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

[fizz57]

Originally Posted by MechEngr

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.

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.

[MechEngr]

Originally Posted by Vendee

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.

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.

[MechEngr]

Originally Posted by fizz57

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.

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?

[MechEngr]

Originally Posted by OldnGrounded

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.

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.

[Fly Aiprt]

Originally Posted by MechEngr

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.

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 ?

[MechEngr]

Originally Posted by jdawg

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

[Stribeck]

Originally Posted by MechEngr

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.

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...

[Longtimer]

Originally Posted by MurphyWasRight

Not enough info released yet to say for sure, could also have been a mistake in a timing driven script that had an incorrect value, in hotel layover terms : clock correct, wrong alarm time set.
Either way not a good day for Boeing.

I almost missed a flight once due to the classic AM/PM mistake, always use 2 alarms for anything critical since then.

Now back to our regularly scheduled "not rocket science" 737/MAX programming.

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

[Fly Aiprt]

Originally Posted by MechEngr

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.

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.

[EvaDestruction]

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.

[KelvinD]

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.

[MechEngr]

Originally Posted by KelvinD

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.

[MechEngr]

Originally Posted by Fly Aiprt

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.

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.

[kontrolor]

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.

[OldnGrounded]

Originally Posted by Drc40

You’re right [that the Flyers' Rights lawsuit] 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.

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.

[OldnGrounded]

Originally Posted by MechEngr

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.

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.

[OldnGrounded]

Originally Posted by Phugoidinator

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.

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

[Grebe]

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



B737 Max threads


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

From ??? in pprune rumours

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."

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 . ..

[jdawg]

Originally Posted by MechEngr

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.

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.

[medod]

Ahhh so it’s Title 14 Part 25.255 the MAX airframe falls foul of. Spent ages looking through Part 25 yesterday but didn’t read that section (Out-of-trim characteristics).

Whatever the exact reasons, the MAX’s big ass engines mean it fails 25.255 without MCAS. That is why I believe the MAX will never fly again in commercial service with less than three AoA vanes.