busy with other things to process the pilot's command first.

YYZjim

As far as the main electric trim (activated by the yoke trim switch), it's all switches and relays as far as the electric part of it goes. No computers needed. Keep in mind that the basic 737 design goes back 50 years, so Boeing figured out how to do a lot of things without IC chips. There have been updates along the way, of course, but the main electric trim system is pretty robust and reliable.

The automatic trim inputs (Speed Trim, Mach Trim, Autopilot) are a different matter.

[patplan]

The poor old processor got overloaded by data came from various sources? I think I posted somewhere that the new routine(s) associated with the MCAS data could overwhelm the control system from time to time, especially during the failure mode [i.e. vane's malfunction].

New flaw discovered on Boeing 737 Max, sources say


...A new flaw has been discovered in the computer system for the Boeing 737 Max (https://www.cnn.com/2019/03/18/world/boeing-737-crashes-similarities/index.html) that could push the plane downward, according to two sources familiar with the testing, an issue that is expected to further delay the aircraft's return to service.

A series of simulator flights to test new software developed by Boeing revealed the flaw, according to one of the sources...

...In simulator tests, government pilots discovered that a microprocessor failure could push the nose of the plane toward the ground. It is not known whether the microprocessor played a role in either crash.
When testing the potential failure of the microprocessor in the simulators, "it was difficult for the test pilots to recover in a matter of seconds," one of the sources said. "And if you can't recover in a matter of seconds, that's an unreasonable risk.".

Boeing engineers are now trying to address the issue, which has led to another delay in recertifying the 737 Max...
===========



- https://edition.cnn.com/2019/06/26/politics/boeing-737-max-flaw/index.html

[patplan]

Also propagated by Reuters...Boeing suffers new 737 Max issue that could delay return...Reuters, which first reported the new issue, said during an FAA pilot simulation in which the stall-prevention system was activated, it took longer than expected to recover the aircraft.

Other sources said the problem was linked to the aircraft's computing power and whether the processor lacked enough capacity to keep up.

Boeing said "we are working closely with the FAA to safely return the Max to service" and that it believed a software fix would address the problem.

But the FAA will be looking into whether it is a hardware issue.

If regulators are unsatisfied with the software fix, the microprocessor unit would have to be replaced and the grounding could stretch on for months longer than previously thought...==========


- https://www.bbc.com/news/business-48752932

[OldnGrounded]

...In simulator tests, government pilots discovered that a microprocessor failure could push the nose of the plane toward the ground.

I'm guessing that, as patplan suggested above, the problem is not an actual (processor) hardware failure. More likely, it's a failure to see or process inbound data, which could be caused by _________ <<< insert favorite cause here.

Whatever it may be, this is another "not reassuring" development, if the reports are accurate.

[billybone]

In view of the current ' discovery ' by FAA re microprocessor issue , this April long detailed article seems to provide some factual data. ??

Fred George Aviation Week & Space Technology Apr 11, 2019

Demo

Fred George Aviation Week & Space Technology Apr 11, 2019

Boeing has demonstrated the old and new versions of the MAX’s Maneuvering Characteristics Augmentation System (MCAS) to pilots and regulators in its 737 MAX engineering cab simulator in Seattle. The MCAS is a new flight-control-computer (FCC) function added to the MAX to enable it to meet longitudinal stability requirements for certification.

However, the system is only needed to enhance stability with slats and flaps retracted at very light weights and full aft center of gravity (CG). The aircraft exhibits sufficient natural longitudinal stability in all other parts of the flight envelope without the MCAS to meet the rules. Boeing emphasizes that the MCAS is not an anti-stall or stall-prevention system, as it often has been portrayed in news reports.

The new software load [P12.1] has triple-redundant filters that prevent one or both angle-of-attack (AOA) systems from sending erroneous data to the FCCs that could falsely trigger the MCAS. It also has design protections that prevent runaway horizontal stabilizer trim from ever overpowering the elevators. Boeing showed pilots that they can always retain positive pitch control with the elevators, even if they don’t use the left and right manual trim wheels on the sides of the center console to trim out control pressures after turning off the trim cut-out switches.

Most important, the MCAS now uses both left and right AOA sensors for redundancy, instead of relying on just one. The FCC P12.1’s triple AOA validity checks include an average value reasonability filter, a catastrophic failure low-to-high transition filter and a left versus right AOA deviation filter. If any of these abnormal conditions are detected, the MCAS is inhibited.

Three secondary protections are built into the new software load. First, the MCAS cannot trim the stabilizer so that it overpowers elevator pitch control authority. The MCAS nose-down stab trim is limited so that the elevator always can provide at least 1.2g of nose-up pitch authority to enable the flight crew to recover from a nose-low attitude. Second, if the pilots make electric pitch trim inputs to counter the MCAS, it won’t reset after 5 sec. and repeat subsequent nose-down stab trim commands. And third, if the MCAS nose-down stab trim input exceeds limits programmed into the new FCC software, it triggers a maintenance message in the onboard diagnostics system.

According to a pilot who was shown the changes in a simulator session, the demonstration begins with the original MCAS software load. During a normal takeoff, at rotation, the left AOA indication moves to its maximum reading—as seen from the flight data recorder in the Ethiopian Airlines accident. Pilots currently do not experience this during initial or recurrent simulator training. The stickshaker fires continuously, using loud sound and control wheel vibration to focus the pilot’s attention on the critically high AOA indication.


The erroneous AOA reading also creates large-scale indicated airspeed (IAS) and altitude errors on the primary flight display (PFD) which can be both distracting and disorienting.

AOA is used by the aircraft’s air data computers to correct pitot and static pressure variations induced by changes in nose attitude in relation to the relative wind. Large errors in AOA can cause 20-40-kt. errors in IAS and 200-400-ft. errors in indicated altitude. This is accompanied by the illumination of annunciators on both PFDs that warn of disparities in the IAS and altitude between the left and right displays. As part of the MCAS redesign, Boeing also is upgrading the MAX with AOA dial indicator displays and AOA disagree warning annunciators on the PFDs.

After the high-AOA indication, pilots then follow the checklist for “airspeed unreliable,” which assures that auto-pilot, auto-throttles and flight directors are turned off. They then pull back power to 80% fan speed, set 10-deg. nose-up pitch attitude and climb to 1,000 ft. above ground level. At that point, they lower the nose, start accelerating and begin retracting slats and flaps at 210 kt. indicated airspeed. When the slats and flaps are fully retracted—the MCAS kicks in.

“It’s a good thing we knew what to expect. Otherwise tunnel vision from the ‘airspeed unreliable’ event could have blinded us to the subsequent MCAS nose-down trim input. When I noticed the trim wheels racing, I grabbed the left wheel. It was easy to stop the trim with hand pressure, but I knew in advance what was happening,” says the pilot flying. “We followed the checklist for runaway stabilizer, checking again for auto-pilot off and auto-throttle off. We turned off both trim cut-out switches and cranked the ‘frisbees’ [manual trim wheels on both sides of the center console] to relieve control pressures. We used manual trim for the remainder of the flight to landing touchdown and rollout. That was quite an eye-opener, as I had never been exposed to that during sim training,” he notes.

It is critical to follow the checklist memory items: Pull back thrust to 75% after retracting slats and flaps and set attitude at 4 deg., nose up. If speed builds up beyond 220-250 kt., controllability becomes increasingly difficult, he adds.

Pilots for three U.S. air carriers tell Aviation Week that during their sim training they had never been exposed to extreme and continuous AOA indication errors, they’ve not experienced AOA induced airspeed and altitude deviations on PFDs and have not had to deal with continuous stall-warning stickshaker distractions. They also note that they have never been required to fly the aircraft from the point at which a runaway stab trim incident occurred all the way to landing using only the manual trim wheels. “We’re just checking boxes for the FAA,” says one Seattle-based pilot.

A full aerodynamic stall with the MCAS inoperative is another exercise pilots experience in the MAX engineering cab simulator. “We reduced thrust at 5,000 ft. and slowed the aircraft at about 1 kt. per sec. We were at a midrange cg [center of gravity] with gear, slats and flats up. We trimmed until we reached 30% above stall speed and then just continued to ease back on the control wheel,” one of the pilots says.

“Pitch feel was natural, progressively increasing as airspeed decayed. Somewhere between the audible low airspeed warning and stickshaker, I felt the slightest lightening on control pressure in my fingertips. Quite candidly, if I had not been watching for it, I don’t think I would have noticed any difference between the MAX and the Next Gen [NG] models. I kept pulling back through stickshaker, then buffet, then elevator feel shift [a function that doubles the artificial control feel forces near stall] and finally until the yoke was buried in my lap. The nose just flopped down gently at the stall, and I initiated recovery as I would in most other airplanes I’ve flown,” he adds.

During design of the MAX, Boeing added two more leading-edge vortilons [generating vortices over the top of the wing at high AOA] in 2018, for a total of six per side and also lengthened and raised the inboard leading-edge stall strips to assure stall behavior would be as docile as that of the NG.

Repeating many of the same maneuvers in the engineering cab simulator with the new software load would have been academic at best, as the triple-redundant AOA validity checks all but assure that the MCAS will not be triggered by erroneous AOA inputs in the future. But, FCC P12.1 changes do not protect against erroneous AOA causing stickshaker or large-scale distortions in indicated airspeed and altitude values. Those malfunctions still can cause distraction and disorientation, especially when flying at night and/or in instrument conditions.

The new MCAS protections built into the P12.1 software load preserve its essential role in enhancing the MAX’s longitudinal stability, while virtually guaranteeing that it won’t be triggered by erroneous AOA. And when it does activate, its nose-down stabilizer trim command authority will be limited to assure the pilots always can control aircraft pitch with the elevators.

However, the FCC software upgrades are not the only critical changes needed to boost safety margins for operators. Pilots who underwent the demonstration also say the sessions underscored the need for additional simulator training for dealing with compound emergencies involving AOA and runaway trim failures.



To find a 'new' issue at this late date indicates BA needs some severe housecleaning..

[Smythe]

FDR...verbose as you are , it is not correct on many fronts..

1. "I am looking at what was stated, and provide the aerodynamic reasoning behind it. Forget about stick feel."
stick force is a consequence of aerodynamics, control system architecture and modifiers.

That is what I stated, stick feel is a result of aerodynamics, not the other way around...

2. (A) "...going supersonic..."

High AOA is inconsistent with high mach numbers in general, wings tend to fall off Par 25 planes when pulling high AOA at high mach, available AOA is constrained by buffet in such cases, so the terminology used is vague... At low speeds, there is no transonic flow on a BAC447-450 type section at any AOA. The BAC airfoil section may have gone supersonic in the Silk air bingle, and possibly Adam Air splash, but otherwise it is rather unlikely to get to a point where the section is supersonic, e.g., has an oblique shock formation. It will usually have a normal shock at around 0.5c-0.6c in cruise, at say M0.78 and above, at 2.3 AOA.


You misquoted what I stated. Surfaces do go transonic, I did not mention only low speed, but high AOA...Boeing does mention low speed stall, AND high AoA AND high G ....not inclusive. Many surfaces go transonic on the wing, fuselage and cowlings...

Boeing DOES state that in testing, they found airflow over the wing went transonic, and tried to alleviate this issue with vortex tabs and changing the wing design...Boeing own words, not mine.

(B) "...lack of laminar flow...";

irrespective of what is being smoked, and how much was spent on the design, there is no laminar flow worth noting on any RPT jet transport. If you want laminar flow, go and look at a standard sail plane. the slat TE destroys laminar flow, as does the first rivet head, screw head etc that exists on the slat. For the first 1% of the chord, which more or less is in the radius of the LE, around the Kutta point, there may be laminar flow dependent on when the plane was last washed. FYI, the slat TE eats up the boundary layer, it is a discontinuity in the surface, and causes separation in the presence of a shear, and that always gives an initial span-wise vortex structure which is highly unstable as is any flow behind an aft facing step... That vortex structure starts to shed with a Strouhal number that is identifiable as a harmonic of the how frequency vibration that arises from the instability of the normal shock and the associated SBLI foot, which is observable oscillating on the wing in steady flight. Look out the window with the sun aligned down the mid chord span and you will have a Schlieren iamge of the shadow of the densty change in the foot of the shock. In essence, at any time, your Boeing, or Bus doesn't have laminar flow anywhere except in the idealised models of the airfoil in simplified CFD modelling. Sorry. That is not to say it can't be improved...


No laminar flow over the wing??? Really...explain this video:
https://www.youtube.com/watch?v=HekbC6Pl4_Y



(C ) "...induces a stall..."

not the way flow works. Sorry. A turbulent BL has one benefit over laminar and that is it can cope with adverse pressure gradient perturbations before becoming messy. Flow behind a shock is separated near the surface, but that is not a stall as such, which is defined in the regs... as a number of specific conditions that occur. separation may be a pain, particularly geographic ones, but it is a normal part of life. Stalls are stalls... per the regs. Taking your comment to an extreme, pulling say 10 AOA at M0.82 will still not end up in a stall, it will take the wings right off the aircraft, but before you get to that point, flow separation will have resulted in severe buffet, and reducing Cl/AOA, which put you back toward the beginning...

Again, as verbose as that is, how does it relate to the MAX stalling at relatively low AoA comparatively? On low speed final, it pushes the nose down 2.5 degrees to avoid stall...what AoA are you at on final?

4. Look at what they tried to do, adding vortex tabs, changing the wing design....

That is what happens with almost all designs, including Busses etc. The dog tooth on the Bus arises from a surprise in testing... VG's are a good tool for curing issues of separation and shock issues, they don't cook means well, but they have their uses. I would be happier if they did do more work on the wing, there is a lot of room for improvement on all of these designs which are the industrial engineers mass produced product rather than the ASW-21 or other embodiment of elegant design.

Of course, that is why they still have vortex tabs running down the wing....but you say this cures issues of separation, but deny laminar flow over the wing?

5. "...Changing the wing design?? Do you change the wing design to make the stick pressure the same, or to prevent stall? The differential pressure on the yoke is a RESULT of the stall..."

There are a number of manners that the design could be tweaked in testing to attempt to achieve compliance with the stick force gradient requirements. As commented above, removing the strake would get rid of the issue, but comes at a cost, whereas MCAS had no cost had it been implemented competently. There are designs out there that have specific application of VGs to meet the same requirement. Almost every aircraft on the ramp has flow modifiers on them, and they are all specific in their application, what the defect was that was underlying their implementation. Each conventional VG has a drag count penalty, according to NASA research of around 0.0002Cd, so they are used sparingly and only when the gain from their use offsets the Cd increase from their installation. Not sure what a differential pressure on a yoke refers to, I only speak english, but if you are referring to the stick force gradient non linearity, that is not caused by stall. Consider that any premature stall around the nacelle will actually improve handling qualities of any aircraft; it will ensure that the lateral control requirements are met, that the aircraft will pitch down, that buffet on the airframe from impingement of wake on the stab and elevators is more likely to be encountered. These are good things. The GTF nacelle doesn't cause a premature stall, it does the opposite... Now if the wing tips stall, then you get great entertainment, and that is not the problem, nor would it arise from the nacelle.... Think of why simple wings have wash out, and why dog tooths, VG's vortilons etc are used on swept wing aircraft. I don't think I agree with your paragraph on that matter...

But we do appear to agree that he stick feel is a result, correct?

6. Stick pressure? I feel that is a half baked response by Boeing to mask the problems with the aerodynamics of the wing/engine design, and simply does not make sense. Maybe that is how is was presented to the FAA, but I dont think that is reality. Boeing will never admit that the aircraft was not aerodynamically stable.

If the aircraft was unstable at any point, which is pretty hard to see how that would be achievable, given the margins that exist between the normal aft envelope and the neutral point, then in any case, no TP would have signed off on the acceptability of the aircraft. It is a very straight forward matter to ascertain if the design is or was unstable, and for the record the data that has been published already is sufficient to show that the aircraft was statically and dynamically stable. Long ago I looked at an RPT aircraft that was on the limit of stability in flight, and it is not hard to detect in the data. THE MAX IS NOT UNSTABLE. PERIOD. Don't take my word for it, look at the public domain data on the control inputs. It had a trim issue, and that is all.

We are over 105 days on the grounding, and the FAA found another issue. Perhaps unstable is a bit harsh, but certainly, in many conditions , it is not predictable....

7. Is MCAS operational in AP? While I keep hearing the mantra, it only operation with AP off, it appears it is operational according to several reports that show turning off the AP resolves the problem. Wasnt it the case with the last crash, that when they turned AP back on, MCAS engaged again?

Again, as the aircraft IS NOT UNSTABLE, an autopilot doesnt need MCAS to meet any stick force per g compliance matters. That is the first big clue that the issue is and has always been about the force gradient compliance. MCAS would be redundant, a double negative, nonsense for an autopilot engaged condition.

What if MCAS is far more reaching than Boeing has stated? It certainly appears to be. Perhaps the half FBW design has inherent flaws that were either unintended or incorrectly implemented in the software?


8. "....In one incident, an airline pilot reported that immediately after engaging the Max 8’s autopilot, the co-pilot shouted “DESCENDING,” followed by an audio cockpit warning, “DON’T SINK! DON’T SINK!”

Autopilots are born of man (or woman etc... )per the bard and fail. The reported issue was a pitch down excursion which is an event that is required to be covered in certification, being an aspect that is considered in the minimum engagement and disengagement heights for autopilots. Had the MCAS not resulted in public misinformation and hysteria, then this particular matter would have been kept in its proper place, that being an APFD anomaly, with no association to the MCAS issue.


Again, over 100 days says that MCAS may be a lot more active than anticipated.

9. “I immediately disconnected AP (Autopilot) (it WAS engaged as we got full horn etc.) and resumed climb,” the pilot writes in the report, which is available in a database compiled by NASA (https://asrs.arc.nasa.gov/search/database.html). “Now, I would generally assume it was my automation error, i.e., aircraft was trying to acquire a miss-commanded speed/no autothrottles, crossing restriction etc., but frankly neither of us could find an inappropriate setup error (not to say there wasn’t one).

The crews getting into the Max deserved more than an Ipad briefing. However, the comment on the APFD anomaly has nothing to do with MCAS.

I completely agree the pilots deserve more. I also think that there are many unintended consequences that can line up in the software...and MCAS is far more active than thought...again, over 100 days, and still finding issues....

10. In reality, MCAS is anti-stall.

Nope, not even close. Refer preceding.

How is the low speed stall issue, not a stall issue? Stick pressure was not mentioned, simply low speed stall. MCAS was designed to push the nose down 2.5 degrees to prevent stall. Where is the interpretation here? 0.6 pitch up on DEP, okay, some lift and stick feel, but jeez, 2.5 on final? (of course, as we all know, its all 2.5 now) What about high G? Where the heck does MCAS come into play in this scenario?

[Maninthebar]

As far as the main electric trim (activated by the yoke trim switch), it's all switches and relays as far as the electric part of it goes. No computers needed. Keep in mind that the basic 737 design goes back 50 years, so Boeing figured out how to do a lot of things without IC chips. There have been updates along the way, of course, but the main electric trim system is pretty robust and reliable.

The automatic trim inputs (Speed Trim, Mach Trim, Autopilot) are a different matter.

In the event that MCAS (or another system) is active the design is that trim signal from the yoke switches overrides* the system-derived input. The decision on which takes priority must be 'processed'. Ergo SOME processing power is involved and a processor, somewhere, is in line.

* I have used the word 'override' but I think 'cancel' might be better in the case of MCAS, or even postpone!

[fdr]

This is an unfortunate development in the process, but two positives exist, 1; the new issue has been discovered before RTS, and 2; it has been acknowledged.

TBC can spend a lot of time sorting out the hardware on this, and probably about the same in getting a software solution, or they could add an interrupt off the ANU side of the trim switches to de-power MCAS in all cases. An isolating system using existing switches would be a quick but dirty solution to the problem. The lights will be burning in Seattle, and hopefully the lighting bill is paid for by stopping paying to lawyers and putting the funds into the company where it is needed, solving the problems of expediency.

[Smythe]

From NPR on the latest...is this getting back to the trim wheel issue?

"Boeing has developed a software fix for that flight control system, called MCAS, but sources familiar with the situation tell NPR that in simulator testing last week, that FAA test pilots discovered a separate issue that affected their ability to quickly and easily follow recovery procedures for runaway stabilizer trim and stabilize the aircraft."

[HighWind]

I had seen this coming..
https://www.pprune.org/10492558-post371.html

This needed computer hardware upgrade might be related to the risks of having a DAL C architecture instead of a DAL A architecture. (And the difference in failure rate)
With the DAL C architecture the pilot have to perform the fault isolation (since the failure rate is to high), and if that are not reacting fast enough they need to be able mussel the aircraft back in control from a full run-away within the envelope.
This is needed to fulfill CS 25.671.

A DAL A architecture can in principle remove the need for trim wheel and cutout switches (if they want to go this route)

[hans brinker]

This is likely one source of misunderstanding here, as there are many posters here unable to fully comprehend why the grounding is ongoing or what the long delay is a result of.

In the absence of conclusive evidence that demonstrates with absolute certainty that the main electric trim system was 100% operative throughout all phases of both fatal accidents [for the record, there is none], investigators are left drawing probable conclusions based on the available evidence:

1> It is not very plausible that two separate crews comprised four trained, experienced, pilots all elected not to attempt to trim ANU (more than a sub-second blip) when presented with life-threatening ongoing opposing trim in clear conditions in sight of the ground, whilst simultaneously commanding nose up at forces likely not previously experienced.
2> The "blips" are present in the last seconds of both fatal accidents.
3> The "blips" are consistent with an initial ANU trim command, but each has no resulting actual ANU trim of any significance whatsoever, certainly nothing like that required to improve the situation they were presented with. See 1> above.
4> The "blips" are entirely consistent with a shorted and/or overpowered motor.
5> The "blips" look exactly as we would expect a command to an overpowered motor to record; a power spike then null.
6> The manual trim was also overpowered under the aerodynamic loads experienced at that time.
7> "Trim with me" is not the announcement one would expect to be made by a PF who's trim is functioning correctly.
8> The actions of the MS crew, including attempting to briefly trim AND in such a situation, are consistent with a crew dealing with an inoperable main electric trim in the ANU moment.
9> Both crews were aware that an 'auto' trim was trimming against them but were ultimately unable to prevent their aircraft from trimming them into the ground to their certain death and the demise of all onboard.
10> XAA's all around the world have grounded all aircraft of this type until further notice.

I could go on, however... given just the above, and all the other facts as known at this time, we can safely conclude that ON THE BALANCE OF PROBABILITY the main electric trim was totally inoperable in the ANU moment during the last seconds or minutes of the fatal accidents.

That conclusion inexorably leads to some uncomfortable truths, and it is understandable that many here do not wish to go there. But, just as the Soviets learnt the hard way at Chernobyl, in the end, only by shining a bright light on all of the flaws can real progress be made towards rectification.

This aircraft remains grounded worldwide until that happens, despite protestations otherwise, whether on this forum or elsewhere.

yeah.....

“oh contraire” (sic):
The trim rim inputs are recorded separate from the THS movement. In both flights there were longer periods of manual inputs resulting in longer movements of the THS. In both flights towards the end there were short manual inputs followed by small THS movement. In both flights following manual inputs there was MCAS trimming AND. At no point in the FDR readout was there a prolonged manual input followed by no movement of the THS (the real indicator of an overpowered trim motor).

The balance of probability is there is an absence of knowledge on your part, and an overwhelming amount of long words to compensate for that.

[fdr]

FDR...verbose as you are , it is not correct on many fronts..


That is what I stated, stick feel is a result of aerodynamics, not the other way around...



You misquoted what I stated. Surfaces do go transonic, I did not mention only low speed, but high AOA...Boeing does mention low speed stall, AND high AoA AND high G ....not inclusive. Many surfaces go transonic on the wing, fuselage and cowlings...

Boeing DOES state that in testing, they found airflow over the wing went transonic, and tried to alleviate this issue with vortex tabs and changing the wing design...Boeing own words, not mine.



No laminar flow over the wing??? Really...explain this video:
https://www.youtube.com/watch?v=HekbC6Pl4_Y





Again, as verbose as that is, how does it relate to the MAX stalling at relatively low AoA comparatively? On low speed final, it pushes the nose down 2.5 degrees to avoid stall...what AoA are you at on final?

.

Of course, that is why they still have vortex tabs running down the wing....but you say this cures issues of separation, but deny laminar flow over the wing?



But we do appear to agree that he stick feel is a result, correct?



We are over 105 days on the grounding, and the FAA found another issue. Perhaps unstable is a bit harsh, but certainly, in many conditions , it is not predictable....



What if MCAS is far more reaching than Boeing has stated? It certainly appears to be. Perhaps the half FBW design has inherent flaws that were either unintended or incorrectly implemented in the software?




Again, over 100 days says that MCAS may be a lot more active than anticipated.



I completely agree the pilots deserve more. I also think that there are many unintended consequences that can line up in the software...and MCAS is far more active than thought...again, over 100 days, and still finding issues....



How is the low speed stall issue, not a stall issue? Stick pressure was not mentioned, simply low speed stall. MCAS was designed to push the nose down 2.5 degrees to prevent stall. Where is the interpretation here? 0.6 pitch up on DEP, okay, some lift and stick feel, but jeez, 2.5 on final? (of course, as we all know, its all 2.5 now) What about high G? Where the heck does MCAS come into play in this scenario?

I'm off on a test flight, however will leave the following reading list for you Smythe, and I will respond later... Being verbose, possibly, however, your posts cover a lot of scope and therefore the responses are necessarily expansive.

Suggested Reading:

Start with Ira & Albert... Theory of Wing Sections, an oldie but a goodie.

then:
Lin, J. C. (NASA). (2002). Review of research on low-profile vortex generators to control boundary-layer separation. Progress in Aerospace Sciences, 38(4–5), 389–420. https://doi.org/10.1016/S0376-0421(02)00010-6

Lin John C. (1999). Control of Turbulent Boundary-Layer Separation using Micro-Vortx Generators. In 30th AIAA Fluid Dynamics Conference. Norfolk, VA: AIAA.

Crawford, B. K., Jr, G. T. D., West, D. E., & Saric, W. S. (2014). Quantitative Boundary - Layer Transition Measurements Using IR Thermography. AIAA SciTech, 2013(July), 233–239. https://doi.org/10.2514/6.2014-1411

Schobeiri, M. T., Öztürk, B., & Ashpis, D. E. (2007). Effect of Reynolds Number and Periodic Unsteady Wake Flow Condition on Boundary Layer Development, Separation, and Intermittency Behavior Along the Suction Surface of a Low Pressure Turbine Blade. Journal of Turbomachinery, 129(1), 92. https://doi.org/10.1115/1.2219762

Sapuppo, J.; Archer, R. D. (1982). Fully laminar flow airfoil sections. Journal of Aircraft, 19(5), 406–409. https://doi.org/10.2514/3.44763

Cohen, G. (2007). Control of shock-induced boundary layer separation at supersonic speeds. Queen Mary University of London. Retrieved from https://qmro.qmul.ac.uk/jspui/handle/123456789/1724

Cowley, S. J. (n.d.). LAMINAR BOUNDARY-LAYER THEORY : A 20TH CENTURY PARADOX ?, (1981), 1–23.

Xingyu, M., Geisler, R., Agocs, J., & Schr"{o}der, A. (2014). Time-resolved tomographic PIV investigation of turbulent flow control by vortex generators on a backward-facing step. 17th International Symposium on Applications of Laser Techniques to Fluid Mechanics, Lisbon, 2014

Heap, H. (n.d.). A REVIEW OF CURRENT LEADING EDGE DEVICE TECHNOLOGY AND OF OPTIONS FOR, 1–13.

Knob, M. (2009). Dynamics of a boundary layer separation. Engineering MECHANICS, 16(1), 29–38.

Schanz, D., Schröder, A., Heine, B., & Dierksheide, U. (2012). Flow structure identification in a high-resolution tomographic PIV data set of the flow behind a backward facing step. In 16th Int Symp on Applications of Laser Techniques to Fluid Mechanics Lisbon, Portugal, 09-12 July, 2012 (pp. 9–12).

Ashpis, D. E. (2005). on Boundary Layer Development , Separation , and Re-attachment along the Suction Surface of a Low Pressure Turbine Blade. In GT2005 ASME Turbo Expo 2005:Power for Land, Sea and Air.

Anyiwo, J. C. (1973). A force field theory. Part I - Laminar flow instability. AIAA Journal, 11(1), 43–49. https://doi.org/10.2514/3.6668

Reed, H. L. (2011). Laminar-to-Turbulent Stability and Transition. Flight Research Laboratory, Texas A&M University

Watmuff, J. H. (1995). Boundary Layer Transition Studies MCAT Institute Final Report. NASA CR-NCC2-698-95-12

Muraca, R. J. (1978). Laminar flow control overview. In 47th AIAA Aerospace Sciences Meeting Including The New Horizons Forum and Aerospace Exposition 5 - 8 January 2009, Orlando, Florida (pp. 1–13). Orlando: AIAA 2009-381.

Jobe, C. E., Kulfan, R. M., & Vachal, J. D. (1979). APPLICATION OF LAMINAR FLOW CONTROL TO LARGE SUBSONIC MILITARY TRANSPORT AIRPLANES. Journal of Aircraft, 16(3), 78–95.

Joslin, D. (1998). Overview of Laminar Flow Control. NASA/TP-1998-208705. Hampton VA.

Hefner, J. N., & Sabo, F. E. (Eds.). (1987). Research in Natural Laminar Flow and Laminar-Flow Control. In NLF Symposium held at Langley Research Center, Hampton Virginia, March 16-19, 1987. Hampton VA: NASA.

Park, G. I., Wallace, J. M., Wu, X., & Moin, P. (2012). Boundary layer turbulence in transitional and developed states. Physics of Fluids, 24(3), 77–86. https://doi.org/10.1063/1.3693146

Thomas, A., Saric, W., & Braslow, A. (1985). Aircraft Drag Prediction and Reduction. Retrieved from http://oai.dtic.mil/oai/oai?verb=getRecord&metadataPrefix=html&identifier=ADA160718 (http://oai.dtic.mil/oai/oai?verb=getRecord&amp;metadataPrefix=html&amp;identifier=ADA160718)

Lange, R. H. (1984). Design integration of laminar flow control for transport aircraft. Journal of Aircraft, 21(8), 612–618.


After a while you will get a sense of the the sensitvity of an laminar flow to transition to turbulent. A simple analysis is done online, where a shape factor, H32, d3/d2
is less than the following value:
https://www.mh-aerotools.de/airfoils/images/frm_lamsep.gif
above that value, transition is expected to occur.

Your video shows a 738 or similar wing in transonic flight, the aircraft is at about M0.785 to M0.795, and has an aoa of around 2.3-2.5. In front of the shock, the flow is turbulent, behind the shock it has a layer of separated flow extending rearwards from the shock boundary layer foot. Not sure hat your point is. Any laminar flow on that wing has stopped on the left hand side, at, or before the flow got to the aft facing step of the slat on the wing. Oddly enough, for the last 5 years of testing the B737 in transonic drag reduction related to this very matter, we have seen various configurations that gave considerable drag reduction. Conventionally, a slat is expected to add 1% to total drag, however that neglects the step effect.

the following image is post flight from NASA on a LR31 test using sublimating chemicals to undertake visualisation of transition. Quoniam res ipsa loquitur. hot wire anemometry etc has often been used to detect transition, however we have found that such methods alter the experiment, as they introduce boundary trips to the experiment.


https://cimg6.ibsrv.net/gimg/pprune.org-vbulletin/769x422/lr31_tip_nlf_viz_3ec5ee704086731292260bb6add93726e214e361.jp g

Is the photo succinct enough?

P.S.

the shockwave on the video is forward of that modelled by the manufacturer in their CFD. Just one of those things of modelling a monolithic meshed section that is simplified in order to attain rational CPU hours. It also exhibits instability, which arises from the shedding of the flow structure immediately behind the slat TE, and that happens about 10x more frequently than the vibration frequency of the shock as imaged. The shock motion does not correlate to engine vibration, or to the acoustic impingement on the bottom of the wing, which was an area of some research in the past by NASA and OEM's. BTW, the shock is observed at the point where the flow decelerates to subsonic, everything to the left of that with the exception of the separation area immediately behind the slat step is sonic, with the exception of the Kutta point which has zero velocity.

Most if not all 3D semi span models do not incorporate the slat as built, which gives a step in a critical part of the airfoil, The neatest example was the EMB170 wing CFD which shows a SC02 type pressure pattern, with a shock location around 0.8c, yet the same conditions in flight video the shock at 0.35-0.4c. The CFD had no slat... The difference in section drag count in detail testing is on the order of 25%-28% optimistic to that with a slat, a solution that tied up my own supercomputer for months to get, and took out the resources of the software supplier to run for a substantial time and cost.

The B737 wing is transonic in cruise, unless at very light weights, low mach, and or low altitude, as most transport aircraft are, by memory, above around M0.745 you will achieve Mcrit on the BAC section at average weights for altitude. Increase AOA, and the speed reduces to achieve Mcrit... and v.v.

Details matter.

[fdr]

As the muppets song goes: "...one of these things is not like the other...." Note the CFD doesn't have a slat... the wing does... There is negligible laminar flow on that wing, and none at all aft of the slat without some assistance like the device I developed, and even then, it is not laminar, it is hybrid.


https://cimg6.ibsrv.net/gimg/pprune.org-vbulletin/1957x1144/emb170_pp_bf4456b09f2e038f4b592d7fad89960ff618f386.png
https://cimg9.ibsrv.net/gimg/pprune.org-vbulletin/900x675/emb170_actual_28239ae609562ae22b9a0185b4b7c3abb7f0fcd7.png

[fox niner]

About 15 years ago, when I was still flying PG and NG 737’s, we had a de-icing procedure that required us to place the stabilizer full nose down.
Including using the trim wheels, all the way to the stop.
It was said that this was necessary, so that the de-icing fluid could run down the aft end of the horizontal tail structure.
This procedure was abandoned later, and repaced by a neutral stab trim setting. So that fluid would not accumulate between the stabilizer and elevator, and subsequently freeze up during flight, impairing elevator authority.
Why am I telling you guys this? Well...
Very often, as I placed the stab trim full nose down with the trim wheel, the following would regularly happen.
It regularly was VERY hard to return the stab trim. INCLUDING using the trim wheels. You had to yank and pull the trim wheels quite a few times so that they would snap free, and the it was usable again and trimmable.
And this was at the gate/de-icing platform. Airspeed was zero.
If this “snap free” hardware problem turns out to be the issue here, than...Boeing is in for a hardware modification. That requires a totally new certification.
I suspect the MCAS is capable of driving the trim wheels all the way forward and capable of causing this same “snap free” lockup as I experinced regularly at the gate.

[PiggyBack]

Most engineers that designed embedded control systems in the 80's and afterwards were weaned on the PDP11 or other DEC machines that used memory-mapped I/O and included it in their designs. Motorola used it on the 6800 and Intel were the ones that promoted separate I/O instructions. Nowadays most control systems use memory-mapped I/O. So as soon as machines were developed that had memory addressing, I/O could not be access in user mode. Maybe memory-mapped I/O was the majority of designs for technical reasons, but I think the fact that you had to buy a $10,000 workstation to develop for the 8008.8080, and Motorola gave away kits of parts for engineers to build and write code on a cross-assembler on their department PDP11 went a long way to giving memory-mapped addressing the edge that it now enjoys.

I am not sure this is the right forum but the reason why memory mapped I/O dominates is nothing to do with the cost of development tools for intel versus motorola or the availability of low cost or free evaluation kits. The reason is that having a seperate I/O space has significant disadvantages. It means that special instructions are required to access I/O which at least in the case of Intel were very limited and restricted compared to memory access instructions. From a softwrae perspective having I/O seperate makes I/O operations special and different from memory acces soperations which either meant the use of machine specific languages such as PL/M or the requirment that I/O is performed through access libraries which is inefficient. The use of a seperate I/O space also precluded or at least makes difficult DMA again affecting or even precluding high speed interfaces. The most obvious evidence that there are good technical reasons why memory mapped I/O is performed is that intel processors have dominated general purpose processor and they support a seperate I/O space but despite this systems designed using intel processor normally have memory mapped peripherals.

On any system which has the concept of multiple users with different access levels and measures to isolate different users from accessing resources they were not authorised access to I/O was necessarily restricted. In systems with dedicated I/O this meant special faciltiies to manage access to I/O use, with memory mapped I/O this is provided as part of a wider memory management function. This is in fact another advantage of memory mapped I/O the unification of control of access to memory with control of access to I/O.

The level of detail discussed is completely irrelevant to the forum both approaches can work and it is not paticularily significant or relevant when considering the overall architecture of a safety related system.

[Less Hair]

What about just publishing the 737 MAX's flight test data? Everybody could see what exactly MCAS is needed for and whether to accept this or not?

[Bend alot]

The poor old processor got overloaded by data came from various sources? I think I posted somewhere that the new routine(s) associated with the MCAS data could overwhelm the control system from time to time, especially during the failure mode [i.e. vane's malfunction].
New flaw discovered on Boeing 737 Max, sources say

...A new flaw has been discovered in the computer system for the Boeing 737 Max (https://www.cnn.com/2019/03/18/world/boeing-737-crashes-similarities/index.html) that could push the plane downward, according to two sources familiar with the testing, an issue that is expected to further delay the aircraft's return to service.

A series of simulator flights to test new software developed by Boeing revealed the flaw, according to one of the sources...

...In simulator tests, government pilots discovered that a microprocessor failure could push the nose of the plane toward the ground. It is not known whether the microprocessor played a role in either crash.
When testing the potential failure of the microprocessor in the simulators, "it was difficult for the test pilots to recover in a matter of seconds," one of the sources said. "And if you can't recover in a matter of seconds, that's an unreasonable risk.".

Boeing engineers are now trying to address the issue, which has led to another delay in recertifying the 737 Max...
===========



- https://edition.cnn.com/2019/06/26/politics/boeing-737-max-flaw/index.html
Seems we need another Band-Aid!

They have not even got in a real MAX yet, so what are we now looking at for flight testing 6 -12 months after the fix of the fix is presented?

[Bend alot]

Double post sorry.

[peteturbo]

If an unidentified failure - cpu overload due to MCAS operations - leads to an unidentified fail state - uncontrollable operation of controls - is this a system operating as expectable since the designers havent designed the system properly?
Systems analysis requires identification of all reasonably foreseeable events, and 'computer operation overload', and the resultant fail state condition must surely be one of them.

[GordonR_Cape]

The poor old processor got overloaded by data came from various sources? I think I posted somewhere that the new routine(s) associated with the MCAS data could overwhelm the control system from time to time, especially during the failure mode [i.e. vane's malfunction].

https://edition.cnn.com/2019/06/26/politics/boeing-737-max-flaw/index.html

Not many details in the articles, so read between the lines: The nose-up trim commands from the yoke have to be processed in software, in order for the pilot to override MCAS. Many people on this thread assume this is instantaneous. If MCAS is busy (10 second countdown), or something else is running in the background, this may overload the processor. Just speculating, but this is exactly the kind of unanticipated scenario that may trigger an malfunction.

[Peter H]

Very often, as I placed the stab trim full nose down with the trim wheel, the following would regularly happen.
It regularly was VERY hard to return the stab trim. INCLUDING using the trim wheels. You had to yank and pull the trim wheels quite a few times so that they would snap free, and the it was usable again and trimmable.
And this was at the gate/de-icing platform. Airspeed was zero.
If this “snap free” hardware problem turns out to be the issue here, than...Boeing is in for a hardware modification. That requires a totally new certification.
I suspect the MCAS is capable of driving the trim wheels all the way forward and capable of causing this same “snap free” lockup as I experinced regularly at the gate.
[SLF & retired s/w engineer]
Sounds like an "interesting" failure mode. However it wouldn't show up in the current-simulator based testing unless Boeing have modelled it, which I really doubt.

PS As this failure-mode might occur whenever the trim end-stops are reached it would be comforting to know that a "responsible adult" in the FAA has investigated
the likelihood and consequences.

PPS As the problem happened regularly during de-icing it should be easy to recreate on ground tests of the linkage. Do the maintenance engineers know about it?

[unworry]

Newspaper reports understating what went down during latest sim tests.

[yoko1]

In the event that MCAS (or another system) is active the design is that trim signal from the yoke switches overrides* the system-derived input. The decision on which takes priority must be 'processed'. Ergo SOME processing power is involved and a processor, somewhere, is in line.

* I have used the word 'override' but I think 'cancel' might be better in the case of MCAS, or even postpone!

A simple relay (well, simple in an aviation context) is all that it takes to remove MCAS from the trim circuit to allow the main electric trim to override and function normally. There was a circuit diagram of the trim logic posted on one of these threads, and I could probably dig one up with a little time if you really need to see it. Again, a reminder that all manner of complex tasks used to be managed in aviation systems before the advent of IC processors.

That being said, the 5-second pause that MCAS is supposed to take after a main electric trim input is likely incorporated in the circuit logic (which resides within the Flight Control Computer or FCC), so it is possible that this aspect of the MCAS side of the system may be affected by processor issues.

[yoko1]

About 15 years ago, when I was still flying PG and NG 737’s, we had a de-icing procedure that required us to place the stabilizer full nose down.
Including using the trim wheels, all the way to the stop.
It was said that this was necessary, so that the de-icing fluid could run down the aft end of the horizontal tail structure.
This procedure was abandoned later, and repaced by a neutral stab trim setting. So that fluid would not accumulate between the stabilizer and elevator, and subsequently freeze up during flight, impairing elevator authority.
Why am I telling you guys this? Well...
Very often, as I placed the stab trim full nose down with the trim wheel, the following would regularly happen.
It regularly was VERY hard to return the stab trim. INCLUDING using the trim wheels. You had to yank and pull the trim wheels quite a few times so that they would snap free, and the it was usable again and trimmable.
And this was at the gate/de-icing platform. Airspeed was zero.


Yep, I remember this. In fact, I'm pretty sure that one of the reasons that the stab trim is even capable of being positioned so far in the nose down direction was to accommodate ground related procedures such as this as I've never seen the stab go much more forward than about 4.0 units in flight. As I recall, the problem was that moisture was getting inside the tailcone area and the stab and/or the limit relay was getting frozen against the stop. This is one of the reasons the procedure was ultimately changed.

[PiggyBack]

We are all speculating based on incomplete information but it has been apparent that current concept is flawed and this could not be resolved by a software fix even if it successfully addressed whatever issues had caused the two crashes. We know that a single fault could cause the stabiliser to be driven to a position where it is hard or impossible to recover. Addressing specific fault scenarios without addressing this wider concern is not a full solution. We have partial information and those involved are competent people so we must assume that this is beng addressed however the timescales for the solution always seemed over optimistic.

[Speed of Sound]

Do we have anyone on here who designs/maintains flight simulators?

The Reuters report says that the ‘additional issues’ were discovered in the sim. Is the hardware in a flight sim identical to the actual hardware in the aircraft? As a flight simulator is primarily a training aid and not a test bed for hardware I would assume that the simulator system hardware emulates, and software models what is in the aircraft, rather than being identical to the actual circuits and microprocessors found on the aircraft.

If this is the case then great care should be taken in drawing any hardware deficiency conclusions away from an actual flying aircraft.

[unworry]

The Reuters report says that the ‘additional issues’ were discovered in the sim.



They were referring to incidents in test flights conducted in the sim ... not issues with the sim platform itself.

Apologies if I'm misreading the jist of your question

[yoko1]

Do we have anyone on here who designs/maintains flight simulators?

The Reuters report says that the ‘additional issues’ were discovered in the sim. Is the hardware in a flight sim identical to the actual hardware in the aircraft? As a flight simulator is primarily a training aid and not a test bed for hardware I would assume that the simulator system hardware emulates, and software models what is in the aircraft, rather than being identical to the actual circuits and microprocessors found on the aircraft.

If this is the case then great care should be taken in drawing any hardware deficiency conclusions away from an actual flying aircraft.

Aviation Week reports that the tests were conducted in Boeing's engineering flight simulator (i.e. a testing platform, not a training device), so I suspect that from a system standpoint they were emulating the aircraft hardware and software as much as feasible.

[Speed of Sound]

Aviation Week reports that the tests were conducted in Boeing's engineering flight simulator (i.e. a testing platform, not a training device), so I suspect that from a system standpoint they were emulating the aircraft hardware and software as much as feasible.

Thanks.

That makes a lot more sense if they are drawing actual hardware performance conclusions from the test ‘flights’. If I was building a commercial flight simulator I would stick all the processing in a modern quad core based computer with a shed load of RAM with a very accurately written programme to model what happens in the actual aircraft (much like a high end gaming flight sim).

[SteinarN]

From Aviation Week (https://aviationweek.com/commercial-aviation/faa-flags-new-computer-issue-737-max-testing?NL=AW-05&Issue=AW-05_20190627_AW-05_808&sfvc4enews=42&cl=article_1&utm_rid=CPEN1000001583807&utm_campaign=20177&utm_medium=email&elq2=98f827d4da10493e8d470a8367404969)

It seems in fact like the latest problem is related to FCC overload. The article says the THS is slow to react to manual trim inputs from the thumb switches when simulating a trim runaway. Thereby forcing the aircraft into a steep dive before the THS finally starts reacting to the thumb switches. It is not clear if this problem can be fixed by a software update or if it requires time consuming hardware changes.

WASHINGTON—FAA (http://awin.aviationweek.com/OrganizationProfiles.aspx?orgId=31159) test pilots have flagged a new issue in the Boeing 737 (http://awin.aviationweek.com/ProgramProfileDetails.aspx?pgId=634&pgName=Boeing+737NG) MAX flight control system that must be addressed as part of changes being made to get the aircraft back into service, Aviation Week has learned.The issue came to light within the last week during tests in Boeing (http://awin.aviationweek.com/OrganizationProfiles.aspx?orgId=12083)’s MAX engineering flight simulator, or e-cab, a source with knowledge of the situation confirmed.

The pilots were simulating a runaway stabilizer scenario and running through the requisite emergency-response checklist. A key early step is to use control column-mounted electric-trim switches to command horizontal stabilizer movement to counter the runaway. A subsequent step, if needed, is to toggle cutout switches that disable the trim motors. According to the source, the FAA pilots found response to the electric-trim inputs took too long. “They had a difficult time quickly resolving the situation,” the source explained. The issue has been traced to how quickly a specific flight control computer chip is processing data, the source said. What is not clear: whether the chip itself needs to be changed, or if a software update will address the issue. A second industry source said that a software fix is possible—and certainly would be preferable for Boeing, which suggested in a statement that a software modification will be sufficient.

Changing chips could further delay the MAX’s return to service, as it would likely require new chip architecture as well as changing chips on nearly more than 500 MAXs in airline fleets or ready to be delivered.“The FAA is following a thorough process, not a prescribed timeline, for returning the Boeing 737 MAX to passenger service,” the agency said in a statement. “The FAA recently found a potential risk that Boeing must mitigate. ”Boeing said the issue is “an additional requirement” that the FAA “has asked the company to address through the software changes that the company has been developing” for the MAX. “Boeing agrees with the FAA’s decision and request and is working on the required software to address the FAA’s request. Addressing this condition will reduce pilot workload by accounting for a potential source of uncommanded stabilizer motion,” it added.

The MAX has been grounded since mid-March following two fatal accidents in five months. Boeing has been working on changes to the MAX’s flight control system, specifically the maneuvering characteristics augmentation system (MCAS) flight control law. MCAS commands automatic horizontal stabilizer inputs in certain flight scenarios, and it activated erroneously in both accident sequences. Its failure can result in a runaway stabilizer scenario, which pilots are supposed to mitigate by following the “stabilizer runaway” checklist. Trimming the aircraft using the control-column switches is a key first step meant to stabilize the aircraft and enable the pilots to safely de-power horizontal stabilizer trim motors using cutout switches mounted on the aircraft’s center console. In both accident sequences—the October 2018 crash of Lion Air Flight 610 and Mar. 10 crash of Ethiopian Airlines (http://awin.aviationweek.com/OrganizationProfiles.aspx?orgId=21507) Flight 302—the crews used the column-mounted switches to counter MCAS. Neither followed the runaway stabilizer checklist step-by-step, and were overcome by MCAS’s repeated inputs that forced the aircraft’s nose down due to erroneous angle-of-attack data being fed to the flight control computer. Both accident sequences ended with uncontrollable dives. The newly discovered issue came up during a very specific failure scenario, and it is not clear whether it has any link to either MAX accident sequence, the first source emphasized.

[Maninthebar]

A simple relay (well, simple in an aviation context) is all that it takes to remove MCAS from the trim circuit to allow the main electric trim to override and function normally. There was a circuit diagram of the trim logic posted on one of these threads, and I could probably dig one up with a little time if you really need to see it. Again, a reminder that all manner of complex tasks used to be managed in aviation systems before the advent of IC processors.


APPEARS to be contradicted by post 730

?

[fizz57]

From Aviation Week (https://aviationweek.com/commercial-aviation/faa-flags-new-computer-issue-737-max-testing?NL=AW-05&Issue=AW-05_20190627_AW-05_808&sfvc4enews=42&cl=article_1&utm_rid=CPEN1000001583807&utm_campaign=20177&utm_medium=email&elq2=98f827d4da10493e8d470a8367404969)
The article says the THS is slow to react to manual trim inputs from the thumb switches when simulating a trim runaway.

One wonders what the FDR would record while the pilots were mashing the thumb switches but only getting a "slow response". Short pulses, perhaps?
(I know, pure speculation on my part :rolleyes: )

[yoko1]

APPEARS to be contradicted by post 730

?

Waiting for the detail on this one. Does not jive with the circuit diagrams I've looked at nor with the previously reported behavior. Another undocumented feature?

[Maninthebar]

Waiting for the detail on this one. Does not jive with the circuit diagrams I've looked at nor with the previously reported behavior. Another undocumented feature?

Maybe the documentation thus far disclosed is 'incomplete'.

That could be described as consistent with other material

[BDAttitude]

Doesn't make me wonder. MCAS wins if the the FCC fails to compute the Main Trim Interlock. Works as designed :uhoh:
https://cimg3.ibsrv.net/gimg/pprune.org-vbulletin/721x531/unbenannt_68eb3644f0bf66ded05f05fde11e4437796c8c45.png

[Speed of Sound]

It is not clear if this problem can be fixed by a software update or if it requires time consuming hardware changes.

Presumably before even thinking about a hardware solution, software engineers will look at a ‘load shedding’ solution which gives priority to trim instructions if the processing is getting close to ‘maxing’ out. This of course will depend on not creating conflict with another simultaneous safety critical instruction.

[MurphyWasRight]

Doesn't make me wonder. MCAS wins if the the FCC fails to compute the Main Trim Interlock. Works as designed :uhoh:

The diagram is missing detail of the electric stab trim motor block.
That is where an absolute override of all automatic trim inputs by pilot inputs "should" be, very simple no computer required.

It the reports are correct it appears it may not be present and the issue is fcc slowness in recognising pilot inputs and disabling auto inputs.

[BDAttitude]

Presumably before even thinking about a hardware solution, software engineers will look at a ‘load shedding’ solution which gives priority to trim instructions if the processing is getting close to ‘maxing’ out. This of course will depend on not creating conflict with another simultaneous safety critical instruction.





Either the interlock parts of the algo have been running erroneously in a much too slow or improperly synchronized task, or the whole system is (and probably has been) MAXed out to an extend that timing overruns happen. If this is the case they are in deep trouble as they would have to be expected in other situations as well. And the questions arising regarding QA ....

[BDAttitude]

The diagram is missing detail of the electric stab trim motor block.
That is where an absolute override of all automatic trim inputs by pilot inputs "should", very simple no computer required.

It the reports are correct it appears it may not be present.and the issue is fcc slowness in recognising pilot inputs and dibling auto inputs.
I'm not sure
1) If it was, the Interlock in FCC would not be neccessary. So the educated guess is, that it is not.
2) Operating the trim switches in AP on leads to disengagement. Another relay to priorize manual input woud be superfuous.

[MurphyWasRight]

I'm not sure
1) If it was, the Interlock in FCC would not be neccessary. So the educated guess is, that it is not.
2) Operating the trim switches in AP on leads to disengagement. Another relay to priorize manual input woud be superfuous.
True, except MCAS is enabled when AP is off, perhaps another overlooked failure mode/path?

[BDAttitude]

True, except MCAS is enabled when AP is off, perhaps another overlooked failure mode/path?
Just wanted to highlight that pre MCAS concurrent activation of manual and AP channel was software interlocked as well.

Another educated guess would be, that not AP has priority over manual, but one direction has priority over the other because it's likely to be implemented by a reversion switch both channels are connected to. So from this scenario ANU would be the "default" direction and AND the "reversed" direction activated by either manual or AP channel being switched AND.

[bill fly]

This is an unfortunate development in the process, but two positives exist, 1; the new issue has been discovered before RTS, and 2; it has been acknowledged.
.

Yes indeed. Let us hope that an honest culture is taking hold - at B and at the FAA - then maybe something good will come from this catastrophe for the future

[OldnGrounded]

Doesn't make me wonder. MCAS wins if the the FCC fails to compute the Main Trim Interlock. Works as designed :uhoh:


Very disturbingly, it looks like that might well be the case. I hope it is not.

[robocoder]

Embedded real-time software design carefully budgets the available CPU for the worst case. It would be deeply disturbing if the problem came from that direction, so I'm going to go out on a limb and guess that that's not it. Those programs tend to be extremely static in the sense that tasks/data do not arrive ramdomly or with variable rates, but everything is done on preallocated resources. All the fancy features of scheduling are dispensed with in favor of extreme predictability.

Frankly, if this is the case, as a software engineer, I would be appalled. This is Real-time Systems 1.01.

​​​

[tdracer]

Presumably before even thinking about a hardware solution, software engineers will look at a ‘load shedding’ solution which gives priority to trim instructions if the processing is getting close to ‘maxing’ out. This of course will depend on not creating conflict with another simultaneous safety critical instruction.

Agreed - this was SOP when doing FADEC software changes to the 1980's era engine controls where memory and throughput were at a premium, and making hardware changes to thousands of in-service units was simply not an option. There was often more focus on what they could get rid of to make room then there was on the planned changes.

BTW, while comparisons have been made to the 1201 and 1202 error codes during the Apollo 11 landing, the reason for that was Buzz left the rendezvous radar on during the landing - something that wasn't per the checklist and hence hadn't been checked. On some of the older FADECs, there was logic to drop 'unneeded' processes (usually condition monitoring stuff) if the processor became overloaded.

[jmelson]

Aviation Week reports that the tests were conducted in Boeing's engineering flight simulator (i.e. a testing platform, not a training device), so I suspect that from a system standpoint they were emulating the aircraft hardware and software as much as feasible.

OK, not Boeing (although it is, now) and not commercial, but military flight control systems, but I did see how they did this at McDonnell Douglas some years ago. They had a big Honeywell computer that was interfaced to the FCC, and had some controls and instrument panel displays provided by another computer, like a Unix workstation (Sun, SGI or so). The Honeywell simulated the aircraft flight dynamics and all the sensor inputs (air data, gyros, feedback from flight control positions sensors, etc.) So, it was to mimic everything the FCC would sense while flying the aircraft. It also recorded all the inputs and outputs so they could be compared with what the FCC was supposed to do.

I'm guessing this is the sort of system Boeing is using to test the 737 Max FCC, and they've got the necessary flight controls rigged to it so the test pilots are doing the normal trim settings, etc. But, it is probably on a desktop with most of the indicators and controls just appearing on a couple screens.

And, from the VAGUE descriptions I've seen so far, it sounds like it could be a problem with priority of various tasks on the FCC. It seems these days an FCC really should NOT be running out of CPU cycles.

Jon

[Mike Flynn]

FAA approval will come first because they are working very closely with Boeing and are the ‘home’ regulator. EASA will lift the grounding after a respectable period of time, mainly for political reasons. The FAA realises how important the MAX is to Boeing and will not let anything slide which may cause the huge embarrassment of the FAA lifting the grounding while other regulators say no, hence the latest ‘hardware issues’ delay. Boeing are smart enough to know that trying to bounce the FAA into an early decision will only serve to increase caution and suspicion in other regulators.

The MAX will only fly again when all parties, including Boeing, are convinced that it is completely safe to do so.

As the saying goes you can put lipstick on a pig but it is still a pig.

The 737 Max is a fifty year old design with a few lights bells and whistles.

All we are seeing with Boeing and the FAA is a method of selling this death trap to the worlds airlines.

They laughed at Airbus and their plastic and electric aeroplanes but sadly now discover they have missed the bus. (pun intended)

[yoko1]

Still waiting for more details, but I found a trim electrical schematic with a slightly different layout, but may add some clarity to the schematic BDAttitude posted above at post #735, in particular the function of the Main Electric Trim Interlock. I'm not claiming to be an expert in reading wiring diagrams, but here's what I see. Flight deck inputs come from the left. The right side represents the trim motor and associated electronic controls. All the FCC inputs (MCAS, STS, Mach Trim, MCAS) appear to come in (off another schematic) into the trim motor box from the right. Various control and limit relays are in the middle.
.


https://cimg6.ibsrv.net/gimg/pprune.org-vbulletin/1968x750/trim_schematic_2_f88a066e18edf70d381e8debb0a701c807fc2479.jp g


.
If I am reading this correctly the "Main Trim Arm" relay is activated by one of the pilot yoke switches which, among other things, sends a signal to and opens the interlock relay contained in the trim motor box on the right side:
.

https://cimg6.ibsrv.net/gimg/pprune.org-vbulletin/217x76/interlock_6ee429633932225d8b532f1a1e4e80a5a4983386.jpeg
.

The interlock relay appears to be a basic relay and not a function of any IC processors. This relay appears to be spring-loaded closed allowing automated trim inputs by default (which makes sense because the STS and Mach Trim systems need to be active full time). When a Main Electric Trim switch is actuated, the interlock opens, and this "open" signal goes off the right hand side of the page presumably into the FCC logic which should stop any automated inputs. There is no way to tell from this diagram how the FCC handles that input.

If for some reason the FCC logic does not process the "stop" signal, then I presume that both the Main Electric Trim and the FCC could attempt to send trim signals to the box marked "Controller" located just above the box labeled "Brushless DC Motor." We can't really tell from this schematic if that could happen, or what the Controller would do if it received conflicting signals.

So sorry, no definitive answer, but perhaps narrows the focus some.

All of this should be qualified by the fact that this so far appears to be an issue with the updated FCC software/firmware. Not sure if it could be extended to the original software.

[armchairpilot94116]

https://www.sfgate.com/business/article/FAA-finds-new-risk-on-737-Max-orders-Boeing-to-14054004.php

Apologies if it has been posted before, but the FAA finds a new problem with the microprocessor that can induce a dive. Not related to the two crashes they say but....are we sure?

Nevermind, posted already