Boeing 737 Max MCAS System

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I'm sorry, but the fact that you continue to insist on saying "increase negative pitch attitude" is what makes your statement plain wrong, instead of just being a different choice of wording. Your statement is equivalent to claiming that adding nose down trim increases negative pitch attitude. That's just wrong. I've already described why. In fact I did so in the very post (post #170) where I made the statement about what the job of MCAS was that you agreed with.

If you had said "MCAS compensates for the effect of the new engines by adding nose down trim", that would be fine and I would never have had a problem with it. But you cannot make the blanket statement that this increases negative pitch attitude. Adding nose down trim can increase negative pitch attitude, under the circumstances I've already described. But that does not mean it always does increase negative pitch attitude, which is what you are claiming. You cannot just equate "adds nose down trim" with "increases negative pitch attitude". That's wrong.
I'm not equating. I'm saying the desired effect of MCAS is to increase negative pitch attitude, because the engines are creating a positive pitch attitude vector. It does that by adding nose down trim. It seems pretty self evident to me.
 
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They certainly are now, yes. But I'm looking at what they said about MCAS before there were any incidents; see above. I think what they said then is good information about what the intent of MCAS was.
OK, but again it's just about how you get there. As I said:

If the primary purpose is to make it feel like earlier versions, that doesn't refute what I'm claiming. I'm claiming how it gets there, and it does that by nose down trim. That effects the forces on the column and the feel, but it effects the aerodynamics of the aircraft first, which however you want to describe it, acts to increase negative pitch attitude. It doesn't provide force feedback for the pilot alone as it's function. The reason it doesn't feel like earlier versions is the engines (mostly). So you can say it's meant to change the feel because of the new engines or you can say it's meant to deal with the new engines because they change the feel. It's the same thing.
 
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I'm not equating. I'm saying the desired effect of MCAS is to increase negative pitch attitude, because the engines are creating a positive pitch attitude vector.
Maybe this is an issue of you using non-standard terminology. The engines, in standard terminology, do not create "a positive pitch attitude vector". They create a pitch up moment. In other words, a torque that must be included in the total balance of forces and torques on the plane. (And this moment, as has been noted, depends on angle of attack and airspeed.) The nose down trim added by MCAS adds a counteracting pitch down moment, i.e., it adds an additional torque to the total balance of forces and torques on the plane, that counteracts the torque added by the engines. But these additional moments do not necessarily translate into actual changes in the pitch attitude of the plane; that depends on the total balance of forces and torques on the plane, which of course will include the forces and torques induced by the pilot's commands made using the yoke.
 
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Maybe this is an issue of you using non-standard terminology. The engines, in standard terminology, do not create "a positive pitch attitude vector". They create a pitch up moment. In other words, a torque that must be included in the total balance of forces and torques on the plane. (And this moment, as has been noted, depends on angle of attack and airspeed.) The nose down trim added by MCAS adds a counteracting pitch down moment, i.e., it adds an additional torque to the total balance of forces and torques on the plane, that counteracts the torque added by the engines. But these additional moments do not necessarily translate into actual changes in the pitch attitude of the plane; that depends on the total balance of forces and torques on the plane, which of course will include the forces and torques induced by the pilot's commands made using the yoke.
OK, I don't think that really changes my view of things much. But I apologize if my incorrect terminology didn't make sense, it made sense to me:

I'm saying the desired effect of MCAS is to counteract the pitch up force caused by the engines. It does that by adding nose down trim.
 
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I'm saying the desired effect of MCAS is to counteract the pitch up force caused by the engines. It does that by adding nose down trim.
Yes, this is fine. The word "force" (or "torque" or "moment") correctly describes what the MCAS and the engines are causing.
 
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Yes, this is fine. The word "force" (or "torque" or "moment") correctly describes what the MCAS and the engines are causing.
I appreciate the detail with the accuracy of the physics terms.

However, I do think you are looking at a fairly specific scenario. In the scenario of the MCAS and the controversy surrounding it, I think it's pretty safe to say that MCAS adds nose down trim, to effectively push the nose down, to add negative pitch attitude. We are seeing in the data that the planes have unstable vertical airspeed, the nose is moving up and down with the pilots essentially losing the tug of war. That's why I think the terminology I was using isn't entirely inaccurate in this case. Can MCAS add nose down trim with the intention of not having any nose down, negative pitch attitude effect? Well no I don't think that's ever the systems intention. You've illustrated that the forces coming from the column could cancel it out in normal operation. Even in the case where the nose down trim is cancelled out, that was from pilot input. The pilot is not part of the MCAS. So the MCAS is sending nose down trim to increase negative pitch attitude. It seems silly to think the system is not trying to effect the pitch attitude. My contention isn't that it always results in a net negative pitch attitude, of course there are millions of scenarios, my contention is that that's what the MCAS intention is which counteracts the pitch up force from the engines. It just all seems somewhat pedantic without adding much to the understanding of the system.
 
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I do think you are looking at a fairly specific scenario.
I'm looking at how MCAS, and the trim system generally, is intended to operate under normal conditions. See below.

n the scenario of the MCAS and the controversy surrounding it
Which involves conditions that are not normal. That's the key point I've been repeatedly trying to make. Yes, under abnormal conditions, the MCAS will issue nose down trim commands that will cause the nose to actually pitch down. But that is not the normal operation of the system; it's happening precisely because the situation is not normal, which means the nose down trim commands are being issued under conditions when the plane's actual angle of attack and airspeed do not indicate that they should be. That means the pilots are not expecting them.

Can MCAS add nose down trim with the intention of not having any nose down, negative pitch attitude effect?
Of course; that's the normal operation of the system. As I've already repeatedly explained. Under normal conditions, the pilot will be using the yoke to command a specific pitch attitude; the only effect of the nose down trim the MCAS adds will be to increase the yoke force the pilot has to exert to maintain that desired pitch attitude. Under conditions where the pilot is not exerting any force on the yoke to command a particular pitch attitude, MCAS will not be operating at all if it's operating correctly--either because the plane is on autopilot, or because the pilot is manually flying the plane straight and level and the yoke is in its neutral position, and the angle of attack and airspeed are not anywhere close to the range where MCAS adds any nose down trim.

You say I have described a "fairly specific" scenario--well, it's the only scenario in which MCAS is supposed to operate at all. MCAS is not even supposed to be operating at all under the conditions in which it issued erroneous nose down trim commands in these incidents. So saying that, well, under the conditions in these incidents the MCAS actually caused the nose to pitch down, does not seem like a good way of describing what MCAS is intended to do.
 
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You say I have described a "fairly specific" scenario--well, it's the only scenario in which MCAS is supposed to operate at all. MCAS is not even supposed to be operating at all under the conditions in which it issued erroneous nose down trim commands in these incidents. So saying that, well, under the conditions in these incidents the MCAS actually caused the nose to pitch down, does not seem like a good way of describing what MCAS is intended to do.
Again I would stand by the statement that it's largely pedantic semantics that really doesn't help describe the system in any meaningful way imo. In fact I think the way you describe MCAS is flawed, you suggest that the normal operation of the system doesn't result in any effect on attitude and the trim commands will be nulled.

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

It's mainly active when the AoA enters a certain max threshold. That is when the plane is pitched upward. It sends nose down trim commands with the intention of producing added negative pitch attitude from the current pitch attitude to prevent a stall. It's not active when cruising, stable, normal flight conditions. It is not active during any pitch change from the yoke / elevators. For all scenarios as far as I see, when it's active it's intention is to bring the nose down and you can call that adding negative pitch attitude. I'll just keep saying it because I believe it to be true and I don't see anything anywhere online that suggests otherwise.

Also it sounds like you're equating MCAS to trim generally. That's not correct.
 
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Under normal conditions, the pilot will be using the yoke to command a specific pitch attitude; the only effect of the nose down trim the MCAS adds will be to increase the yoke force the pilot
First of all, as is very clearly established, MCAS doesn't operate under normal conditions and normal pitch adjustments from the yoke / pilot. I think this is the nucleus of your misunderstanding of MCAS.

This is also where I think you're mistaken:
...the only effect of the nose down trim the MCAS adds will be to increase the yoke force
The yoke force increase is directly a product of the nose down trim, which in effect keeps the pilot from increasing the pitch attitude, which in effect is adding negative pitch attitude. It's just in the way you seem to be envisioning the system and the order of things. Add the pedantic semantics of trim generally and you're really in a tangled web.
 
jim hardy
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I'm saying the desired effect of MCAS is to counteract the pitch up force caused by the engines. It does that by adding nose down trim.
Yes Torques and moments are the terms we'll hear.


upload_2019-3-16_19-20-14.png


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
 

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MCAS doesn't operate under normal conditions and normal pitch adjustments from the yoke / pilot.
I don't know what you think is "normal". MCAS certainly operates at sufficiently high angles of attack, and it is perfectly possible to achieve those in normal flight. The most common way is during a bank. Perhaps I should have described banking scenarios instead of straight pitch up scenarios previously to help make that clear.

Remember that angle of attack is not the same as pitch attitude. Angle of attack is the angle between the wing and the relative wind. Pitch attitude is one thing that can affect angle of attack, but not the only one.

I think this is the nucleus of your misunderstanding of MCAS.
I have no idea in what way you think I am misunderstanding MCAS. We both agree on the key point, that MCAS adds nose down trim, which adds a nose down pitch moment. That in itself is sufficient to understand what is happening in the incidents we have been discussing, at least as far as MCAS is concerned: MCAS keeps adding nose down trim, which keeps adding a pitch down moment, which makes it harder for the pilots to recover. If the pilots don't realize what is happening in time and disable the entire stability trim system, the plane will crash.

The yoke force increase is directly a product of the nose down trim
Yes.

which in effect keeps the pilot from increasing the pitch attitude
If the pilot really wants to, he can continue to increase the yoke force and pitch the plane up further.

which in effect is adding negative pitch attitude.
This is just your idiosyncratic, non-standard use of language. "Adding a pitch down moment" does not, in standard language, equate to "adding negative pitch attitude". Nor does "keeping the pitch attitude from increasing further" equate to "decreasing the pitch attitude". But if it makes sense to you, whatever. Just don't expect me to use your idiosyncratic language. I'm going to continue using the language we have agreed on, which is that MCAS adds nose down trim, which adds a pitch down moment.

It's just in the way you seem to be envisioning the system and the order of things.
The way I am "envisioning" the system is the standard terminology. Your terminology is not.

Add the pedantic semantics of trim generally and you're really in a tangled web.
Again, I have no idea where you are getting this from. I have correctly explained, several times now, what adding nose down trim does. You have even agreed with the basic statement of that that I made. I have also linked you to a detailed series of articles by a pilot that explains how pilots use the trim system generally. You have no basis whatever that I can see for claiming that I am misunderstanding anything. I am simply using standard terminology with which you appear to be unfamiliar, so you are inventing your own idiosyncratic terminology instead.
 
russ_watters
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The thread is moving rather fast now, so I apologize if I repeat what others have said...
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.
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.

Example (numbers made-up for the example):
Imagine you are flying a normal plane that stalls at 14 degrees aoa. You've just taken-off, trimmed for takeoff which keeps it relatively neutral with little back-pressure for a moderate climb. You're at 5 degrees aoa and you pull back to increase to 7 degrees. It takes 5lb of force. You pull back more to increase to 9 degrees. Now it takes 10lb of force....twice more to get to 13 degrees and it takes 20lb of force. Say you want to keep increasing the aoa. You pull back harder. 30lb. 40lb. 50lb. You put your foot on the control panel for more leverage (it's happened); 90lb. 100lb. No matter how hard you pull, the plane doesn't increase its angle of attack past 13 degrees. That's what stall prevention systems are for*.

Now a 737Max without MCAS: you pull up from 5 degrees to 7 degrees, with 5lb of force. Without MCAS the plane's natural feel would be backwards. Instead of being harder to raise the nose, it would be easier. Now the nose keeps rising and you aren't adding more backpressure. So you release the backpressure and the nose keeps rising. Now it's at 11 degrees and you're pushing forward with 5lb of force. 13 degrees and you're pushing forward with 10lb of force. That's pitch instability and that's why MCAS exists. It's probably pretty disconcerting for a pilot.

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.

*Caveat: I'm not clear on what full-fledged stall prevention system the 737 has, if any. Unfortunately the recent crashes have saturated google searches on the subject. Airbus planes, as an example of the alternative, use full fly-by-wire, so there is no "feel" issue and at the same time there is no direct relaying of the input to the output. The physical augmentation of MCAS or other stall prevention is fully dealt with in software. There's no force, just stick position. But it's the same idea.
 
russ_watters
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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.
Let me amplify this with a description of how manual trim adjustments tend to work. Let's say you are trimmed for high speed and you reduce your speed. Now in order to stay level you need to add backpressure because the stabilizer also needs a higher (negative) angle of attack to provide the same stabilizing force. After you've stabilized in this new condition, you adjust your trim to reduce that force to zero. The plane's attitude doesn't change, only the force required to hold it does.

Again: the MCAS failure that led to these crashes was not due to its normal operation, but rather due to a runaway trim issue. Instead of making small adjustments to alter the feel, it rapidly trimmed full nose down, which did cause the nose to drop.

Let me say it another way: if the nose drops due to normal MCAS operation, it is being too aggressive.
 
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I'm not clear on what full-fledged stall prevention system the 737 has, if any.
I'm not either, and I haven't been able to find any useful information online. I would really like to know (a) what stall prevention the 737 previously had, (b) what stall prevention the 737 MAX has, and (c) how the AoA sensors are used in either a or b.
 
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don't know what you think is "normal". MCAS certainly operates at sufficiently high angles of attack, and it is perfectly possible to achieve those in normal flight. The most common way is during a bank. Perhaps I should have described banking scenarios instead of straight pitch up scenarios previously to help make that clear.

Remember that angle of attack is not the same as pitch attitude. Angle of attack is the angle between the wing and the relative wind. Pitch attitude is one thing that can affect angle of attack, but not the only one.
So first off you pretty clearly suggested that MCAS is active in any pitch adjustment via the yoke not only in incidents of high AoA. That is incorrect.

Second, it's fairly safe to say when there is a high AoA there is going to be a positive pitch attitude. I did not equate them.
 
russ_watters
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Second, it's fairly safe to say when there is a high AoA there is going to be a positive pitch attitude.
While this is often true, it is not always true and it is a big mistake to try to equate them.
 
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first off you pretty clearly suggested that MCAS is active in any pitch adjustment via the yoke not only in incidents of high AoA
If you insist on taking particular things I said out of context, I suppose you could claim this. But I don't think that's a fair way of reading what I've said in this thread, taken as a whole.
 
jim hardy
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it's fairly safe to say when there is a high AoA there is going to be a positive pitch attitude. I did not equate them.
unless ,maybe you're pulling out of a steep dive..
 
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While this is often true, it is not always true and it is a big mistake to try to equate them.
You omitted the part where I said I did not equate them. I just said it's safe to say. I'm open to a scenario you can illustrate where this isn't the case, I'm not a pilot after all.
 
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You omitted the part where I said I did not equate them. I just said it's safe to say. I'm open to a scenario you can illustrate where this isn't the case, I'm not a pilot after all.
I guess in the context of the two 737s were talking about I see it as fairly probably that when MCAS engages due to a high AoA, it's going to want to bring that positive pitch attitude down.
 
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I don't know how much clearer I can say it.
You don't have to be any clearer about the "force" part:

MCAS when active is almost always going to try to be compensating for the pitch up force of the engines, it is therefore going to be producing a pitch down force
Yes, indeed. This is correct, I've already agreed it's correct, and nobody else in the thread has questioned it.

which in turn is going to create a negative pitch attitude force
Isn't this just repeating "pitch down force"?

It may not always net out with the nose lowering, but that's what MCAS is trying to do.
And here is where you keep going wrong. If you would just leave this part out, there would be no issue at all. But you keep insisting on putting it in, which is why I, and now @russ_watters , are objecting.
 
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If you insist on taking particular things I said out of context, I suppose you could claim this. But I don't think that's a fair way of reading what I've said in this thread, taken as a whole.
I'm not taking you out of context, here:

Thinking of the system as "engaging" is misleading. The MCAS system is always adjusting the trim in manual flight to compensate for the pitch up moment of the engines. Its purpose is not "spot some particular condition we don't want and adjust to get out of it". Its purpose is "change the way the plane feels to the pilot to make it like previous 737s". If the system were only active part of the time in manual flight mode, the "feel" of the plane would change from one flight regime to another. That would not be good.
You are clearly mistaken here. It is indeed only active "part of the time".
 
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in the context of the two 737s were talking about
In those cases, the MCAS should never have engaged at all. The fact that it did was an error, not normal operation. I have made this point repeatedly, but you still don't appear to understand the implications.
 
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And here is where you keep going wrong. If you would just leave this part out, there would be no issue at all. But you keep insisting on putting it in, which is why I, and now @russ_watters , are objecting.
So to be clear, if you don't think when MCAS activates it's trying to lower the nose, ultimately - to produce a negative attitude adjustment. Then what do you think the system is trying to do? Simply provide stick force?
 

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