A moot point - Airline accountants and all the airlines today (and in the future) just want fuel economy and $$savings, hence the fitting of the big new CFM LEAP engines to basically a 737-800, of which they are now the root cause of the unstable MAX air frame, necessitating the MCAS software requirement.

Let me remind you that the "S" in MCAS stands for System.

MCAS did fail. The code may have done what it was designed for but the system (which includes the AoA vane) failed.

Error in post corrected.
 

This is my first post here so I apologise for any errors in the way that I have done so. Advice welcome.

As I understand it the 737 Max cannot be certified to fly at all without MCAS because it does not meet the requirements of FAR 25.203(a) which says "No abnormal nose-up pitching may occur. The longitudinal control force must be positive up to and throughout the stall"

Is there any definitive confirmation of this fact?

If it is NOT true, it would seem to me that Boeing's best way by far to get the planes flying again would be to remove or completely disable MCAS and admit that it is a different aicraft from older 737s and agree to pay for all training required to fly it.

Re the Canadian plan, it is exactly what I had proposed sometime in some thread on this board. Kick MCAS out of the aircraft and train for the issues of stick force linearity or pitch up tendencies due to engine location or whatever the aerodynamic aberrations are. If that training requires a new type rating, then so be it.

My LA4-200 Lake Amphibian would pitch down with power and up when you decreased the throttle due to the rear facing engine located more or less at the CG. Exactly the opposite of every other aircraft that I had flown. That aerodynamic tendency was covered on day #1 of training.

Martin & Lake how do you propose to comply with the Grandfather rights?

"Seeking Alpha" provides an opinion on the cash flow of the Max program, and supports your comments on the balance of advanced payments versus production costs. Their analysis shows that the advanced payment revenue falls below the production costs (42/M) in Jan.

The fact that B keeps the non-MCAS (raw) pitch-up characteristics a secret leads to the conclusion that those characteristics are much worse than just a mild non-linearity. That is probably also the reason why they multiplied the MCAS action by 4 without telling the FAA (and anybody else).
Drastical pitch up cannot be certified (thank God) as it is bl..dy dangerous even when pilots get told/trained. A commercial jet is not a Lake Amphibian and it is operated on instruments , not on seat of pants VFR operation where a pitch deviation up or down is easily identified and corrected.

It certainly leads, at least, to serious skepticism. For some reason, despite lots of people repeatedly asking about this, the data has not been provided.

The Flyers' Rights lawsuit may pry some of it loose.

Here are two important parts of the story that we do not know as yet so far as I am aware.
1.) What are the aerodynamic characteristics or aberrations that led to MCAS?
2.) How many times (if any) has the MCAS been activated besides the two Lion and one ET flights? Has MCAS ever been activated when there was no malfunction in the AoA?

Bend a Lot, while I am aware of grandfather rights, I have no idea how much content on the MAX would be verboten or obsolete if a separate type rating was required. Or invent a "sub-type rating". And I have no idea what the type rating has to do with the type certification of the aircraft! Just because an aircraft requires a type rating, why would that affect the type certification of the aircraft?

jimtx, #74 ‘just use the AP’.
This is similar to the calls for FBW; unfortunately the AP argument is flawed.

The Air Data Computers use AoA for corrections. The output of the two ADCs is used by the AP (Flight Guidance Computer) which compares values. If the values of speed / alt disagree ‘by xx’, AoA input error, then the AP will disengage - ‘invalid data’, and the relevant Disagree alerts given.
For Speed and Alt the crew can crosscheck the StBy systems: a checklist item. There is no third AoA, thus the consequential and distracting alerting remains on; no AP available: abnormal conditions #17 (https://www.pprune.org/rumours-news/628134-max-s-return-delayed-faa-reevaluation-737-safety-procedures-mk-ii.html).

The ‘don’t pull too hard’ quip might represent residual problems relating to the design of the trim system and subsequent use without MCAS - see below.

MCAS Annunciator

Has anyone proposed an annunciator to show when MCAS is active instead of trying to determine if it is a case of runaway trim or not?
Just a thought.

PP

Icarus2001, #76, following on Water pilot, #72,
The description of trim operation is based on normal operation; relating it to basic flying training. However, the 737 trim system differs from basic training aircraft . The 737 design can inadvertently create bad habits tending to fly the aircraft with trim (Boeing cautions against this).
In normal operation the pilot-aircraft-trim interaction is similar to any other system, the effects of the different design are not readily apparent. However, with large changes in stick position / force, or if the trim malfunctions then the pilot-trim interaction changes (https://www.skybrary.aero/bookshelf/books/2627.pdf).
…
The 737 Max pitch stability is like slice of stale bread, curled up at the edges. MCAS logic applies trim to flatten out the edge bits; small trim changes have a large ‘restorative’ effect, which works.
If large, unwarranted MCAS trim changes occur anywhere in the flight envelope then both aircraft control and restorative trim application is difficult - accidents.
After modification, comparison of sensor input, errors inhibit further MCAS trim movement after one small application; this value will place the aircraft ‘out of trim’. If this occurs in the ‘flat’, centre area of the bread slice, applying trim to flatten a non existent curl might dent or hole the bread causing further problems.
…
Before the Max can return to service the latter case must be assessed as satisfactory - within an agreed range of piloting competence. However, if the small amount of trim from MCAS ‘failure’ creates a false impression of pilot-aircraft-trim, via residual stick force, then the piloting task is increased. How this might apply to manual trim wheel operation is not clear, but evidence suggest its not beneficial.
Similarly application of corrective electric trim to reduce any offset could be masked by the higher stick forces. This situation could be further complicated if the aircraft is flown with trim, small trim blips inadvertently amalgamate to a longer application, applied instinctively in surprising situations - several alerts and distracting stick vibration (see trim system design link above, NB accidents - and recent FlyDubai).

Previous posts https://www.pprune.org/rumours-news/628134-max-s-return-delayed-faa-reevaluation-737-safety-procedures-mk-ii.html#post10642164 and https://www.pprune.org/rumours-news/628134-max-s-return-delayed-faa-reevaluation-737-safety-procedures-mk-ii.html#post10643566 assume that all electrical trim operations are inhibited with MCAS inhibit. The rationale is that if the dual FGC is used as the monitor for MCAS, and electric trim and AP trim are within the same box, then the most effective safeguard is to switch everything off. If so, could subsequent pilot evaluation via checklists determine which part of the system has failed and then some parts reinstated; particularly advantageous if electric trim is effective in restoring and continuing safe flight.

The above is influenced by not understanding why the Max trim-inhibit switches differ from the NG, separating elect trim and AP trim (extensive Tech Log discussion); did Boeing expect some aspects to be restorable - is the Max more dependent on electric trim than the NG, thus it is desirable to restore it to reduce workload in abnormal conditions; particularly where trim runaway still has to be considered.
It is in these areas which checklist drills and actions could be generating more certification discussion.
.

If by "grandfather rights" you are referring to the Changed Product Rule, I think service history, 2 catastrophic accidents, provides the necessary justification to impose the most current safety standards.

From Investor's Business Daily, yesterday:

There's no drastic pitch-up. How does this get started and why is it repeated? Comparing the plan form to the NG and the differences are difficult to make out.Slight changes in configuration tend not to make drastic changes in performance. More than that, the Max has a thrust line closer to the CG than any other 737, so the pitch coupling should be less or about the same. The larger engine can produce more thrust, but unless the plane is a lot heavier than previous planes the only way to use that thrust is in uncomfortable sea level take-offs or in normal rate departures from high-hot-humid airports. The size increases the coupling of fuel burn to air movement for an increase in thrust specific fuel consumption, aka, fuel efficiency. So, they keep the thrust the same and burn less fuel.

I'm sure the change in factor was not yelled from the roof tops because no one had a fault-tree analysis linking the AoA system to the MCAS output to a definitive emergency outcome. Had they passed along the change in rate without anyone having done the analysis, it would have not raised any questions, just like it did not in the first place. The more aggressive change should have acted to draw the attention of the flight crew to the more rapid pitch trim change and then to the cause of it, rather than being a slower and less obvious one; it appears in the first and second MCAS events that this is exactly what happened. In the third event, the pilots seemed unaware of the change in trim for a relatively long time and did not notice they were left with a large increment.

So far no one has volunteered just how the control forces for pitch mis-trim can be so low so the pitch trim error is unrecognized and so high they cannot be overcome.

Don't think of it as an issue of "thrust line" nor "thrust to weight" but rather nacelle lift farther forward and higher than the NG. This in fact causes a pitch up moment force behind the curve.

I'm not sure I'll fully agree with the above.
Small changes in configuration *can* lead to drastic change in flight qualities. Heavy tail buffet in a transport plane has been known to be due to windshield/windshield wipers perturbations, and had to be cured with VG's on top of the cockpit.
Some very little variations in airfoil, washout, dihedral, sweep or mechanical details can trip unwanted flow behaviour.
Boeing said the stability issue was due to the airflow over nacelles and adjacent wing portion rather than thrust line.
Not sure there is a way to foresee mach tuck etc. just by comparing 3-view drawings so I'd say there is more to the 737 stability issue than is officially published.

We don't know how pronounced the pitch-up tendency may be. We haven't seen data from testing of the bare airframe.

It got started because both Boeing and the FAA have indicated that the size/shape/positioning of the LEAP engines creates additional lift at some attitudes/corners of the envelope and that as a result of that, MCAS is required to provide linear stick force to discourage pilot input bringing the aircraft dangerously close to stall. There is a fair amount of skepticism about the assertion that a system with full authority over the horizontal stabilizer was truly developed and implement merely to control stick force.

I don't think anyone has suggested (at least in many months) that thrust is the issue here. The issue, as far as we can tell, is increased lift due to the engine nacelles and their position.

You’re right in it might provide insight but when? Considering how long it takes lawsuits to get moving I can’t see it happening before the FAA makes a decision on the MAX. My guess is most of the speculation here is 100% dead on about the aerodynamic issues with the MAX sans the exact specifics/numbers. I think the best hope we have for answers is the FAA. I would hope they publish a detailed analysis of MCAS and why it’s needed, how it engages, etc when the commentary period opens.

Maybe I’m overly optimistic about the FAA but at this point they have to think 100% transparency is paramount to regaining the lost trust.

On another note, the announcements of shutdowns is now directly impacting many more thousands of workers. Their livelihoods are being compromised, families will have to endure cuts and the number will only increase. This will result in enormous political pressure. Might even become an election issue if this drags on and the out of work numbers continue to rise. The economic impact of Boeing and its ancillary businesses is far and wide. Time is now becoming a factor and I hope it doesn’t cloud the eyes of regulators.

OldnGrounded, could it also be that higher and more forward position of the nacelles can cause turbulence over the wing at higher angles of attack? Difficult to model or predict what turbulence/loss of laminar flow can do to lift generated by wing.

Just to clear up another of your misconceptions (I see that the first has already been addressed), no-one is suggesting that the trim forces are too low and too high under the same flight conditions.

MCAS met it's system specification and worked according to what the developers expected of it. The expectations were incomplete. I believe I covered the need for the specification to be managed and that humans are involved in that. So, no, the MCAS system did not fail. The development system for MCAS did. Now that a new factor has been added, the new system will likely handle the new factor.

It's not a misconception. There have been people suggesting that the pilots could not detect the need to trim; since that need is primarily sensed by control forces, the trim must have been too light to notice. The accident reports suggest the pilots were pulling hard, suggesting the the control forces were high to resist.

So how is it that the pilots did not realize from the control forces in nearly level flight that they needed to trim, particularly on ET302 where the control forces continued to increase with increasing speed. What are those forces?

No one?

"My LA4-200 Lake Amphibian would pitch down with power and up when you decreased the throttle due to the rear facing engine located more or less at the CG. Exactly the opposite of every other aircraft that I had flown. That aerodynamic tendency was covered on day #1 of training."

Seems like a suggestion that thrust was a factor, so it's not been months.

Of course we are playing on words here.
Considering that the MCAS was - among other rather vague purposes - meant to provide the same aircraft behavior as the 737NG, one could object that the end result wasn't especially "NG-like", hence the idea that the MCAS failed and has to be changed.
And the change doesn't look like trifle, does it ?

I think of increased lift as due to the huge increase in diameter and not the small change to a more forward or higher location.

The fact that engine drag can induce a pitching moment is well known. Below is a quote from the book An Introduction to the Longitudinal Static Stability of Low-Speed Aircraft by F.G. Irving, written in 1966:

"... effects induced by the power plant. If the resultant drag force does not act through the c.g., it produces a pitching moment."
(www: sciencedirect.com/book/9781483200194/an-introduction-to-the-longitudinal-static-stability-of-low-speed-aircraft, from chapter 11 abstract)

Regarding plan form comparison of the 737 NG to the MAX:
The engine drag (NG vs MAX) can be estimated by calculating the drag for a cylinder inclined at 15 degrees AoA with the dimensions of the CFM 56-7b (737 NG) compared to the LEAP-1B (737 MAX). It will indicate a 40 % increase in drag at 200 m/s and 15 degrees AoA for the MAX. The magnitude is in the order of 10's of kN per engine, and unlike the thrust pitch coupling effect mentioned, the drag has the potential to be much more severe as it does not act along the axis of the engine. Consider that drag increases with AoA, as more surface area is exposed, but interestingly due to the high position of the MAX engines, it is likely that the moment arm of the drag will also increase with AoA! I haven't really seen this second point being discussed, but personally I think this is a key that could explain why MCAS was so aggressive.

To summarize: It is clear that the Max engines produce a significantly increased drag compared to the NG, and it is clear that this causes a pitching moment that can increase non-linearly with AoA. How bad is it? It will depend on the exact relation between the aircraft CG and the nature of the engine drag. Only Boeing can answer this question, and the fact that they have declined to do so for about 1 year tells me that it is probably pretty bad...

simple fix, program all of your time devices in 24hr clock.

According to Boeing, it is due to the change for an higher and more forward position.
Of course this is caused byt the engine larger diameter and short landing gear.
Worthy of note, the 320 Neo has no issues, though its engine have a larger diameter yet.

And so we have here an excellent discussion by very knowledgeable individuals as to how best to solve aerodynamic and design issues that should have been discussed by boards of engineers at Boeing many years ago. If Boeing had played by the rules and certified it as a new type, if Boeing had been honest and professional. Which it wasn't.

Declaration: I am not a pilot! (Have flown gliders only)
Having got that out of the way, I have to ask a question that has been bothering me since this issue first arose: From what I read here, the MCAS problems arise from erroneous AoA indications (plus the obvious follow ons such as the correct action to take etc). Why does the only indication of AoA have to be from a fallible instrument stuck outside on the airflow? Outside of aviation, there are many devices that can perform the same function, such as inclinometers etc, either working from a bubble indication or an object floating in a liquid. Why can a pilot not have an instrument, mounted inside the cockpit, that will give him an indication of AoA, regardless of what the airflow over an outside instrument tells him., At least, he would then have the opportunity to think about whether or not something is wrong.

Angle of attack is independent of pitch and acceleration. Inclinometers and bubbles indicate the direction of local acceleration. This is only reliable if the vehicle they are in cannot accelerate and add or subtract from the acceleration due to gravity. The closest one can get is to differentiate (mathematical process) to determine the path and compare that to the apparent pitch alignment to that path. Even then it's not a certainty as that depends on the air being stationary and uniform, which it usually isn't.

Essentially the only way to account for the local relative wind's likely effect on the aerodynamics of the plane is to use an instrument that is directly affected by the local relative wind. Some less-direct aerodynamic sensors do exist, such as laser Doppler meters that measure the effect of the local wind on dust particles in the air, and I think there are hot-wire probes that detect the thermal convective characteristics, but those are more prone to getting broken and are much more expensive their fragility.

The 302 Neo constantly adjusts the stabilizer trim because it operates in MCAS 100% of the time, they just don't call it MCAS. It hasn't had any problems because it is not being sold to airlines which need to directly transfer 737 NG pilots to new seats. As a result they don't have the 737 cockpit automation, for good or bad, and don't fit into the ground-handling system the 737 does.

Guardian article, doesn't look good. I think Boeing must change its corporate culture. It went south after merging with McDonnell Douglas. Toxic culture spilled all over. And Boeing should feel the consequences of their greed. Because it was pure corporate greed that drove to this situation. I hope EASA will be even tougher on MAX. This is just insane - Boeing makes an airplane with fatal flaw, the consequences of which could even be mathematicaly proven (crash every 2-3 years) and that even didn't bother them. I hope the managment will be sued big time and fired sans golden parachutes this time. It is high time that strictier regulation is put over whole industry. This time everything went too far.

That might be true, but this isn't the sort of litigation that *usually* runs on forever, at least not at the district court stage. The Flyers' Rights folks are arguing that the prompt release of the information is critical, precisely because of the (presumably) impending FAA action. They're just seeking an order from the court requiring the agency to release the information as required by FOIA. It might not take too long.

And the FAA will be in a really gigantic mess if they approve return to service while withholding the data from independent experts and something bad happens. At least some people there probably understand that.

Well, perhaps I was insufficiently careful with my wording, but, seriously, if you read the very long record of these threads, it will be clear to you that what I wrote is a reasonable description of the development of the prevailing understanding around here.

I'm sure it's generally possible, but determining whether that could apply to the MAX is way beyond my aerodynamic knowledge.

A pile of links that may help
 

The length of this thread and its previous makes it difficult to refer to specifics and what the past and current semi- technical discussions have been- since most of the current questions have long since been addressed- discussed- ad naseasum

So I'm going to post a few links to different blogs- which if 0ne takes the time to read and figure out how to go to previous and following on the site(s) listed will at least provide mostly rational discussion.


[QUOTE]

https://www.satcom.guru/2019/10/flaw...-disaster.html

https://leehamnews.com/2019/11/29/bj...-crash-part-5/


https://www.moonofalabama.org/2019/0...g-737-ngs.html



https://www.pprune.org/tech-log/6214...x-threads.html


And from a long ago post somewhere in prune news and rumours one can find the below




And re stick force found

https://www.satcom.guru/2019/03/taki...aiting-on.html

........ This aspect is essential in ensuring the pilot does not have to push the yoke to stop the plane from pitching up, which violates positive stick force per g as required in 25.255....

... stick force per g must be positive . ..

Understood. But the more forward nacelle during high angles of attack is a longer lever arm. Additionally the higher up on the pylon the nacelle is mounted also contributes a longer lever arm pitch up moment at high aoa. Again it's the the lift from air impinging on the nacelle as well as the lift from the nacelle.