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

[Grebe]

" Isnt lying to Congress an offense?"

Depends on who gets tagged with " lack of candor"

Boeing is now searching for the Janitor who misplaced the ' documents' or forgot to keep the shredder turned on

[edmundronald]

Originally Posted by gums

Salute!

Thanks for the input, PEI
OTOH, we see many raw data flight data recorder traces and have had super analysis amongst us, especially on the Tech Log. So seeing the data is not without precedent here, although the masses that read the New York Slime would not have a clue ( as they do not about many things)

My point and maybe Stri and FDR and Edmund , et al be that the plane may be just fine at idle thrust versus high thrust or even cruise settings. How bad is the linearity? And I do not believe "Peter" has the actual plots from flight test data, Finally, the change to the amount of stab movement, etc after the initial implementation is, or should be, of interest to all of us.

And "all of us" includes old farts like me that are now only SLF. As a SLF I once got a free bottle of champaign from the Western airline crew ( back a long time ago before being absorbed) for commenting to nearby SLF that the crack maintenance folks were repairing a hydraulic leak on the left main gear brakes. We had to delay about a half hour, and the nearby SLF were getting restless, heh heh. Crew told me that my commentary helped calm down the SLF and made things easier for all.

I may only be a knowledeable piece of SLF nowadays, and it scares me more than the average piece sitting next to me when I see what this MCAS debacle has wrot.

Gums sends, and Merry Christmas to all!!!!

Gums,
Math jargon Sri’s , “may exhibit non-linear feedback” is the engineer’s equivalent of saying may become EXTREMELY unpredictable.

- one wants a system to be controllable - ie can be brought/forced to a desired eg. nice stable state starting from any state inside the envelope.
- if the system is “linear” then a branch of engineering maths called LINEAR control theory applies, the domain where it is controllable is known, and eveything is “copacetic”, automation can be easily used, and the system may be approved on the virtues of its design.
- if the system is “non-linear” and even worse if it exhibits non-linear feedback, then can as they say in the US it can “go postal”, and standard school maths can’t describe it nor control it. An auditor won’t certify such a system unless it is extensively tested -if the auditor be honest. And automation can be very unhappy about the surprises baked into such a unpredictable system.

The suspicion is that MCAS is there to keep the MAX centered in an envelope where it appears linear, but that without MCAS it might get into states where it would be really hard to control (except for Chuck Yeager and you). The unfortunate joke is that the extant MCAS took several airframes into states where they were hard to control. Of course one could argue one can always deviate into a non-linear behaviour eg. stall, but the suspicion here is that the volume of linearity is more limited than it should be, and that really ugly things happen in places where certified airframes should still be acting nice.

I’m sorry if the above is erroneous or unclear, my opinions are usually worth exactly what people are paying for them.

Merry Xmas

[ktcanuck]

Originally Posted by edmundronald

Gums,
Math jargon Sri’s , “may exhibit non-linear feedback” is the engineer’s equivalent of saying may become EXTREMELY unpredictable.

- one wants a system to be controllable - ie can be brought/forced to a desired eg. nice stable state starting from any state inside the envelope.
- if the system is “linear” then a branch of engineering maths called LINEAR control theory applies, the domain where it is controllable is known, and eveything is “copacetic”, automation can be easily used, and the system may be approved on the virtues of its design.
- if the system is “non-linear” and even worse if it exhibits non-linear feedback, then can as they say in the US it can “go postal”, and standard school maths can’t describe it nor control it. An auditor won’t certify such a system unless it is extensively tested -if the auditor be honest. And automation can be very unhappy about the surprises baked into such a unpredictable system.

The suspicion is that MCAS is there to keep the MAX centered in an envelope where it appears linear, but that without MCAS it might get into states where it would be really hard to control (except for Chuck Yeager and you). The unfortunate joke is that the extant MCAS took several airframes into states where they were hard to control. Of course one could argue one can always deviate into a non-linear behaviour eg. stall, but the suspicion here is that the volume of linearity is more limited than it should be, and that really ugly things happen in places where certified airframes should still be acting nice.

I’m sorry if the above is erroneous or unclear, my opinions are usually worth exactly what people are paying for them.

Merry Xmas

If you are saying that MCAS was a brute force way of stopping the Max getting into a [secret] corner of the envelope that was nasty but ended up, through some failure, putting the poor souls into a different nasty situation, I agree entirely.

[edmundronald]

Originally Posted by ktcanuck

If you are saying that MCAS was a brute force way of stopping the Max getting into a [secret] corner of the envelope that was nasty but ended up, through some failure, putting the poor souls into a different nasty situation, I agree entirely.

Yes. And we’re saying we believe that the envelope where the MAX is natively nice is too small to allow it to be certified, and this is what Boeing is hiding with passive collusion from the FAA. If -when - international certification authorities do tests without MCAS, the deficiencies will become apparent, and goodbye certification. My guess - I’m just a random SLF- is that civilian airliners that are flown by standard pilots are supposed to have very forgiving aerodynamics, and large time constants, and that as soon as AoA climbs strongly or goes negative and the nacelle effects come into play there is nothing forgiving about this airframe any more.

The answer to this question would be provided by flight test data without MCAS. I think that at this point certification without this data being published is not feasible, at least outside the US.

Also, I would think that any pilot who gets into a MAX at this point should demand to know the real intended function of MCAS. It’s possible that at high AoA, elevator authority effects are very substantially different from the NG, which is why MCAS was redesigned to react fairly strongly. Please take this with a large grain of salt, I am not a pilot.


Edmund

[MechEngr]

Originally Posted by DaveReidUK

I'm not sure what the relevance of the "international investigation community" is.

ICAO Annex 13 is clear - only the party leading an investigation (here, the Ethiopian AIB) is authorised to release information, and it decides what will be released when, not the operator, manufacturer or any of the other parties affiliated to the investigation.

That would be public releases, not to withhold information from other investigative agencies in the ICAO.

[George Glass]

Curious that the initial report on the Ethiopian accident has been removed from the ecaa.gov.et website. Anyone know where its gone ?

[MechEngr]

Originally Posted by edmundronald

Yes. And we’re saying we believe that the envelope where the MAX is natively nice is too small ...

Also, I would think that any pilot who gets into a MAX at this point should demand to know the real intended function of MCAS. It’s possible that at high AoA, elevator authority effects are very substantially different from the NG, which is why MCAS was redesigned to react fairly strongly. Please take this with a large grain of salt, I am not a pilot.

Edmund

An airplane in flight is essentially a teeter-totter balanced on the Center of Pressure (CP) for the entire airplane and is considered to be the point where the lift acts. One element of this is that lift from positive AoA produces a nose-down pitch torque. To counter that, the horizontal stabilizer produces a nose up pitch torque by pushing down on the aft end of the fuselage. When these are balanced everyone is happy, or at least the AoA isn't changing. Of note is that the stabilizer functions as a wing that produces lift opposite to the lift of the wing and has its own local AoA.

If one looks at all possible stable AoAs one would like to see a linear relation between the AoA of the wing and the stabilizer trim position required to balance it. But the Max seems to have another player on the teeter-totter and that is the nacelle of the engine which starts producing noticeable nose up pitch torque at high AoA. So the stab trim position is no longer quite as linear as it was. Since the elevator also affects the pitch torque, that's where the effect could be noticed by the pilot as they move the controls without adjusting the trim. The function of MCAS is to make it so the pilot doesn't experience this new player.

Because it's just to offset the new player and depends on AoA and airspeed (because the amount of lift the stabilizer produces depends on those things to generate nose-up torque) it really isn't moving much or fast; it just has to be fast enough to keep up with AoA changes to the aircraft and to speed changes, neither of which ought to be particularly high. It gets more pronounced at low speeds because not only does the requisite high AoA needed to provide lift at low speeds increase the effect from the engine nacelles, the lower speed also means the stabilizer has less dynamic pressure to work with. This is no different than, say, rudder authority at low speed; the rudder has to move a lot farther to get the same effect at low speed than high speed.

AFAIK that's the intended MCAS function. To meet a linearity requirement for pilot controls by rebalancing a larger input from the engine nacelles than was existent, but could be ignored, on earlier models. It's not fast enough for a negative stability situation, so that's not it.

[MechEngr]

Originally Posted by George Glass

Curious that the initial report on the Ethiopian accident has been removed from the ecaa.gov.et website. Anyone know where its gone ?

Maybe the link is gone; where did you look that you did not find it? Not sure if I get to post links:
http://www.ecaa.gov.et/Home/wp-conte...MAX-ET-AVJ.pdf

[edmundronald]

Originally Posted by MechEngr

An airplane in flight is essentially a teeter-totter balanced on the Center of Pressure (CP) for the entire airplane and is considered to be the point where the lift acts. One element of this is that lift from positive AoA produces a nose-down pitch torque. To counter that, the horizontal stabilizer produces a nose up pitch torque by pushing down on the aft end of the fuselage. When these are balanced everyone is happy, or at least the AoA isn't changing. Of note is that the stabilizer functions as a wing that produces lift opposite to the lift of the wing and has its own local AoA.

If one looks at all possible stable AoAs one would like to see a linear relation between the AoA of the wing and the stabilizer trim position required to balance it. But the Max seems to have another player on the teeter-totter and that is the nacelle of the engine which starts producing noticeable nose up pitch torque at high AoA. So the stab trim position is no longer quite as linear as it was. Since the elevator also affects the pitch torque, that's where the effect could be noticed by the pilot as they move the controls without adjusting the trim. The function of MCAS is to make it so the pilot doesn't experience this new player.

Because it's just to offset the new player and depends on AoA and airspeed (because the amount of lift the stabilizer produces depends on those things to generate nose-up torque) it really isn't moving much or fast; it just has to be fast enough to keep up with AoA changes to the aircraft and to speed changes, neither of which ought to be particularly high. It gets more pronounced at low speeds because not only does the requisite high AoA needed to provide lift at low speeds increase the effect from the engine nacelles, the lower speed also means the stabilizer has less dynamic pressure to work with. This is no different than, say, rudder authority at low speed; the rudder has to move a lot farther to get the same effect at low speed than high speed.

AFAIK that's the intended MCAS function. To meet a linearity requirement for pilot controls by rebalancing a larger input from the engine nacelles than was existent, but could be ignored, on earlier models. It's not fast enough for a negative stability situation, so that's not it.

MechEngr,

You have very cogently made the case -by reasoning- for the need for something like MCAS; of course if a new force is introduced it would be desirable to offset it if one wishes to use an extant control model.
However you haven't really explored the issues which lead to this need, in other words what piloting a MAX without MCAS would look like.

Also it is not at all obvious -in fact extremely improbable- that a given control model can be perfectly grafted on a different airframe by means of a simple mechanical accessory. Of course one could make the case that the mechanical aid creates a similar linearised behavior around equilibrium, bu it would still be required to demonstrate that the relinearised zones share the same volumes of state space, and that these volumes are the only regimes which are of practical importance during flight. Hence it would be very difficult to demonstrate by maths alone, without extensive testing that the MAX shares a control model with the NG and thus can share its certification. There is no evidence to date of the results of such testing.

My understanding that under current regulatory règimes for civilian aircraft, acceptance of the airframe design WITH MCAS is conditioned on the behavior WITHOUT MCAS. Namely if without MCAS the airframe cannot be flown easily ie. can tend to stall or dive or do other wierd things, then the design would be classed as necessitating active control (MCAS) and couldn't be certified. Which is why Boeing so prudently talks of "linearizing" stick feel.

I continue to believe that in a rush to acquire a grandfathered certification, Boeing was overly economical with the truth in its descriptions of the 737 dynamics, which explains Boeing's reluctance to reveal the exact design goals of MCAS.


Most of the pilot readers here would appear to have no issue with MCAS certification if it were "just" a stick-pusher, but if it is a more complex entity as now seems likely, then a more careful certification process seems necessary.

Edmund

[MechEngr]

Originally Posted by edmundronald

MechEngr,

You have very cogently made the case -by reasoning- for the need for something like MCAS; of course if a new force is introduced it would be desirable to offset it if one wishes to use an extant control model.
However you haven't really explored the issues which lead to this need, in other words what piloting a MAX without MCAS would look like.

Also it is not at all obvious -in fact extremely improbable- that a given control model can be perfectly grafted on a different airframe by means of a simple mechanical accessory. Of course one could make the case that the mechanical aid creates a similar linearised behavior around equilibrium, bu it would still be required to demonstrate that the relinearised zones share the same volumes of state space, and that these volumes are the only regimes which are of practical importance during flight. Hence it would be very difficult to demonstrate by maths alone, without extensive testing that the MAX shares a control model with the NG and thus can share its certification. There is no evidence to date of the results of such testing.

My understanding that under current regulatory règimes for civilian aircraft, acceptance of the airframe design WITH MCAS is conditioned on the behavior WITHOUT MCAS. Namely if without MCAS the airframe cannot be flown easily ie. can tend to stall or dive or do other wierd things, then the design would be classed as necessitating active control (MCAS) and couldn't be certified. Which is why Boeing so prudently talks of "linearizing" stick feel.

I continue to believe that in a rush to acquire a grandfathered certification, Boeing was overly economical with the truth in its descriptions of the 737 dynamics, which explains Boeing's reluctance to reveal the exact design goals of MCAS.


Most of the pilot readers here would appear to have no issue with MCAS certification if it were "just" a stick-pusher, but if it is a more complex entity as now seems likely, then a more careful certification process seems necessary.

Edmund

The question is about what it would feel like. I think it would feel as much like an NG at a similar load as anything because pilots spend so little time at high AoA and also little time hand flying that they have no good basis for comparison. We already have record of a flight without MCAS in the first Lion Air situation non-crash flight. The pilots made no comment on any difficulty of hand flying besides the obvious loss of electric trim.

You cannot use a stick pusher to change the balance of aerodynamic forces on the the plane, which MCAS does and a stick pusher does not. And if that stick pusher had the same authority to apply full-down elevator with as much force on the column as MCAS produced because, for the same AoA system malfunction, the controls felt the plane was in a deep stall, there would still be the same crashes.

[Capn Bloggs]

The question is about what it would feel like. I think it would feel as much like an NG at a similar load as anything because pilots spend so little time at high AoA and also little time hand flying that they have no good basis for comparison. We already have record of a flight without MCAS in the first Lion Air situation non-crash flight. The pilots made no comment on any difficulty of hand flying besides the obvious loss of electric trim.

They were nowhere near the corner of the flight envelope where MCAS (normally) operates.

[edmundronald]

Originally Posted by MechEngr

The question is about what it would feel like. I think it would feel as much like an NG at a similar load as anything because pilots spend so little time at high AoA and also little time hand flying that they have no good basis for comparison. We already have record of a flight without MCAS in the first Lion Air situation non-crash flight. The pilots made no comment on any difficulty of hand flying besides the obvious loss of electric trim.

You cannot use a stick pusher to change the balance of aerodynamic forces on the the plane, which MCAS does and a stick pusher does not. And if that stick pusher had the same authority to apply full-down elevator with as much force on the column as MCAS produced because, for the same AoA system malfunction, the controls felt the plane was in a deep stall, there would still be the same crashes.

The question is what it flies like, not what it feels like. MCAS may re-linearise MAX to resemble NG in normal flight, but we don't know when and whether the behavior of Max can depart from that of the NG.

A good example of this might be stall onset and deep stall recovery. Are they the same?

Another nastier example is recovery from a strong accidental pitchdown.


We can continue to argue this forever; it'll only be put to rest when either Boeing publishes the data, a non-us regulator rebels and does a test flight, or someone constructs a detailed aerodynamical simulation of the Max.

Edmund

[fdr]

Originally Posted by edmundronald

...The suspicion is that MCAS is there to keep the MAX centered in an envelope where it appears linear, but that without MCAS it might get into states where it would be really hard to control (except for Chuck Yeager and you). The unfortunate joke is that the extant MCAS took several airframes into states where they were hard to control.

I’m sorry if the above is erroneous or unclear, my opinions are usually worth exactly what people are paying for them...

Merry Xmas

I suspect that in the fullness of time, when modelling of the flow is conducted by some august body with unsteady modelling code, that the effect of the cowling will end up being noticeable but not significant in itself, that MCAS the cure is worse than the risk that it protects against. My estimate is that it will not have ever resulted in an unstable condition, that the underlying effect was notchiness of the nacelle acting as a lifting body with vortex structures over the wing, and that the vortex interaction on the wing is a more significant component of the pitching moment; the combined effects of planform increase forward of the cg, vortex structures over the wing increasing lift inboard (forward shift of Cp and reducing Cm on a swept wing) and increased downwash at the tail from the inner section of the wing arising from the nacelle vortex interaction with the wing at high AOA. With all of those effects, still suspect that the total effect was modest and never resulted in a pitch up. I'll wager USD100 on that to the first DNS/LES modelling that shows differently.

All that really means, the cure may well have been far worse than the symptom.

[xetroV]

Originally Posted by PEI_3721

The certification arguments for MCAS were flawed, but the principles were proven in flight test, certification, and subsequently commercial operations. MCAS worked as designed; however history shows that the supporting design was not fail safe, nor in that sense, were the interpretations, judgements, and approvals in the regulatory process; both systemic failures.

Are there any records of MCAS firing up as designed during actual commercial operations? I wonder how an aircrew that somehow finds itself in the part of the flight envelope where MCAS would be activated would react to the sudden (presumably unexpected) automatic nose-down trim correction. MCAS may work as designed in that case, but what about the startled human element?

[infrequentflyer789]

Originally Posted by turbidus

AFTER all this time, they "FIND" documents that show troubling and further communications with Forkner?????

You can do a word search for "Forkner" and find them all????

what sort of software do the run there??? (oh...never mind)

Careful, the wording used was "additional documents that were discovered by its internal investigation ... that the company’s lawyers say it needs to disclose".

The finding may have been done long ago, the "need to disclose" may be the recent change. Finding (electronic) documents is relatively easy, can be done with software. Deciding which documents you "need to disclose" or which documents are protected from disclosure or even may not be disclosed, is a lot harder, cannot be done (in most cases) with software, and requires wetware. Expensive, legal, wetware.

There is also a high stakes poker game going on between Boeing, Forkner and Justice Dept. / investigators. Forkner plead the fifth, clearly implying he has something to hide, something to sell, and something to gain from immunity - but no one knows what, maybe it's all a bluff. Boeing has to guess, because Forkner apparently isn't using Boeing legal counsel (no **** sherlock...) - which means they can't be sure if he is going to helpfully fall on his sword or come round behind them and stab them with it.

[fdr]

Originally Posted by xetroV

Are there any records of MCAS firing up as designed during actual commercial operations?

Doubt it. Had there been, it would have been extenuating to the implementation of MCAS, so would have come to light to mitigate the decisions that were made.

[petit plateau]

Originally Posted by fizz57

The non-linearity issue was brought up by FCEng84 way back when the whole thing started in one of the now-closed threads. It is the stick force reversal and instability that are speculations, and came later.

That poster was usually a very reliable and well-informed source. Unfortunately he hasn't posted for a very long time (probably the price for being too well-informed).

A few of us commented on the issue regarding trimming non-monotonic curves at the time. The wise old pilots ignored. So we shut up.

[DaveReidUK]

Originally Posted by xetroV

Are there any records of MCAS firing up <em>as designed</em> during actual commercial operations? I wonder how an aircrew that somehow finds itself in the part of the flight envelope where MCAS would be activated would react to the sudden (presumably unexpected) automatic nose-down trim correction. MCAS may work as designed in that case, but what about the startled human element?

I'd guess that deployment of MCAS for real would result in at least an ASR.

Last time I did a search of NASA's ASRS, there weren't any reports of such.

[JPcont]

Originally Posted by edmundronald

Gums,
Math jargon Sri’s , “may exhibit non-linear feedback” is the engineer’s equivalent of saying may become EXTREMELY unpredictable.

- one wants a system to be controllable - ie can be brought/forced to a desired eg. nice stable state starting from any state inside the envelope.
- if the system is “linear” then a branch of engineering maths called LINEAR control theory applies, the domain where it is controllable is known, and eveything is “copacetic”, automation can be easily used, and the system may be approved on the virtues of its design.
- if the system is “non-linear” and even worse if it exhibits non-linear feedback, then can as they say in the US it can “go postal”, and standard school maths can’t describe it nor control it. An auditor won’t certify such a system unless it is extensively tested -if the auditor be honest. And automation can be very unhappy about the surprises baked into such a unpredictable system.

The suspicion is that MCAS is there to keep the MAX centered in an envelope where it appears linear, but that without MCAS it might get into states where it would be really hard to control (except for Chuck Yeager and you). The unfortunate joke is that the extant MCAS took several airframes into states where they were hard to control. Of course one could argue one can always deviate into a non-linear behaviour eg. stall, but the suspicion here is that the volume of linearity is more limited than it should be, and that really ugly things happen in places where certified airframes should still be acting nice.

I’m sorry if the above is erroneous or unclear, my opinions are usually worth exactly what people are paying for them.

Merry Xmas

I consider this a bit too simplified approach. Basically every real world control loop is more or less non-linear. Yet, typically the control design is based on the linear analysis. However, the design rules includes stability margin. This stability margin is related to the Lyapunov stability analysis.

On the other hand, even the linear system might have uncontrollable and unobservable states. In a closed loop system, there tends to be observability problems that restricts the the usability of the closed loop data.

If we consider MAX case, there are two independent feedback controllers: the pilot and the MACS. So both of them have to operate with restricted information. I see this as the root cause of the problems. When the system starts to work in an unpredictable way the failure analysis might become unreliable.

I don’t understand why the MAX is still grounded. The control problems should be relatively easy to solve (with additional actuators) when the understanding of the underlying problem is present.

My guess is that MACS was used to fix something else that stick force problem (in the control engineering referenced as open loop gain). My guess is that the system was there to prevent to enter to a envelope where the the real problems begins. For the political reasons it was named and defined to system that makes the “feeling” like NG.

My guess is that the problem were solved if the politics, the "salesman's lies” and the lawyer advices were were forgotten and the raw engineering job from clean table were started. Some kind of spoiler in the wing root or somewhere near the pylon might be needed. There should be courage to think out of the box and forget the short term costs.

[hec7or]

Originally Posted by DaveReidUK

I'd guess that deployment of MCAS for real would result in at least an ASR. L

time I did a search of NASA's ASRS, there weren't any reports.

A real activation of MCAS as designed, would be almost imperceptible to crew, (I've seen it demonstrated in the MAX simulator) - eg; during flapless maneuvering at low IAS/High AoA where MCAS will activate momentarily and cease activation as soon as AoA reduces to design limits- much like a speed trim correction. It may well have activated many times without any crew being aware of its operation.
I think what is meant here is a runaway MCAS activation following AoA sensor failure, - an erroneous activation which will not stop trimming as the perceived AoA would not reduce even when trim reaches the forward limit, because the sensor has failed, which Boeing mistakenly assumed would be recognised as a trim runaway.