How Safe is the Boeing 737 Max's MCAS System?

[PeterDonis]

pilots report it happens with autopilot ON , and to my understanding MACS is supposed to be OFF when on autopilot.

Yes, this is a key observation, and, as @cyboman posted, it's a reason to think the issue is not just an MCAS issue.

 

[PeterDonis]

I still think the MCAS and stall prevention shouldn't only look at AoA. If pitch attitude is detected to be literally pointing into the ground or even below the horizon for that matter, no system should be sending nose down commands in that scenario. It also shouldn't be sending repeated commands if it has sent N commands after all being overridden by pilot input.

Bear in mind that we don't have access to the internal algorithms. So we don't know that these systems look only at AoA. Nor do we have access to the flight recorder data that shows what the sensor readings were. So we don't know on what basis the system did what it did. Nor do we know what it would do under other circumstances. We don't know exactly when it sends repeated commands or what exactly stops it from doing so. We're speculating based on what information is available, which is limited (and sometimes, as has been commented, contradictory).

 

[nsaspook]

More analysis; somewhat speculative, so take with a grain of salt:

My interpretation of Boeing's/the FAA's interpretation of the MCAS system's criticality is that failure of the system is not an emergency situation. This is evidenced by the previous day's Lion Air flight, which had the same failure and continued on to its destination after effectively dealing with it. The flight manual doesn't describe it as an emergency requiring an immediate landing, so they didn't. This is probably because "runaway trim" is a thing that happens from time to time. Any plane that has automation or even just electronic trim controls could have this, and it does happen from time to time. It can be overridden and the flight continued...but it does sometimes result in crashes, so it is a tough call as to whether or not it should be considered an emergency.

The difference with the MCAS system is that instead of just going full nose down trim and staying there until the pilot shuts off the system, it allows the pilot to override it with the trim levers -- and then repeats the full nose down trim a few seconds later. The apparent fixing of the problem and re-occurrence would cause a confused pilot to delay disabling the system, eat-up altitude and increase the likelihood of a crash.

That seems to be true as stability augmentation is not new or novel in aircraft and there are rules about how to handle failures.
https://www.law.cornell.edu/cfr/text/14/23.672

As usual it's all about trust. At MAX 8 introduction, if Boeing told the FAA "we've got this' stability 'defect' under control so pilots get what they expect from flight controls I think we all would have said OK two years ago. Today they have some 'splainin' to do.

 

[PeterDonis]

Some sources, such as the above, say the MCAS moves the entire stabilizer, not just adjusts the trim tab (the graphic doesn't match the text of the article).

If I'm reading some previous posts correctly, the automated trim system, which is what the MCAS is using, does move the entire stabilizers (both sides); the yoke moves a portion of them (the elevators).

 

[russ_watters]

If I'm reading some previous posts correctly, the automated trim system, which is what the MCAS is using, does move the entire stabilizers (both sides); the yoke moves a portion of them (the elevators).

Looks like you are correct. See:

Also, the following site provides a discussion (with photos) of the 737 flight control system clearly showing the all-moving stabilizer for pitch trim and the elevators for pitch control. The 737 elevators also have balance tabs, which look like trim tabs, but have a different purpose.
http://www.b737.org.uk/flightcontrols.htm
On the 737, Mach trim, speed trim, and MCAS all work through an all-moving stabilizer, not any sort of elevator trim tab.

https://leehamnews.com/2019/03/10/use-caution-about-ethiopian-737-8-crash/

The B737-700 is fitted with tabs on the trailing edges of the elevator control surfaces. These act as balance tabs to reduce the control forces required to move the elevators and are critical for manual control of the aircraft in the event of a double hydraulic system failure. Two control rods link each tab to the elevator control system such that when the elevators are deflected the tabs also deflect.

http://code7700.com/aero_balance_tabs.htm

https://www.pmdgsim.com/2016/10/why-does-boeing-use-balance-tabs-on-737_21.html

This doesn't necessarily change the interpretation of the crash, but it is good to get the terminology and parts correct...

This link is incredibly detailed, created after the Lion Air crash, though I haven't read through the whole thing yet:
https://www.satcom.guru/2018/11/stabilizer-trim.html

 

[PeterDonis]

It's painting a picture of corporate negligence caused by cutting corners and expediency and ultimately greed. That's harsh but it's definitely evident in the lack of training and poor documentation.

Again, bear in mind that we do not have all the facts yet. Also, bear in mind that this thread is focused on the technical aspects; allegations of "corporate negligence" and "greed" are off topic since those aren't technical judgments.

 

[cyboman]

Again, bear in mind that we do not have all the facts yet. Also, bear in mind that this thread is focused on the technical aspects; allegations of "corporate negligence" and "greed" are off topic since those aren't technical judgments.

Absolutely, that's why I said it's "painting a picture". And it'd say it's only fairly conclusive so far Boeing's documentation in the flight manuals for the new MCAS was fundamentally lacking as was adequate pilot training. You can gather that not just from the media but from the pilot reports.

I wouldn't say such suggestions are off topic. My original post started with technical questions but the ramifications of some of the light we shed on such technical considerations can lead to suggestions that the engineers or the company were potentially negligent in the name of profits. No one is suggesting we analyse the nature of corporate negligence and greed.

 

[PeterDonis]

I wouldn't say such suggestions are off topic.

That statement of mine was not a personal opinion; it was a reminder about our moderation policy. If we cannot keep this thread focused on technical questions and refrain from making allegations that are not technical, the thread will be closed. I don't want that to happen and neither do you. So please take heed.

some of the light we shed on such technical considerations can lead to suggestions that the engineers or the company were potentially negligent in the name of profits.

Regardless, such suggestions do not belong here, since this is a technical discussion. Negligence is a legal judgment, not a technical judgment.

 

[cyboman]

If I'm reading some previous posts correctly, the automated trim system, which is what the MCAS is using, does move the entire stabilizers (both sides); the yoke moves a portion of them (the elevators).

This is what you and and Russ I think are muddling and causing some confusion. From my understanding of the system, the MCAS and/or/with the automatic stall prevention system do pitch the nose of the aircraft down. MCAS is an augmentation system, it's not simply a force feedback system as you seem to imply. It was implemented in order to deal with the changed aerodynamics of the plane due to the larger engines which cause a positive pitch attitude - they cause the aircraft nose to rise. This can cause a stall under normal operating conditions with a pilot without special training for the changed aerodynamics of the plane. It may provide force feedback to warn the pilot as a secondary effect, but it's primary effect and role is to change the aerodynamics of the aircraft by adjusting trim and in effect pushing the nose down. This as has been pointed out was put in place as opposed to providing additional training to pilots certified to fly previous 737s, such that it's invisibly working behind the scenes to make the craft appear to handle like previous models. Truly invisible in the sense it wasn't even included or noted in the flight manuals.

 

[nsaspook]

Another jewel from the link of @russ_watters

https://www.satcom.guru/2018/11/737-mcas-failure-is-option.html

 

[cyboman]

That statement of mine was not a personal opinion; it was a reminder about our moderation policy. If we cannot keep this thread focused on technical questions and refrain from making allegations that are not technical, the thread will be closed. I don't want that to happen and neither do you. So please take heed.
Regardless, such suggestions do not belong here, since this is a technical discussion. Negligence is a legal judgment, not a technical judgment.

Fair enough. I don't see how saying the lack of documentation amounts to potential negligence is pushing the thread into some dangerous non-technical realm. To be fair, a good part of this thread has been speculation. In effect, that's all we can really do. But I understand your wish to reemphasize nothing is conclusive yet.

Further, I will steer clear of directly suggesting any negligence on Boeing's part if that is off limits.

 

[PeterDonis]

I don't see how saying the lack of documentation amounts to potential negligence is pushing the thread into some dangerous non-technical realm.

That's why I made it clear that, as far as PF's moderation policy is concerned, it does. PF's moderation policy might not agree with your personal judgment, but it is what it is, and I wanted to make sure it was clear to everyone.

 

[cyboman]

I will be careful not to suggest any possible negligence on Boeing's part as it's been pointed out to me this is seen as defamatory and not permitted on these forums.

 

[cyboman]

Bear in mind that we don't have access to the internal algorithms. So we don't know that these systems look only at AoA. Nor do we have access to the flight recorder data that shows what the sensor readings were. So we don't know on what basis the system did what it did. Nor do we know what it would do under other circumstances. We don't know exactly when it sends repeated commands or what exactly stops it from doing so. We're speculating based on what information is available, which is limited (and sometimes, as has been commented, contradictory).

That's a good point. Perhaps I am speculating too much here. In fact, I did read somewhere at one point, I believe, that the system is taking in other data, I believe it was altitude and air speed. It will be interesting to see as the investigations conclude, what changes they are proposing to the system. We know from the air authority memo I posted earlier that they are looking at a max command limit, as well as activation and sensor changes to MCAS. It seems to me pitch attitude with reference to the horizon would be very relevant data to such a system. Ideally, I would suggest GPS data should be used for redundancy.

 

[PeterDonis]

From my understanding of the system, the MCAS and/or/with the automatic stall prevention system do pitch the nose of the aircraft down.

I've already explained repeatedly what the system does and under what circumstances the nose will in fact go down. So have others. But I'll try once more.

The fundamental error you are making with regard to MCAS is to confuse trim adjustments with pitch commands. Trim adjustments are made for the purpose of changing the force required to hold the yoke at a particular point in its travel. They are not made for the purpose of changing the plane's pitch attitude. It is possible for the plane's pitch attitude to change as the result of a trim adjustment, if the pilot does not adjust the force he is exerting on the yoke. But that is a side effect of the trim adjustment; it's not the primary purpose of the trim adjustment.

It was implemented in order to deal with the changed aerodynamics of the plane due to the larger engines which cause a positive pitch attitude - they cause the aircraft nose to rise.

This is oversimplified. The pitch up moment due to the new engines (it's not just that they're larger, they're also further forward) is not constant; it depends on airspeed and angle of attack. We already know the MCAS trim adjustment depends on angle of attack; I would expect it to depend on airspeed as well. (And possibly on other conditions like angle of bank, since that will affect the stall speed.)

This can cause a stall under normal operating conditions with a pilot without special training for the changed aerodynamics of the plane. It may provide force feedback to warn the pilot as a secondary effect, but it's primary effect and role is to change the aerodynamics of the aircraft by adjusting trim and in effect pushing the nose down.

No, you have this backwards. The primary purpose is to provide force feedback to the pilot as a function of airspeed and angle of attack that is similar to previous 737s. That's how the need for special training was to be avoided. That's why it uses the trim system, since, as noted above, the trim system's primary purpose is to change the yoke force as a function of yoke travel. Pushing the nose down, if it happens, is a secondary effect, as I've already said.

If the primary purpose of MCAS were to "change the aerodynamics of the plane" (which I think is a misleading way of looking at it anyway) and force the nose down, it would function more like automatic stall prevention in a fly-by-wire system: if it detected that the plane was too close to a stall, it would simply override the pilot's input and use the yoke control system to force the nose down--i.e., it would force the yoke to a different position than the pilot wants to put it. But it doesn't do that.

 

[cyboman]

If the primary purpose of MCAS were to "change the aerodynamics of the plane" (which I think is a misleading way of looking at it anyway) and force the nose down, it would function more like automatic stall prevention in a fly-by-wire system: if it detected that the plane was too close to a stall, it would simply override the pilot's input and use the yoke control system to force the nose down--i.e., it would force the yoke to a different position than the pilot wants to put it. But it doesn't do that.

Hmm, this really is getting confusing. I thank you for your continued efforts to explain it. I am understanding the MCAS system better and better as a non-pilot. However, it seems there is different ways this system is described. For one, it's not clear that the MCAS is completely independant of the automatic stall prevention.

I've seen it written many places that MCAS "commands nose down trim". That would suggest it is in effect commanding nose down by adjusting trim. Not providing force feedback to communicate to the pilot to provide nose down trim.

This new location and size of the nacelle causes it to produce lift at high AoA; as the nacelle is ahead of the CofG this causes a pitch-up effect which could in turn further increase the AoA and send the aircraft closer towards the stall. MCAS was therefore introduced to give an automatic nose down stabilizer input during steep turns with elevated load factors (high AoA) and during flaps up flight at airspeeds approaching stall.

On the face of it this seems like a sensible, beneficial system. However following the accident to Lion Air MAX-8 PK-LQP on 29 October 2018, shortly after take-off, in which it appears that the Captains AoA sensor was faulty, it is believed that the MCAS used this erroneous AoA data to command nose down stabiliser which was not counteracted sucessfully by the crew until the aircraft impacted the water.

From: http://www.b737.org.uk/mcas.htm

 

[PeterDonis]

I've seen it written many places that MCAS "commands nose down trim". That would suggest it is in effect commanding nose down by adjusting trim. Not providing force feedback to communicate to the pilot to provide nose down trim.

"Nose down trim" means adjusting the trim so that the point in the yoke's travel where the force needed to hold it there is zero, is moved towards the "nose down" end of the yoke's travel. It does not mean "push the nose down". The pilot's term for pushing the nose down is "pitch down", or more verbosely "commanding pitch down" (by pushing forward on the yoke). A pilot does not add nose down trim to push the nose down.

Perhaps it's worth going into some more detail about how pilots manually adjust trim. Let's suppose the pilot wants to increase the cruising speed of the plane (for example, perhaps the plane has just leveled off after a climb, and now the pilot wants to increase from best climb speed to best cruise speed, which is typically higher). The pilot will push forward on the yoke and increase the throttle, because the correct angle of attack for the faster airspeed is lower, and more engine power will be needed to compensate for increased drag at the higher speed. The pilot will need to maintain force on the yoke in order to hold the nose at the new angle of attack. Once the plane has stabilized at the higher airspeed, the pilot will gradually add nose down trim to reduce the force required to hold the yoke in position, until that force is reduced to zero, i.e., until the yoke will stay in the desired position, holding the plane at the desired angle of attack, without any force exerted by the pilot. At that point, the plane is trimmed for the new condition (lower angle of attack, higher airspeed).

Notice that, to actually move the nose, the pilot used the yoke, not the trim system. During the entire process of adjusting the trim, the nose did not move at all. The plane remained at a constant angle of attack. The only thing that changed was the force the pilot needed to exert on the yoke to hold it in position. That is how the trim system is intended to be used. And none of that changes in the presence of MCAS. The only thing that changes is that the magnitude of the force the pilot is exerting on the yoke at certain angles of attack will be different than it would have been if MCAS weren't there.

Don't be confused by the fact that, in the presence of faulty AoA sensor data, MCAS can cause the nose to pitch down. As I've already explained, the reason for that is that the pilot is not expecting the trim change and so is not adjusting the force he's exerting on the yoke, the way he would if the trim change were expected. (Note that in the process I described above, the pilot is continually adjusting the force he exerts on the yoke as he adjusts the trim.) But that is an error condition, and should not be used as the basis for an understanding of how MCAS, or the trim system in general, is intended to work during normal operation.

 

[PeterDonis]

Btw, a good detailed treatment of flying from a pilot's point of view is here:

https://www.av8n.com/how/

 

[cyboman]

"Nose down trim" means adjusting the trim so that the point in the yoke's travel where the force needed to hold it there is zero, is moved towards the "nose down" end of the yoke's travel. It does not mean "push the nose down". The pilot's term for pushing the nose down is "pitch down", or more verbosely "commanding pitch down" (by pushing forward on the yoke). A pilot does not add nose down trim to push the nose down.

Perhaps it's worth going into some more detail about how pilots manually adjust trim. Let's suppose the pilot wants to increase the cruising speed of the plane (for example, perhaps the plane has just leveled off after a climb, and now the pilot wants to increase from best climb speed to best cruise speed, which is typically higher). The pilot will push forward on the yoke and increase the throttle, because the correct angle of attack for the faster airspeed is lower, and more engine power will be needed to compensate for increased drag at the higher speed. The pilot will need to maintain force on the yoke in order to hold the nose at the new angle of attack. Once the plane has stabilized at the higher airspeed, the pilot will gradually add nose down trim to reduce the force required to hold the yoke in position, until that force is reduced to zero, i.e., until the yoke will stay in the desired position, holding the plane at the desired angle of attack, without any force exerted by the pilot. At that point, the plane is trimmed for the new condition (lower angle of attack, higher airspeed).

Notice that, to actually move the nose, the pilot used the yoke, not the trim system. During the entire process of adjusting the trim, the nose did not move at all. The plane remained at a constant angle of attack. The only thing that changed was the force the pilot needed to exert on the yoke to hold it in position. That is how the trim system is intended to be used. And none of that changes in the presence of MCAS. The only thing that changes is that the magnitude of the force the pilot is exerting on the yoke at certain angles of attack will be different than it would have been if MCAS weren't there.

EDIT: I should add your explanation here of trim was very enlightening. It helped me understand exactly how the trim is effecting the forces on

Don't be confused by the fact that, in the presence of faulty AoA sensor data, MCAS can cause the nose to pitch down. As I've already explained, the reason for that is that the pilot is not expecting the trim change and so is not adjusting the force he's exerting on the yoke, the way he would if the trim change were expected. (Note that in the process I described above, the pilot is continually adjusting the force he exerts on the yoke as he adjusts the trim.) But that is an error condition, and should not be used as the basis for an understanding of how MCAS, or the trim system in general, is intended to work during normal operation.

Reading a bit more about trim: I'm understanding that the trim basically functions to assist the pilot by adjusting the aerodynamics such that the pilot won't need to continually be applying input on the column using elevator control to keep the pitch neutral. Or pitch down or up, whatever the intended maneuver, but the trim seeks to eliminate any control pressure. I imagine this makes maneuverability better and maintains control. And eliminates the need for the pilot to continually input force on the column to maintain a constant attitude.

My issue with your explanation is that it seems to imply trimming is fundamentally a force feedback mechanism. That seems erroneous. It seems to me trim effects the forces operating on the column but that is due to changing the aerodynamics of the horizontal stabilizer. MCAS is adjusting trim and effecting column forces due to it adjusting the horizontal stabilizer position. The changed aerodynamics is applying a force with a vector up or down on the tail, in turn effecting how much column force is required to move the elevator and effect pitch.

By changing the position of the horizontal stabilizer (nose trim) you're actually changing the lift vector of the tail down force coming from the horizontal stabilizer. The way I see it this is changing the forces required to alter the pitch via the column / elevator control but it is not a force feedback mechanism, it's due to the changes in the rear horizontal stabilizer airfoil.

Let's imagine a scenario: The yoke is at neutral and the trim is at zero or no trim and the plane is cruising and stable. If you were to apply maximum trim down, it's not just going to effect column forces. The plane is going to start pitching down. The nose will start to fall. Maybe not as much as if the pilot pushed the yoke completely forward but it will effect the planes pitch as the lift vector on the horizontal stabilizer has changed. The control surface of that airfoil, of that stabilizer has changed.

Force feedback usually describes a mechanism that simulates real world forces like in fly-by-wire systems. I don't think trim falls into that definition. It is effecting column forces but that's because it's actually changing the aerodynamics of the plane, the resultant forces felt on the column from trim changes are not simulated they are from the changes in the planes aerodynamics.

Here's another way I see it, trim is applying a force vector on the plane itself not on the yoke. The trim changes create a different lift vector on the tail which is in effect could be thought of as adding an input to the pitch input coming from the column, though imperfect from an engineering view, it could be seen that the two are two inputs and the resultant sum is the final pitch vector. Such that if a pilot pulls back on the yoke say with 1 unit, but adjusts trim at 0.5 units up, then to keep that attitude he now only needs to pull with 0.5 units. So the force feedback the pilot feels on the column, as I see it, is a product of the changed aerodynamics of the horizontal stabilizer. Not the other way around.

 

[cyboman]

Btw, a good detailed treatment of flying from a pilot's point of view is here:

https://www.av8n.com/how/

This looks very comprehensive. I'll check it out! I got to say, this whole discussion has really renewed my interest in flight, which I'm sure like many, has been with me since childhood.

 

[cyboman]

A caveat to that though is that evidently some part of the stall prevention system was active at least for Lion Air, as the "stick shaker" stall notification system was active throughout the flight. So it is possible that both the MCAS and the stall prevention system were active; a 1-2 punch of down nose. This part is somewhat speculative though. I don't think it was mentioned in the preliminary Lion Air report.

That sounds like a fairly compelling hypothesis. I think you alluded to earlier that it's not entirely clear how disparate these two systems are. Seeing as we haven't seen this in previous models without MCAS. It may be that there were other changes to the flight laws and software when they incorporated MCAS. Perhaps the integration of these two systems is where the bug(s) exists. There certainly is a precedent for that sort of complication in software engineering. It could of been two completely separate teams working on each system and the integration of the two was the weak point.

 

[cyboman]

Here's some more information: http://knkt.dephub.go.id/knkt/ntsc_aviation/baru/pre/2018/2018 - 035 - PK-LQP Preliminary Report.pdf.

Wow, nice find.

 

[cyboman]

If the primary purpose of MCAS were to "change the aerodynamics of the plane" (which I think is a misleading way of looking at it anyway) and force the nose down, it would function more like automatic stall prevention in a fly-by-wire system: if it detected that the plane was too close to a stall, it would simply override the pilot's input and use the yoke control system to force the nose down--i.e., it would force the yoke to a different position than the pilot wants to put it. But it doesn't do that.

When we look at the data we have, the MCAS system or the automatic stall prevention, whichever, engaged 26 times with nose down maneuvers in the Lion Air case, before crash the FDR showed the pilots exerting 100lbs of force to pitch up. Even if the system is not overriding consistently like in fly-by-wire. It's reengaging often enough after the pilots try to override with manual force over and over again, enough to might as well be considered "overriding".

Also, simply because MCAS adjusts trim via the a horizontal stabilizer, I would contend it doesn't make it less dangerous or flight critical than if it took the yoke control and adjusted only the elevators of the stabilizer.

 

[jim hardy]

EDIT i see I'm late to the MCAS discussion . @ PeterDonis has it right . Maybe someone will find these few words useful so i'll leave them..

The MCAS is not doing stall detection. It's adjusting nose down trim based on angle of attack to compensate for the new engines.

Indeed.
My present understanding is
its purpose is to correct for aerodynamic lift of the new engine nacelles, not the engines' greater thrust.
Being further forward from center of lift they produce more nose up torque as AOA increases.
MCAS applies a little bit of nose down trim so that in a climb the force pilot feels on the yoke is same as with the old engines.
That's why it's supposed to be inactive when autopilot is ON -Otto doesn't need it..
The concept is "control force feedback" see https://www.aopa.org/news-and-media...light-training-magazine/flight-control-forces
source: a comment i saw at PPRUNE , so it's secondhand . Use with caution.

old jim

 

[OCR]

The concept is "control force feedback" see...

I am a member of the AOPA...

It's definitely a legit and well respected organization...

Aircraft Owners and Pilots Association - WikipediaHere is additional subject matter information concerning some of the issues in this thread...

Editor's note: This article was updated with additional MCAS information March 15

Pilots offer insights on Boeing 737 crashes - AOPA.

 

[OCR]

Air France 447 crashed because the flying pilot held full back-pressure on the control stick and stalled the plane from cruise until it hit the ocean about 4 minutes later... It's difficult to know what the pilot was thinking, but it is possible he didn't realize it was possible to stall the plane.

The NOVA and MAYDAY documentaries both contain fairly accurate and factual reports.

Besides the one on Wikipedia, this seems to be the most concise and accurate investigations I know...

ASN Aircraft accident Airbus A330-203 F-GZCP São Pedro and São Paulo Archipelago
Also, be aware, the PDF - Final Report... Published July 2012, is 224 pages long.

.

 

[jim hardy]

Here is additional subject matter information concerning some of the issues in this thread...
Pilots offer insights on Boeing 737 crashes - AOPA

Great find there.

He noted that the control yokes on models with the new technology have a lighter feel than on previous 737 models and can be touchy at high angles of attack. “The idea of the new system is, if it trims a little down, you’ll pull the same [force] as in the old airplanes.” In previous versions of the 737, “When you pull five pounds of force on the yoke, you get five degrees of pitch change, and when you pull 10 pounds, you get 10 degrees” of pitch change.

However, “On the MAX, it only takes about a 10-pound pull to get 15 degrees of pitch” because the aircraft responds quicker to input.
.....
In the air, the 737 MAX “flew just like a Boeing. I like to hand-fly it myself and they did a pretty good job of the control input and on response. It didn’t feel much different” than other 737s.
......
the technology runs the stabilizer pitch down for several seconds and it “reassesses and will start again until it believes the airplane has reached a safe angle of attack, and it operates without the pilots knowing [about it]

so we have a plane that's "hotter" to the touch , but i find in implausible that experienced pilots would get into a PIO.
Though one has to wonder - both incidents were shortly after takeoff . Unresolved item in my alleged brain, speculation that i'll have to resolve for myself by further reading.
A friend of mine's son flies those things, he says he's had two surprises from the new ones but they were not problematic. When i next see him i will get myself edified.

But we do have a system that is capable of getting into a tug-o-war with a pilot
pilot commands nose up with elevator, MCAS commands nose down with horizontal stabilizer, pilot adds more up elevator, MCAS adds more down stabilizer
and they wind up with the two control surfaces generating large opposing forces , consistent with reports pilots were pulling the yoke with all their might.

Could it progress to the point elevator is in stabilizer's wake and loses effect?
that's a question not an assertion

Would that wake effect get worse with more airspeed ? As they found when WW2 fighter planes got close to mach 1?

I read that MCAS can only add ten degrees of down stabilizer and that's what i attempted to sketch
surely that's not enough to change tail surface from a downward to upward net force ? Bleriot proved before WW1 that a lifting tail is dangerous,

I don’t recommend flying the Bleriot any higher than you are willing to jump!

sorry for the speculation - but speculating is part of troubleshooting.
Usually somebody with more knowledge can dispense with the useless speculation
if above is just plain silly please advise and i'll delete it.

old jim

 

[cyboman]

EDIT i see I'm late to the MCAS discussion . @ PeterDonis has it right . Maybe someone will find these few words useful so i'll leave them..

Indeed.
My present understanding is
its purpose is to correct for aerodynamic lift of the new engine nacelles, not the engines' greater thrust.
Being further forward from center of lift they produce more nose up torque as AOA increases.
MCAS applies a little bit of nose down trim so that in a climb the force pilot feels on the yoke is same as with the old engines.
That's why it's supposed to be inactive when autopilot is ON -Otto doesn't need it..
The concept is "control force feedback" see https://www.aopa.org/news-and-media...light-training-magazine/flight-control-forces
source: a comment i saw at PPRUNE , so it's secondhand . Use with caution.

old jim

Again I'm uncomfortable with the summation of MCAS being a force feedback mechanism or primarily a "control force feedback" system. It effects stick forces but it's primary purpose was to fix the unstable aerodynamics and default pitch up attitude caused by the new engines. Summarizing it as force feedback implies it does nothing to the flight characteristics but provide feedback to the pilot via column forces. That is not the case.

The system pitches the nose down using adjustment of the horizontal stabilizer, this in turn provides the pilot the column force to know that he is pulling too close to the stall margin. But the system IS pitching the nose down.

Here is a comment from here: https://airfactsjournal.com/2019/03/can-boeing-trust-pilots/

“Artificial feel” trim has been present in 737s since ages. MCAS is a new, additional module not for stick feel, but to add envelope protection at low speed and high AOA. It was required due to the aerodynamic instability caused by the larger, more forward engine nacelles.

MCAS pushes the nose down to reduce the risk of stalling:

 

[cyboman]

But we do have a system that is capable of getting into a tug-o-war with a pilot

I think this is a very concise way of describing the entire situation.

 

[PeterDonis]

I'm understanding that the trim basically functions to assist the pilot by adjusting the aerodynamics such that the pilot won't need to continually be applying input on the column using elevator control to keep the pitch neutral.

No, not to keep the pitch neutral; to keep the pitch wherever it needs to be to maintain the desired angle of attack.

the trim seeks to eliminate any control pressure. I imagine this makes maneuverability better and maintains control. And eliminates the need for the pilot to continually input force on the column to maintain a constant attitude.

Exactly.

My issue with your explanation is that it seems to imply trimming is fundamentally a force feedback mechanism.

It's fundamentally for the purpose you gave in the previous quote just above, to which I just responded "Exactly". Whether you want to call this a "force feedback mechanism" is a matter of choice of words. The point is that its primary purpose is not to "move the nose". Its primary purpose is to keep the nose where the pilot has already put it, without the pilot having to continually maintain pressure on the yoke.

By changing the position of the horizontal stabilizer (nose trim) you're actually changing the lift vector of the tail down force coming from the horizontal stabilizer. The way I see it this is changing the forces required to alter the pitch via the column / elevator control but it is not a force feedback mechanism, it's due to the changes in the rear horizontal stabilizer airfoil.

The distinction you're making here is invalid. You're confusing different levels of description. Saying that the trim system is a "force feedback mechanism", or more precisely that it "seeks to eliminate any control pressure" once the desired pitch attitude/angle of attack is achieved, is a high-level description of the purpose and function of the trim system. Saying that the trim system changes the rear horizontal stabilizer airfoil properties is a low-level description of how the system achieves its function. Both are true; it's not a question of one vs. the other. But you can't eliminate either one or the other and still have a proper understanding of what the system is doing.

Compare, for example, with this description of the yoke: The purpose or function of the yoke is to allow the pilot to command a particular pitch attitude (and also roll rate, but we're focusing on pitch here, i.e., pulling or pushing the yoke, not turning it). The way the yoke achieves this is...by changing the rear horizontal stabilizer airfoil. So we have two systems that both change the rear horizontal stabilizer airfoil; so you can't properly understand either one by just saying "it changes the rear horizontal stabilizer airfoil". You have to understand why each one is doing that. The two systems are doing it for different purposes, so it's the purpose that distinguishes the two systems, not the physical action they are performing.

Let's imagine a scenario: The yoke is at neutral and the trim is at zero or no trim and the plane is cruising and stable. If you were to apply maximum trim down, it's not just going to effect column forces. The plane is going to start pitching down.

If you don't do anything with the yoke to keep it at neutral, yes. But you can stop the pitching down by applying force to the yoke. And because the purpose of the yoke is to command a particular pitch attitude, whereas the purpose of the trim system is not, any actual pilot, if he wants to pitch the nose down, is going to push on the yoke, not apply nose down trim. The nose down trim will come later, if the pilot decides he wants to maintain a different pitch attitude/angle of attack for an extended period of time.

Force feedback usually describes a mechanism that simulates real world forces like in fly-by-wire systems.

It also describes mechanisms for changing the forces felt by pilots in non-fly-by-wire systems. Trim adjustments are just one such mechanism. Several sources state that the reason Boeing put MCAS on the 737 MAX was to make the force feedback similar to older 737s by automatic trim adjustments. So I don't think you can make a blanket statement that "trim is not a force feedback mechanism".

trim is applying a force vector on the plane itself not on the yoke

This makes no sense; changing the trim will change the force the pilot feels on the yoke. For example, in your scenario where the yoke is at neutral and trim is zero, if nose down trim is applied, there will be a force on the yoke that pushes it forward. The pilot will have to apply a counter force, pulling back on the yoke, if he wants to keep it at neutral. There is no way to apply a force vector to the plane itself through the horizontal stabilizer without applying it to the yoke, because the yoke is mechanically connected to the horizontal stabilizer. And of course the yoke also applies a force vector to the plane itself, through the horizontal stabilizer.

So the force feedback the pilot feels on the column, as I see it, is a product of the changed aerodynamics of the horizontal stabilizer. Not the other way around.

Who ever said it was the other way around? Nobody has claimed that the force feedback the pilot feels on the yoke causes the horizontal stabilizer to move.

simply because MCAS adjusts trim via the a horizontal stabilizer, I would contend it doesn't make it less dangerous or flight critical than if it took the yoke control and adjusted only the elevators of the stabilizer.

Nobody has claimed otherwise. Certainly nobody has claimed that MCAS is not dangerous or flight critical because it only adjusts the trim system. I don't know who you are trying to argue with here.

It effects stick forces but it's primary purpose was to fix the unstable aerodynamics and default pitch up attitude caused by the new engines.

"Default pitch up attitude" is a bit misleading. There is a pitch up moment that varies with angle of attack and airspeed; it is greater at higher angles of attack and higher airspeeds. In other words, at higher angle of attack and airspeed, it takes less force on the yoke to pitch the nose up and more force on the yoke to pitch the nose down. MCAS adjusts this by adding nose down trim. In the absence of any yoke force from the pilot, this will cause the nose to pitch down; but since MCAS is only active in manual flight, it is never going to be the case that the pilot will be applying no yoke force at high angle of attack and airspeed (that would only happen if, for example, the plane was making a climb to altitude after takeoff on autopilot, where MCAS would not be operating). So under normal operation, the effect of MCAS is not going to be to pitch the nose down: it's going to be to increase the yoke force required to keep pitching the nose up, under circumstances where the pilot is already applying yoke force to pitch the nose up. That makes the force feedback to the pilot more like that of previous 737s, so the pilot can properly judge from the yoke force feedback how close the plane is to a stall.

I highlighted "under normal operation" to emphasize, once again, that in all of these incidents, MCAS was not operating normally; it used faulty AoA sensor input to introduce large and repeated nose down trim adjustments when it was not correct to do so. Under those conditions, yes, MCAS will cause the nose to pitch down, and can, as @jim hardy commented (and you agreed), get into a "tug of war" with the pilot. But the "tug of war" is caused by faulty sensor data, not by the mere fact of MCAS making trim adjustments.

Summarizing it as force feedback implies it does nothing to the flight characteristics but provide feedback to the pilot via column forces. That is not the case.

This statement is correct. It would be possible to introduce force feedback that was not associated with any change in the aerodynamics of the plane, but that is not what MCAS, or more generally the 737 trim system, does.

 

[jim hardy]

still some dots left unconnected for me.

A pitch augmentation control law (MCAS) was implemented on the 737 MAX to improve aircraft handling characteristics and decrease pitch-up tendency at elevated angles of attack. It was put through flight testing as part of the certification process prior to the airplane entering service. MCAS does not control the airplane in normal flight; it improves the behavior of the airplane in a non-normal part of the operating envelope.

and i question this statement

Pilot has to deactivate MACS or satisfy it that plane is where MACS wants it, else they''ll get in a tug of war.

I can wait. These disasters are always like a row of dominoes stacked up . Done enough troubleshooting to know that the apparent conflicts don't resolve until you arrive at complete understanding.

 

[cyboman]

If you don't do anything with the yoke to keep it at neutral, yes. But you can stop the pitching down by applying force to the yoke. And because the purpose of the yoke is to command a particular pitch attitude, whereas the purpose of the trim system is not, any actual pilot, if he wants to pitch the nose down, is going to push on the yoke, not apply nose down trim. The nose down trim will come later, if the pilot decides he wants to maintain a different pitch attitude/angle of attack for an extended period of time.

You've stated before consistently MCAS does not pitch the nose of the aircraft down. I completely disagree. It adjusts trim, which is not primarily used by the pilot to change pitch, but it does effect the final pitch. As in my example, you've agreed, yes adding pitch down trim will push the nose of the aircraft down. I was not in anyway suggesting using trim is standard procedure to effect pitch attitude, not sure where you got that. I'm pointing out that trim can and does change the aerodynamics of the plane and effects pitch and can push the nose down. It has nothing whatsoever to do with pilots yoke control. If MCAS fails and sends repeating nose down trim commands and the pilot doesn't touch the yoke, the plane is going to pitch down. I feel you're finding some odd semantic way out of that argument, like "ya but the pilot should counteract with elevator control on the yoke", sure but that doesn't change that trim is pushing the nose down. It's very possible and I'd argue likely in the Lion Air situation that the repeated MCAS commands could add so much nose down trim that the column forces would be impossible to counteract.

Again, I'm asserting MCAS and trim adjustments on the stab DO pitch the nose down. You deny this for some reason.

This makes no sense; changing the trim will change the force the pilot feels on the yoke. For example, in your scenario where the yoke is at neutral and trim is zero, if nose down trim is applied, there will be a force on the yoke that pushes it forward. The pilot will have to apply a counter force, pulling back on the yoke, if he wants to keep it at neutral. There is no way to apply a force vector to the plane itself through the horizontal stabilizer without applying it to the yoke, because the yoke is mechanically connected to the horizontal stabilizer. And of course the yoke also applies a force vector to the plane itself, through the horizontal stabilizer.

No it does make sense. Changing the trim works in this order: it changes the trim, the horizontal stabilizer airfoil is changed, the aerodynamics of the plane have changed and the lift vector on the tail, the yoke feels these changes. There is no primary direct pathway from the trim to the yoke first. The trim operates on the trim. I don't know how you you see it as the trim operates on the yoke. Especially in our example of a 747 (perhaps it's different in a simpler craft), the yoke is hydraulic for one so I doubt MCAS is directly effecting the yoke in some direct pathway before the horizontal stab. Two, the yoke controls the elevator, NOT the entire horizontal stabilizer, which MCAS does.

Who ever said it was the other way around? Nobody has claimed that the force feedback the pilot feels on the yoke causes the horizontal stabilizer to move.

This was in response to when you stated in response to my previous post where you said I had it backwards.

This statement is correct. It would be possible to introduce force feedback that was not associated with any change in the aerodynamics of the plane, but that is not what MCAS, or more generally the 737 trim system, does.

You seem to be contradicting yourself here. If I'm not mistaken you've stated clearly that you believe that MCAS does not pitch the nose of the plane down and that it only effects column forces and if the pilot does not counter them with the yoke then it will pitch down. I've provided pretty ample evidence that the primary role of MCAS is to literally pitch the nose down via horizontal stab movement.

 

[cyboman]

still some dots left unconnected for me.
and i question this statement
View attachment 240372

Pilot has to deactivate MACS or satisfy it that plane is where MACS wants it, else they''ll get in a tug of war.

I can wait. These disasters are always like a row of dominoes stacked up . Done enough troubleshooting to know that the apparent conflicts don't resolve until you arrive at complete understanding.

From my understanding, adjusting the trim manually does disable MCAS. But it can reengage after 5 seconds or so if all the same logic like AoA and airspeed are true. This is what's likely causing massive confusion for the pilots. To really disable it apparently, I've pointed out in a previous post, you actually need to correct the trim electrically first, then disconnect it using cutouts, then use manual trim for the rest of the flight. And then in a post I pointed out a pilot commenting that disabling those cutouts is what a pilot would do first in a nanosecond and that they don't disable MCAS. So it's confusing. It paints a picture of a scenario where MCAS keeps reengaging every 5 seconds and the pilot cannot overcome the nose down attitude. Speed is increasing so column forces are increasing, altitude is dropping, tons of alarms would be sounding and then you need to follow this flow chart of memory items to disable the MCAS. All during a flight critical phase of take off with little altitude to work with. It would be a terrible situation to be in as a pilot.

 

[PeterDonis]

You've stated before consistently MCAS does not pitch the nose of the aircraft down.

I have stated that the purpose of MCAS is not to pitch the nose down. It's to adjust the trim. I have never said that there are no circumstances under which adding nose down trim will cause the nose to pitch down. Of course there are. But adjusting the airplane's pitch attitude is not the primary purpose of the trim system. That's what the yoke is for. I have already explained in detail why I say that.

I am not disagreeing with you about the aerodynamics of the plane. I am emphasizing different aspects of the overall problem than you are.

Changing the trim works in this order: it changes the trim, the horizontal stabilizer airfoil is changed, the aerodynamics of the plane have changed and the lift vector on the tail, the yoke feels these changes.

Yes, that's basically the order of events. However, you are leaving out a key aspect: how fast will the change in the horizontal stabilizer airfoil actually change the airplane's pitch attitude, in the absence of any input from the pilot on the yoke? And how does that time frame compare with how fast the pilot will feel the increased force on the yoke and adjust his own force to compensate if he wants to? Remember that, under normal operation, MCAS will only be adding nose down trim if the airplane is actually at a high angle of attack or airspeed while in manual flight, and that will only happen if the pilot is already pulling back on the yoke to pitch the nose up.

From what I can gather, the answer to the question I just posed is that the pilot will be able to increase his pulling force on the yoke fast enough to keep the additional nose down trim from the MCAS from actually affecting the airplane's pitch attitude. So under those circumstances, which are what would be expected under normal operation, the MCAS will not cause the nose to pitch down. Instead, what will happen is simply that the pilot will feel more feedback force on the yoke. For example, that's what is described in the article OCR linked to on "pilots offer insights"; a test pilot is quoted there describing the MCAS entirely in terms of how it made the airplane feel in flight--that it made it feel like other 737s.

 

[cyboman]

I have stated that the purpose of MCAS is not to pitch the nose down. It's to adjust the trim. I have never said that there are no circumstances under which adding nose down trim will cause the nose to pitch down. Of course there are. But adjusting the airplane's pitch attitude is not the primary purpose of the trim system. That's what the yoke is for. I have already explained in detail why I say that.

I am not disagreeing with you about the aerodynamics of the plane. I am emphasizing different aspects of the overall problem than you are.
Yes, that's basically the order of events. However, you are leaving out a key aspect: how fast will the change in the horizontal stabilizer airfoil actually change the airplane's pitch attitude, in the absence of any input from the pilot on the yoke? And how does that time frame compare with how fast the pilot will feel the increased force on the yoke and adjust his own force to compensate if he wants to? Remember that, under normal operation, MCAS will only be adding nose down trim if the airplane is actually at a high angle of attack or airspeed while in manual flight, and that will only happen if the pilot is already pulling back on the yoke to pitch the nose up.

From what I can gather, the answer to the question I just posed is that the pilot will be able to increase his pulling force on the yoke fast enough to keep the additional nose down trim from the MCAS from actually affecting the airplane's pitch attitude. So under those circumstances, which are what would be expected under normal operation, the MCAS will not cause the nose to pitch down. Instead, what will happen is simply that the pilot will feel more feedback force on the yoke. For example, that's what is described in the article OCR linked to on "pilots offer insights"; a test pilot is quoted there describing the MCAS entirely in terms of how it made the airplane feel in flight--that it made it feel like other 737s.

I would have to go through your previous posts but from what I could gather you made it clear that MCAS does not cause the aircraft to pitch down. That through some secondary effect this can happen. Also you say the purpose is not to pitch the nose down. It's to adjust trim. Huh? It adjusts trim TO pitch the nose down. Not pitch the nose up or keep it neutral or provide stick feel. It says exactly that in the graph I pasted. It provides pitch down commands to counteract the pitch up effect of the engines. Sigh...