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

In summary, the MCAS system was not the cause of the crash and it is possible for the plane to fly without the system if the angle of attack sensor is not working correctly. However, the plane is more likely to stall if the angle of attack sensor is not working correctly and the pilots need to manually fly the plane back to correct pitch attitude.
  • #211
cyboman said:
So you ban people with different opinions here?

You said you wanted us to agree to disagree. That means we stop talking about the points on which we disagree, and focus on the points on which we agree--which should be quite sufficient to discuss the main topic of this thread. It has nothing to do with banning, and I didn't mention banning, so I don't see why you bring it up.
 
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  • #212
Think feedback . seen 185 ?
 
  • #213
jim hardy said:
https://www.bbc.com/news/world-africa-47553174 red marks mine jh
https://www.physicsforums.com/attachments/240391
That would of been a terrifying ride.
PeterDonis said:
You said you wanted us to agree to disagree. That means we stop talking about the points on which we disagree, and focus on the points on which we agree--which should be quite sufficient to discuss the main topic of this thread. It has nothing to do with banning, and I didn't mention banning, so I don't see why you bring it up.

Well because you used the qualifier "If" and you're staff. I'm not sure what other recourse you'd be suggesting.
 
  • #214
cyboman said:
So you ban people with different opinions here?
Well... it's been known to happen... kinda depends on how obnoxious they are.
 
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  • #215
Tom.G said:
Well... it's been known to happen... kinda depends on how obnoxious they are.

Well, I'd hope I'm not perceived as being obnoxious. That's why I suggested agreeing to disagree. I'd like to think I've added some valuable points to the thread.
 
  • #217
cyboman said:
I'd like to think I've added some valuable points to the thread.

You have. That's not at issue.

cyboman said:
That's why I suggested agreeing to disagree.

Yes, and I was trying to clarify what that means.
 
  • #218
PeterDonis said:
You have. That's not at issue.
Yes, and I was trying to clarify what that means.

I was perhaps being oversensitive then. I apologize.
 
  • #219
cyboman said:
In this case, when pulling out of a dive, if operating correctly, would the MCAS system be applying nose down trim?

Yes, it could, because, remember, angle of attack is the angle between the wing and the relative wind. If you're diving with the nose down and you pull back on the yoke to bring the nose level, the plane is still descending, so even though pitch attitude is zero or close to it, angle of attack is still high, because the relative wind is not coming from straight ahead of the plane, it's coming from below the plane. So under those conditions, MCAS could indeed be adding nose down trim, if the descent rate was high enough. If the AoA sensors are operating correctly, MCAS should stop doing that once the plane stops descending--or at least when the descent rate drops low enough to put angle of attack below the threshold when MCAS starts adding nose down trim.
 
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  • #220
cyboman said:
I was perhaps being oversensitive then. I apologize.

No worries.
 
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  • #221
PeterDonis said:
Yes, it could, because, remember, angle of attack is the angle between the wing and the relative wind. If you're diving with the nose down and you pull back on the yoke to bring the nose level, the plane is still descending, so even though pitch attitude is zero or close to it, angle of attack is still high, because the relative wind is not coming from straight ahead of the plane, it's coming from below the plane. So under those conditions, MCAS could indeed be adding nose down trim, if the descent rate was high enough. If the AoA sensors are operating correctly, MCAS should stop doing that once the plane stops descending--or at least when the descent rate drops low enough to put angle of attack below the threshold when MCAS starts adding nose down trim.

OK, I think I follow your explanation of the AoA being high even with the attitude close to zero due to the decent. But why if the plane is descending, and the pilot is trying to essentially pitch up during the recovery of the dive would the pilot want nose down trim?
 
  • #222
jim hardy said:
Yes Torques and moments are the terms we'll hear.View attachment 240387

You might think of that nacelle as giving "Positive Feedback" when changing pitch.
A bit of Nose Up input to the yoke makes the nose rise,
and that small nose up causes upward force on the nacelle which unopposed will rotate the plane even farther nose up.

That's what positive feedback does, it increases the gain of a closed loop.
It also changes its time response, a small change will keep on growing for some time**(see below)

So a pilot accustomed to the old plane takes off in a new one,
pulls on yoke with force that's always given him 10 degrees and gets maybe twenty and it's still increasing ,
were there no passengers he might exclaim YEE HAW what a ride !.

More likely he'll push the yoke back and if he overshoots level that could start an oscillation.
Honestly that's what i first thought when i saw those vertical speed graphs, PIO , but as Ernie Gann says it's too easy to blame the pilot.

So Boeing came up with MCAS to undo the positive feedback from nacelle lift and prevent that scenario.
they created some negative feedback via MCAS
A bit of nose down down from stabilizer will surely cancel out the nacelle lift, it's the right direction for negative feedback
If done well it should prevent that extra nose-up from ever happening in the first place,

as an old controls guy i would be worried about the relative speeds at which yoke input moves the elevator and trim motor moves the stabilizer.
Delay between an offsetting force and a restoring force makes a system prone to oscillate
the offsetting force from pitch, nacelle lift, is immediate,
so the time between yoke and offsetting force is just the plane's rate of change pf pitch
while the restoring force, stabilizer trim doesn't begin until the plane has already pitched and the trim motor has moved the stabilizer.
So restoring force is delayed by at least trim motor speed
furthermore, restoring force is not linear but comes in bursts every few seconds
and what is the pilot apt to do in those few seconds when his plane's controls feel haywire? .

I have to believe all that was considered and analyzed by experts , and I'm no expert by any means
so that's why i hold all these questions open in my mind - facts will connect the dots in time.
** another boring anecdote i had a parallel situation back in the early 1980's.
We were asked to change a setting in our voltage regulators that seemed simple enough. Var compensation.
The change turned what had been a small amount of negative feedback into a small amount of positive feedback.
It sure seemed innocuous so we did it without fanfare, but did stand by in the control room the first time operators put the unit online with the new setting.

Well !
The operator, a distinguished old timer about sixty years of age, closed the breaker , admitted steam to pick up megawatts - all was smooth.
then he switched on the voltage regulator, gave it a tweak to pick up some megavars , switched it back off jumped back and exclaimed "What have you guys done to my voltage regulator ? That tweak should have been twenty megavars, i got fifty and still climbing! So i switched it back off."

I realized immediately what had happened. Changing negative feedback into positive not only changed the gain of the closed loop it extended its time response several fold.
I hadn't thought to warn him he'd see either one of those effects.

So what could i do ?
Being twenty years his junior , all i could do was say out loud in front of everybody : "I owe you an apology, Sir. The regulator did just what it should do.
But my head was buried so far in the equations i never once thought how viscerally different your machine is going to act with these new settings.
When you tweak that regulator knob, what used to give you just a pinch of megavars will now give you a handful of them and it'll take several seconds for them to settle at the new value.
I humbly apologize to you right here for not thinking of that beforehand. It was my oversight.
If you trust me let's try it again ."

He nodded and said "Okay son we'll give 'er another try. But you'd better not trip my unit."
He "gave 'er another try" and everything was fine. He spent several minutes at the knob getting a feel for the new response.
Then he turned to me, smiled, and said "That seems okay, Son. But it was sure a surprise the first time."

Still i got to explain to the plant manager how we'd changed something on his machine and not apprised his operators as to the expected effect on their indications.

So that's why i am perhaps oversensitive to changing things inside a closed loop that involves a human. I ate crow that day.

If indeed MCAS is involved in these crashes, somebody else just learned the same lesson i did all those years ago.
Purpose of this digression is to encourage thinking how a closed loop operates, and its extreme sensitivity to positive feedback.I used no math - there's plenty of tutorials out there.

if this is just clutter please advise and i'll delete.old jim

This is in no way clutter. It's really fascinating and it illuminates another perspective of understanding MCAS that is entirely relevant. Control theory is definitely integral to this discussion. I need to read this post more carefully. And your personal story is really neat. I think it illustrates the bridge between design / implementation phase and real world testing and how robust the latter really needs to be in systems like these.
 
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  • #223
cyboman said:
But why if the plane is descending, and the pilot is trying to essentially pitch up during the recovery of the dive would the pilot want nose down trim?

He wouldn't. But the MCAS might input it anyway. We would have to know the precise control parameters for MCAS to know under what dive conditions this could happen.
 
  • #224
I wonder if it'd be a fun exercise to create our own flight laws or code for a hypothetical MCAS function. We could sort of put together some of the points we've suggested so far. This could take the form of pseudo code or even just plain english recommendations to changes to the MCAS and other systems or overall flight systems or just high level recommendations given what we know. Or both. It's largely speculative at this point but it may be a neat exercise just for fun. Since this thread has gotten fairly long, it'd be a neat way to sort of pool our findings and insights.
 
  • #225
PeterDonis said:
He wouldn't. But the MCAS might input it anyway. We would have to know the precise control parameters for MCAS to know under what dive conditions this could happen.

Right, so it's likely MCAS hopefully would be taking into account airspeed and altitude and this would detect it's in a dive or not and still produce nose down trim commands? Perhaps it's capturing other data like attitude. It really would be nice to know more about the details of this system. Do you think Boeing will release that type of detail in light of the investigations? It seems they may not be obligated to do so. From the varied sources we see online, it looks like it primarily looks at AoA, but I've seen at other places stating it reads altitude and airspeed as well, which makes sense.
 
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  • #226
What's interesting (and terrifying) is that from what I'm hearing, in the event MCAS (allegedly) results in full trim down on the stab, the force needed to keep the aircraft level via the elevator would be impossible. That is, the pilot would not be able to withstand full nose down stab trim. And apparently the breaking news finding with Ethiopia is they recovered the jackscrew from the flight, and it was in full nose down trim position.
 
  • #227
cyboman said:
it's likely MCAS hopefully would be taking into account airspeed and altitude and this would detect it's in a dive or not and still produce nose down trim commands? Perhaps it's capturing other data like attitude.

Yes, all of this is possible.

cyboman said:
It really would be nice to know more about the details of this system. Do you think Boeing will release that type of detail in light of the investigations?

I don't know if Boeing will, but I would expect the final investigation reports to include whatever information about the workings of MCAS is relevant to the incident.
 
  • #228
PeterDonis said:
Yes, all of this is possible.
I don't know if Boeing will, but I would expect the final investigation reports to include whatever information about the workings of MCAS is relevant to the incident.
I was reading on a pilots forum (*note, as much as we can trust it was actually a pilot on the internet), one pilot suggested that MCAS itself was problematic and that they should of never implemented a system that manipulates the stab in order to maintain flight characteristics similar to previous models. He suggested that they should of redesigned the entire horizontal stabilizer itself. He was suggesting that MCAS was sort of a cost efficient way to deal with the pitch up force of the engines and the changed aerodynamics. What are your thoughts on this?

Further, interestingly along the lines of how you view the MCAS system, another pilot suggested that the feedback mechanisms could of actually been incorporated into the elevator feedback system. But that system is not tied into the FCC. So it was easier to create a piece of software (MCAS) that would manipulate the stab instead of producing that force feedback data within the elevator feedback system which I think wouldn't have access to AoA from the FCC.

EDIT: By feedback mechanisms here, it should be qualified: The feedback required to communicate to the pilots the change in aerodynamics due to the engines and changes in the MAX
 
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  • #229
cyboman said:
I was reading on a pilots forum

Can you give a link?

cyboman said:
He was suggesting that MCAS was sort of a cost efficient way to deal with the pitch up force of the engines and the changed aerodynamics. What are your thoughts on this?

I think cost efficiency was very likely a key driver behind MCAS, yes. A redesign of the entire horizontal stabilizer would have been a lot more expensive, and might well have been more risky from a certification point of view. Being able to keep the 737 MAX under the existing 737 certification was evidently another key driver.
 
  • #230
PeterDonis said:
Can you give a link?
I think cost efficiency was very likely a key driver behind MCAS, yes. A redesign of the entire horizontal stabilizer would have been a lot more expensive, and might well have been more risky from a certification point of view. Being able to keep the 737 MAX under the existing 737 certification was evidently another key driver.

It's from this site: https://www.pprune.org. I think there are multiple threads. But if you do a search like: site: https://www.pprune.org +horizontal stabilizer or +MCAS you might find it. The threads are huge and I'm sorry I can't be more specific I have a ton of tabs open. But I did read what I alluded to somewhere in that thread.

Yes I agree certification does seem to be a big part of this story.
 
  • #231
To further clarify, from what I remember reading, the elevator force feedback system is not tied into the FCC which has access to the AoA sensor data.
 
  • #232
OK I found one of the references:
True. But the Airbus planes are stable throughout their flying envelope. The Max alas is not. The FAA should have never allowed a software fix as a remedy for a basic design flaw. And there must have been engineers at Boeing who were very upset of having been overruled by bean counters and MBA types. Admittedly the MCAS as it is today is a remedy (to the symptoms) worse than the disease. But there should never have been an MCAS in the first place, the right thing to do was to redesign the horizontal stabilizer.

here: https://www.pprune.org/rumours-news/619272-ethiopian-airliner-down-africa-46.html
 
  • #233
cyboman said:
I found one of the references

Ok, got it. I find it interesting that he says "the Airbus planes are stable throughout their flying envelope". As I understand it, this "stability" is due to a much greater use of automated computer functions to compensate for things.
 
  • #234
I've tried to find the post regarding the elevator force feed
PeterDonis said:
Ok, got it. I find it interesting that he says "the Airbus planes are stable throughout their flying envelope". As I understand it, this "stability" is due to a much greater use of automated computer functions to compensate for things.
That's true, but the interface between pilot and machine is the crux here. More automation isn't the clear answer. It's the difficult realm that is the ether between human control and machine control that is the issue and requires some aggressive engineering and ingenuity.
 
  • #235
cyboman said:
the interface between pilot and machine is the crux here

Yes, agreed. And from some past Airbus incidents (AF 447, Quantas 71/72) it appears that there are issues with that interface on Airbus aircraft as well.
 
  • #236
PeterDonis said:
Yes, agreed. And from some past Airbus incidents (AF 447, Quantas 71/72) it appears that there are issues with that interface on Airbus aircraft as well.
Yes, and perhaps instead of pushing the possibility of autonomous cars, we should be focusing on how computer human machine interaction is challenging in aviation and further our need of refinement of airliner control systems and the human-machine interface. It seems we may be out ahead of our skis in this respect.
 
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  • #237
I missed your post when Peter and I were going back and forth yesterday. My intention isn't to revive the disagreement but I will reply to your post.

russ_watters said:
No. The short version of the difference is:
1. MCAS applies force against the pilot's input. To override it, you simply pull harder.
2. Stall prevention completely prevents you from exceeding a certain aoa. It will not let you increase the aoa, no matter how hard you pull.

1) I don't share your or Peter's view: My view is MCAS doesn't apply any force to the yoke directly, that's a flawed way of looking at it. MCAS commands the horizontal stab to rotate to a nose down position. This changes the lift force on that airfoil and that effects the pitch attitude. Those trim changes will subsequently change column forces on the yoke. But it's not accurate imo, to see MCAS or even trim for that matter as fundamentally applying force to the yoke. Trim controls speed and attitude of the aircraft. I understand from a flying point of view, it's used for "trimming" and managing yoke forces. Consider that trim maintains constant attitude by causing negative and positive pitch corrections all through flight (on the 737).

During flight on the 737, the trim is constantly being adjusted by the computer (not MCAS in this example). You can see the trim wheels spinning in videos. Mostly because of the changing CoG and center of pressure due to moving occupants and the variable airspeeds. These changes in moment require different force from the stab. The computer is working to keep attitude constant by adjusting the stab. This could and does happen with the pilot exerting absolutely no force on the yoke. Again, the stab effects pitch attitude. Therefore, MCAS effects pitch attitude.

Another example to help support my point:

On military aircraft during wartime, trim tabs often served as unintentional backup control systems for aircraft with damaged controls. Since trim tabs are usually controlled by their own dedicated system of control cables, rods, and/or hydraulic lines, aircraft that had suffered loss of primary controls could often be flown home "on the trim tabs", or by using trim adjustment as a replacement for the non-working primary controls. Such control is effective, if slower and more limited than primary controls, but it does allow the aircraft to be controlled and directed.
https://en.wikipedia.org/wiki/Trim_tab#Uses in aircraft

If trim was used as a back up to control pitch in military aircraft. I would say it's safe to say it effects pitch attitude.

2) I was watching a video of a pilot explaining MCAS. He actually takes the view that it is basically a stall prevention system, which is fundamentally trying to get the nose down. The larger engines change the stall characteristics of the plane. The yoke only controls the elevators, which may not be enough, so the stab is needed to increase pitch attitude trim.

Consider this, if the fin, or the vertical stab could rotate in addition to it's rudder, don't you think that would effect yaw attitude? It's all the same control surface. The elevator is just a rudder of the stab airfoil.

russ_watters said:
With MCAS: You pull up from 5 to 7 degrees, with 5lb of force. The MCAS system adds trim to ensure you still feel backpressure. Now you pull up from 7 to 9 degrees. The MCAS adds more trim so it takes 10lb of force. You pull up further; the MCAS adds more trim so now it takes 15lb. In no case does the nose drop because of normal MCAS system operation.

russ_watters said:
Let me say it another way: if the nose drops due to normal MCAS operation, it is being too aggressive.

If the nose drops during MCAS operation and prior the plane was pitching up and about to stall I would say MCAS is working exactly as described, not aggressive. This would be normal operation.
 
  • #238
cyboman said:
But why if the plane is descending, and the pilot is trying to essentially pitch up during the recovery of the dive would the pilot want nose down trim?
PeterDonis said:
He wouldn't.
Yes you would. This scenario is the whole reason this MCAS behavior exists!

The 737Max doesn't feel like a normal plane. A normal plane, if you try to pull up hard, will fight you. A normal plane doesn't want to go nose up - doesn't want to stall - and the more you pull on the yoke the harder it gets. The 737Max is the opposite: initially it takes more force to pull up, and then when you reach a certain point, the force required decreases and eventually reverses: you have to push forward to keep the nose from rising more. That reversal of behavior makes it more likely the pilot will unintentionally keep raising the aoa and stall the plane.

The fact that in your scenario the pitch angle is below the horizon makes no difference whatsoever in the desirability of the behavior. Just because the nose is pointed down below the horizon doesn't mean you can't still stall.
cyboman said:
1) I don't share your or Peter's view: My view is MCAS doesn't apply any force to the yoke directly, that's a flawed way of looking at it. MCAS commands the horizontal stab to rotate to a nose down position.
I didn't say the MCAS applies force to the yoke. That's the effect, not the direct action. The direct action is adjusting the trim, the same as if the pilot manually rotated the trim wheel. Applying (changing) force to the yoke happens due to the change in aerodynamics. Changing the force is the desired result of adjusting the trim, but changing a control surface position is the direct action.
Therefore, MCAS effects pitch attitude.
You are really arguing this point harder than you need to. It isn't a critical point (or maybe its a different point...). It's a cart or horse, chicken or egg argument that doesn't matter. The point that matters is that unlike a stall prevention system, MCAS will not prevent you from continuing to raise the nose.
2) I was watching a video of a pilot explaining MCAS. He actually takes the view that it is basically a stall prevention system, which is fundamentally trying to get the nose down.
So again, the difference between "trying to get the nose down" and "helping you avoid letting the nose rise too high" is not very critical. The critical difference between the MCAS and a normal stall prevention system is the authority. You can still stall a plane with MCAS, it's just harder. But you can't stall a plane with a normal stall prevention system.
If the nose drops during MCAS operation and prior the plane was pitching up and about to stall I would say MCAS is working exactly as described, not aggressive. This would be normal operation.
I don't think that would happen.

Maybe the issue you are having here is that the MCAS doesn't activate in a steady-state situation.

Assume a pilot is applying a certain force to maintain a certain pitch attitude and AOA. Everything is steady-state. Then someone or something adds nose-down trim. The nose will drop if the pilot doesn't make any adjustments to his yoke force.

But that's not what happens with MCAS. With MCAS, the pilot is in the act of increasing his nose-up force when the MCAS kicks-in to oppose what he's doing. The nose should not drop in that situation, it should just rise less than if MCAS wasn't there.

Without MCAS, the pilot would pull back, the force would suddenly drop, and the pilot would have to reverse and push forward to keep the nose from rising more than he wanted it to.
 
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  • #239
On the news today, they are talking about AOA sensors again.

However, an intermittent loss of feedback of horizontal stabilizer position would seem to cause the same bad effects, regardless of MCAS/autopilot/trim/stick inputs.

The FDR data should resolve all these theories.
 
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  • #240
russ_watters said:
Just because the nose is pointed down below the horizon doesn't mean you can't still stall.

Yes, you're right, I was failing to follow my own advice and confusing pitch attitude with angle of attack.
 
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  • #241
According to this article in the Seattle Times [1], Boeing did a safety analysis on the 737 MAX that included several key flaws:

(1) The analysis assumed that MCAS could only move the horizontal stabilizers a limited amount; in fact it was capable of moving them about four times as much;

(2) The analysis failed to take into account that MCAS could make repeated trim adjustments if the pilots responded with trim adjustments of their own (which means that after repeated trim adjustments MCAS could have moved the stabilizers to the physical limit of their travel);

(3) The analysis assessed a failure of the system as "hazardous", but failed to notice that at that risk rating the system is not allowed to act based on input from a single sensor.

If these items are borne out by further investigation, this is very troubling news. The main open question appears to be whether this safety analysis is the final one that the FAA based their certification on, or whether there were other later ones prior to certification.

[1] https://www.seattletimes.com/business/boeing-aerospace/failed-certification-faa-missed-safety-issues-in-the-737-max-system-implicated-in-the-lion-air-crash/
 
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  • #242
PeterDonis said:
Yes, you're right, I was failing to follow my own advice and confusing pitch attitude with angle of attack.
Note also that we've been focusing on a one-axis pitch-up scenario. The issue also applies -perhaps even more - in a turn. Regardless of if the nose is high, low or level, increasing the angle of bank requires increasing the aoa in order to keep the nose from dropping. It's difficult because in a turn there is no way to know what your stall speed is to maintain level flught. You might think you are plenty fast, but then you start to turn and stall.

I recently saw a light plane crash report where the pilot was executing a missed approach and forgot to apply full power. She started a turn, retracted the flaps, and the plane stalled/spun.
 
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  • #243
russ_watters said:
Note also that we've been focusing on a one-axis pitch-up scenario. The issue also applies -perhaps even more - in a turn. Regardless of if the nose is high, low or level, increasing the angle of bank requires increasing the aoa in order to keep the nose from dropping. It's difficult because in a turn there is no way to know what your stall speed is to maintain level flught. You might think you are plenty fast, but then you start to turn and stall.

I recently saw a light plane crash report where the pilot was executing a missed approach and forgot to apply full power. She started a turn, retracted the flaps, and the plane stalled/spun.
BTW, I'm currently learning to fly in a plane without a functioning stall horn. This discussion has me questioning whether that's wise...
 
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  • #244
Never had one in the gliders I flew. At low speeds if everything went quiet it was time to worry. At high speeds you got buffeting.
 
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  • #245
russ_watters said:
I didn't say the MCAS applies force to the yoke. That's the effect, not the direct action. The direct action is adjusting the trim, the same as if the pilot manually rotated the trim wheel. Applying (changing) force to the yoke happens due to the change in aerodynamics. Changing the force is the desired result of adjusting the trim, but changing a control surface position is the direct action.

I've been asserting basically what you've said here from the beginning. I feel like there is a bit more agreement now however. The crux of our disagreement as I see it was me seeing MCAS as effecting attitude and not just effecting yoke force. So when you say "yoke force is the effect", I also see a change in pitch force from the change in trim an effect, that has an effect on pitch, that's where I think we differ in our viewpoint of the system. I was arguing against the idea that it only applies force feedback and it's only the pilot which effects pitch attitude. I've been saying MCAS effects attitude by changing the aerodynamics a lot, that's sort of central. There were times when I was refuted, by saying "thinking of it changing aerodynamics is not the right way".

I admit my wording, "MCAS adds negative pitch attitude" is confusing and imperfect - that's how I was visualizing it. I don't mean it always has the effect of causing a negative pitch attitude, as in below the horizon (I think I stated that somewhere), but instead that it adds negative pitch (it does that by creating a pitch down moment (thx @PeterDonis)). So the moments created by yoke input, the moment of the stab (commanded by MCAS), and all the other moments in the equation in sum create the result force that determines pitch attitude of the plane.

russ_watters said:
You are really arguing this point harder than you need to. It isn't a critical point (or maybe its a different point...). It's a cart or horse, chicken or egg argument that doesn't matter. The point that matters is that unlike a stall prevention system, MCAS will not prevent you from continuing to raise the nose.

This point was really the necleus of our disagreement the way I see it. It could be seen as unimportant I suppose. I guess the importance of the point is it stems from seeing the system as not ultimately having a direct effect on attitude. It seems to suggest MCAS doesn't have any pitch authority. Actually it has more than the pilot using only the yoke since it's moving the entire horizontal stab. That's likely why in the failures the pilots could not overcome the nose down pitch force from the stab.

russ_watters said:
So again, the difference between "trying to get the nose down" and "helping you avoid letting the nose rise too high" is not very critical. The critical difference between the MCAS and a normal stall prevention system is the authority. You can still stall a plane with MCAS, it's just harder. But you can't stall a plane with a normal stall prevention system.

Put that way, I think they are almost the same thing. Your wording, or the latter, again seems to suggest MCAS has no pitch authority.

russ_watters said:
But that's not what happens with MCAS. With MCAS, the pilot is in the act of increasing his nose-up force when the MCAS kicks-in to oppose what he's doing. The nose should not drop in that situation, it should just rise less than if MCAS wasn't there.

This is semantic I think. If the nose is rising less with the system activated, that means it's dropping in relation to where the nose would otherwise be. This is why almost everywhere it's described as "pushing the nose down". You can say "MCAS keeps the nose from rising" that's fine but our difference is you don't see MCAS as effecting pitch attitude. If it keeps the nose from rising, it's effecting pitch attitude. The view MCAS doesn't effect pitch, that it's only the pilot using elevators which does, isn't accurate because there is a pitch force from the trim (in our scenario aka MCAS).
 

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