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

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

Old 23rd Jun 2019, 16:50
  #581 (permalink)  
 
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Originally Posted by Smythe View Post
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.
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Old 23rd Jun 2019, 17:53
  #582 (permalink)  
 
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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.
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Old 23rd Jun 2019, 18:21
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Originally Posted by Smythe View Post
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.

Last edited by yoko1; 23rd Jun 2019 at 18:56.
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Old 23rd Jun 2019, 19:41
  #584 (permalink)  
 
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Originally Posted by yoko1 View Post
​​​​​​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.


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. 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.


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Old 23rd Jun 2019, 20:14
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Originally Posted by Smythe
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.

Last edited by Smythe; 23rd Jun 2019 at 20:33.
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Old 23rd Jun 2019, 20:31
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The nacelles themselves start to create aerodynamic lift and move the nose up at high alpha.
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Old 23rd Jun 2019, 20:45
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Originally Posted by Kerosene Kraut View Post
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.
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Old 23rd Jun 2019, 20:55
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Originally Posted by Smythe View Post
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.
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Old 23rd Jun 2019, 21:13
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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.
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Old 23rd Jun 2019, 21:38
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Originally Posted by Smythe View Post
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.
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Old 23rd Jun 2019, 22:59
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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.

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Old 23rd Jun 2019, 23:11
  #592 (permalink)  
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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 [email protected] 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.
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Old 24th Jun 2019, 02:05
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Originally Posted by Loose rivets View Post
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.
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Old 24th Jun 2019, 03:29
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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.
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Old 24th Jun 2019, 03:35
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Originally Posted by Loose rivets View Post
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.

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Old 24th Jun 2019, 04:44
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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 ??
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Old 24th Jun 2019, 05:37
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Originally Posted by yoko1 View Post
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.
Originally Posted by yoko1 View Post
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.

Last edited by BDAttitude; 24th Jun 2019 at 05:50.
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Old 24th Jun 2019, 06:51
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Peter Lemme ...

... has updated his findings about the incidence of runaway trims and stabilizer jams.

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

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Old 24th Jun 2019, 09:46
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Originally Posted by BDAttitude View Post
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
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Old 24th Jun 2019, 10:55
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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


Originally Posted by yoko1 View Post
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.

Last edited by edmundronald; 24th Jun 2019 at 11:15.
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