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

[cyboman]

Huh? The MCAS system is a part of the trim system.
Sure. But "sending nose down trim commands" is not the same as "pitching the nose down". It's not even the same as "commanding the nose to pitch down". You do that using the yoke, not the trim system. Ask a pilot. Or read the "See How It Flies" articles.

MCAS adjusts trim TO pitch the nose down. You're just making semantic arguments that don't really say anything in my opinion. The commands MCAS sends results in the nose pitching down. I can say it a dozen different ways. I don't think you want to admit that for some reason. Your assertion that MCAS is primarily to provide column force feedback is completely incorrect in my opinion.

 

[cyboman]

We'll see if anyone replies. I would be interested in the answer.

I would assume you disagree with Albrecht who states MCAS is not for stick feel.

 

[cyboman]

eyewitness reports of smoke and debris from the plane on its way down are another item in my list of unresolved observations.

old jim

Just as a side note I read somewhere other witnesses said they didn't see smoke. So there's some conflicting reports among the witnesses.

Though totally possible, it seems unlikely such a catastrophic mechanical failure causing fire / smoke would happen in a plane that new.

 

[PeterDonis]

You're just making semantic arguments that don't really say anything in my opinion.

No, I'm disagreeing with a particular categorical statement you keep making:

The commands MCAS sends results in the nose pitching down.

This is not true categorically, it's only true with a particular qualifier: if the pilot is not applying force to the yoke to compensate. And as I've already explained, in normal operation, any time the MCAS is sending nose down trim commands, the pilot will be applying force to the yoke to compensate, so the result of the MCAS trim commands will not be to pitch the nose down, it will be to increase the feedback force on the yoke that the pilot feels.

You have not bothered to respond to my repeated statements along these lines even once.

 

[PeterDonis]

I would assume you disagree with Albrecht who states MCAS is not for stick feel.

That's why I replied to him. Because I've seen plenty of other pilots (including the one in the article OCR linked to, which I mentioned before) say that it is. Not to mention Boeing saying that the reason they put MCAS in was to make the 737 MAX feel like previous 737s so pilots wouldn't have to be retrained. I want to see how he responds to that.

 

[cyboman]

No, I'm disagreeing with a particular categorical statement you keep making:
This is not true categorically, it's only true with a particular qualifier: if the pilot is not applying force to the yoke to compensate. And as I've already explained, in normal operation, any time the MCAS is sending nose down trim commands, the pilot will be applying force to the yoke to compensate, so the result of the MCAS trim commands will not be to pitch the nose down, it will be to increase the feedback force on the yoke that the pilot feels.

You have not bothered to respond to my repeated statements along these lines even once.

I have been engaging with this very assertion over and over. You're basically saying that if the system is acting without any additional input, it will result in the nose pitching down. And saying it's not suppose to operate that way, it's commands for nose down trim are supposed to be overridden by pilot yoke input. At least that's what it sounds like you're saying but I'm sure you'll clarify further somehow. In any case, I don't agree, the simple way to refute this is the engines are known to create a positive pitch attitude force. MCAS is designed to counteract that. You can get into the details of the use cases of how MCAS operates as it seems you do. But the primary function of MCAS is to correct that pitch up effect of the engines not provide force feedback or stick feel.

 

[cyboman]

That's why I replied to him. Because I've seen plenty of other pilots (including the one in the article OCR linked to, which I mentioned before) say that it is. Not to mention Boeing saying that the reason they put MCAS in was to make the 737 MAX feel like previous 737s so pilots wouldn't have to be retrained. I want to see how he responds to that.

You think Boeing saying "make it feel like previous 737s" scientifically proves MCAS is primarily an artificial feel system and isn't meant to correct pitch attitude?

 

[jim hardy]

That's the reason behind it, to make the yoke feel similar to earlier planes when performing maneuvers involving steep pitch and bank..

Without it the plane would respond differently than earlier ones.

It's a closed loop : pilot - yoke - control surfaces - pitch - back to pilot
and in a closed loop you can observe it anywhere and you'll have a signal coming in and a signal going out

@PeterDonis is i think examining the loop at the node pilot-yoke
@cyboman i think is examining it at the node pitch-back to pilot
MCAS adds a computed signal at the node yoke-control surfaces so pilot receives same pitch to yoke response as in earlier planes and they don't have to re-certify plane or pilot. ..

automatic control is confusing that way. You can start anyplace in the loop and write a transfer function.
it's hard to express in words but the math is just that way.
You can't open the loop to analyze it. I'd say that's the crux of your disagreement.old jim

 

[cyboman]

That's the reason behind it, to make the yoke feel similar to earlier planes when performing maneuvers involving steep pitch and bank..

Without it the plane would respond differently than earlier ones.

It's a closed loop - pilot - yoke - control surfaces - pitch - back to pilot
and in a closed loop you can observe it anywhere and you'll have a signal coming in and a signal going out

@PeterDonis is i think examining the loop at the node pilot-yoke
@cyboman i think is examining it at the node pitch-back to pilot
MCAS adds a computed signal at the node yoke-control surfaces.

automatic control is confusing that way. You can start anyplace in the loop and write a transfer function.
it's hard to express in words but the math is just that way.old jim

That's very interesting. I really like that loop diagram. I think I need to understand it better.

This I think helps to illustrate my point: MCAS is working after the yoke, it's pathway is to the control systems, then the feedback eventually get's back to the pilot via the yoke. But the MCAS system is not operating on the yoke and it's fundamental role is not force feedback.

So shouldn't it be: pilot - yoke - control surfaces - pitch - yoke - back to pilot

And isn't pitch part of control surfaces?

pilot - yoke - (MCAS) - control surfaces - yoke - back to pilot

 

[jim hardy]

So shouldn't it be: pilot - yoke - control surfaces - pitch - yoke - back to pilot

i don't see a second yoke in the loop.

But indeed MCAS is in the loop where you drew it.

 

[OCR]

Great find there.

Thank you...

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

I'm not sure... but here is the basics of the phenomenon you refer to...

Mach tuck - Wikipedia

...experienced pilots would get into a PIO.

And the acronym PIO, explained by a very short video... .

Pilot induced-Oscillation

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

If you’re faced with a forced landing, fly the thing as far into the crash as possible... fly it until the last piece stops moving.

.

 

[jim hardy]

then the feedback eventually get's back to the pilot via the yoke

pitch feedback gets back to pilot via horizon , either the real one or artificial one (or AOA gage if there is one)
and probably his "seat of the pants" feel.

 

[cyboman]

i don't see a second yoke in the loop.

But indeed MCAS is in the loop where you drew it.

I see, I suppose I was thinking about where the feedback occurs on the yoke. So the pilot pulls up on the yoke, it adjusts the control surfaces, that effects plane pitch attitude, those forces then are felt on the yoke, and to the pilot.

 

[cyboman]

No, I'm disagreeing with a particular categorical statement you keep making:
This is not true categorically, it's only true with a particular qualifier: if the pilot is not applying force to the yoke to compensate. And as I've already explained, in normal operation, any time the MCAS is sending nose down trim commands, the pilot will be applying force to the yoke to compensate, so the result of the MCAS trim commands will not be to pitch the nose down, it will be to increase the feedback force on the yoke that the pilot feels.

You have not bothered to respond to my repeated statements along these lines even once.

Further, by sending nose down trim commands to counteract the positive pitch attitude caused by the new engines, MCAS has the effect of providing a feel closer to previous models. But it's not the other way around as you would seem to suggest. It doesn't provide force feedback so the pilot can correctly deal with the positive pitch attitude from the engines using the yoke. If that were the case it wouldn't be messing with the stab.

 

[jim hardy]

Bob Hoover is my hero. Saw his Shrike show in Homestead 1976.

Somewhere i have a photo of his yellow P51 upside down, gear and flaps extended, maybe fifty feet over the shrubs at end of Homestead General's runway 36.
He continued the roll and touched down perfectly.

I guess it just 'felt good' .

sorry for digression - 'old firehorse' instinct...

old jim

 

[OCR]

Done enough troubleshooting to know that the apparent conflicts don't resolve until you arrive at complete understanding.

.

.

...

 

[cyboman]

Bob Hoover is my hero. Saw his Shrike show in Homestead 1976.

Somewhere i have a photo of his yellow P51 upside down, gear and flaps extended, maybe fifty feet over the shrubs at end of Homestead General's runway 36.
He continued the roll and touched down perfectly.

I guess it just 'felt good' .

sorry for digression - 'old firehorse' instinct...

old jim

Haha, well I'm going to have to read about Hoover now.

 

[OCR]

Somewhere i have a photo of his yellow P51 upside down, gear and flaps extended

.

 

[cyboman]

View attachment 240379.

That's one damn fine looking aircraft. I bet it's a blast to fly.
Looks like it's got a nice powerplant:
Powerplant: 1 × Packard V-1650-7 liquid-cooled V-12, with a 2 stage intercooled supercharger, 1,490 hp (1,111 kW) at 3,000 rpm;[118] 1,720 hp (1,280 kW) at WEP

 

[PeterDonis]

I have been engaging with this very assertion over and over. You're basically saying that if the system is acting without any additional input, it will result in the nose pitching down. And saying it's not suppose to operate that way, it's commands for nose down trim are supposed to be overridden by pilot yoke input.

Yes, because the system only operates in manual flight, and the definition of manual flight is that the pilot is supposed to control the plane's pitch attitude with yoke input.

I don't agree, the simple way to refute this is the engines are known to create a positive pitch attitude force.

The engines create a pitch up moment which depends on angle of attack and airspeed, yes. But I don't see how that refutes what I was saying. See below.

the primary function of MCAS is to correct that pitch up effect of the engines

Correct it how? By pitching the nose down? No, that would be ridiculous. If the pilot wants a 10 degree pitch up attitude, the pilot wants a 10 degree pitch up attitude. We're talking about manual flight, remember. It's not the job of MCAS, or any automated system, to decide what the plane's pitch attitude should be. It's the pilot's job. The job of the MCAS is to correct for the pitch up effect of the engines so that the pilot can do his job the same way he did on previous 737s. That's the manufacturer's stated intent.

Or look at it another way. When you say "the pitch up effect of the engines", what does that mean? Does it mean that, if the pilot pulls back on the yoke to hold the plane at 10 degrees pitch up (we're assuming straight and level flight to start with), the engines somehow intervene and move the plane to, say, 15 degrees pitch up instead? No, of course not. The pilot is commanding a particular pitch attitude with the yoke. The difference the engines make is that, in the absence of MCAS, the force the pilot needs to exert to pull back on the yoke to hold a 10 degree pitch up attitude is less than it was on previous 737s. (Note: I've picked the 10 degree number at random, I don't know if it's actually within the range where MCAS will operate. I've just assumed that it is for purposes of this example.) With MCAS operating, the force the pilot needs to exert to pull back on the yoke to hold a 10 degree pitch up attitude is increased, because MCAS adds nose down trim. The same effect could be achieved in the absence of MCAS by the pilot manually adding nose down trim. But either way, the effect of MCAS is not to pitch the nose down to compensate for the effect of the engines, because the effect of the engines was not to pitch the nose up in the first place. The pilot did that.

You think Boeing saying "make it feel like previous 737s" scientifically proves MCAS is primarily an artificial feel system and isn't meant to correct pitch attitude?

I think the manufacturer's stated intended purpose for the system carries a lot more weight than the statements of random people on the Internet, if we're trying to figure out what the intended purpose of the system is.

 

[PeterDonis]

It's a closed loop : pilot - yoke - control surfaces - pitch - back to pilot
and in a closed loop you can observe it anywhere and you'll have a signal coming in and a signal going out

@PeterDonis is i think examining the loop at the node pilot-yoke
@cyboman i think is examining it at the node pitch-back to pilot
MCAS adds a computed signal at the node yoke-control surfaces so pilot receives same pitch to yoke response as in earlier planes and they don't have to re-certify plane or pilot. ..

I think this is an illuminating way to look at it, particularly the point about being able to look at any node in the loop and have a signal coming in and a signal going out. So there isn't really a "start" or "end" of the loop: it's a loop, running continuously.

I would just add a couple of comments:

First, the trim system in general changes the yoke - control surfaces transfer function. (In the 737, it does this by changing the angle of the horizontal stabilizer. In other airplanes it might do it with trim tabs. But it's functionally the same as far as your closed loop flow chart is concerned.) MCAS is just a special case of this. In one of the previous links, it is noted that there are multiple automatic trim adjustment functions on the 737, to compensate for changes in speed and other flight parameters. All of them have the same effect in terms of the flow chart: they change the yoke - control surfaces transfer function.

Second, the loop as you've written it really tracks two separate things: the actual pitch attitude of the plane, and the force the pilot has to exert on the yoke. Both of those responses are changed by the trim system, and the locus of the change in both cases is the yoke - control surfaces transfer function. To make this more explicit, I'll rewrite the loop in two ways, one for the pitch attitude and one for the force:

(Pitch attitude) pilot moves yoke - yoke movement moves control surfaces - control surface movement changes pitch - pilot sees changed pitch attitude

(Yoke force) pilot exerts force on yoke - yoke force moves control surfaces - control surface movement changes feedback force on yoke - pilot feels changed feedback force

Both of these loops are "running" at the same time, and the pilot's actions in the two loops are not independent: either one determines the other. So from the pilot's point of view, he is expecting two kinds of feedback from every yoke action: a change in the plane's pitch attitude, and a change in the feedback force on the yoke. What MCAS, or trim adjustment generally, does is change the relationship between those two pieces of feedback. That relationship is part of what pilots are talking about when they talk about how it "feels" to fly the plane.

 

[cyboman]

The job of the MCAS is to correct for the pitch up effect of the engines so that the pilot can do his job the same way he did on previous 737s.

Completely, agree. I've been pretty clear on that point. I don't think that was your clear position before. It was more aligned with providing stick feel or column force feedback.

I think the manufacturer's stated intended purpose for the system carries a lot more weight than the statements of random people on the Internet, if we're trying to figure out what the intended purpose of the system is.

I would note that we might want to take any statement from Boeing with a grain of salt as they are in a complicated situation. In light of what's happened, any statements regarding their systems are going to be very carefully worded and I'm going to guess their going to keep the details pretty close to their chest as much as they can.

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.

I think we've reached the useful limit of our disagreement and it makes sense to not waste any more energy on it. I think we've both made our positions clear. If we come across any additional info that clarifies the primary function of MCAS and other details than maybe we can revisit.

 

[PeterDonis]

shouldn't it be: pilot - yoke - control surfaces - pitch - yoke - back to pilot

No; I think the (valid) issue you are raising here is better addressed by viewing the loop as two concurrent control loops, as described in the post I made just now in response to @jim hardy . One loop deals with the change in pitch attitude, the other deals with the change in feedback force.

 

[PeterDonis]

I don't think that was your clear position before.

It has always been my position, but evidently it wasn't clear to you before. I'm glad it's clear now and that we have agreement on this point.

I would note that we might want to take any statement from Boeing with a grain of salt as they are in a complicated situation.

As far as I can tell, their statement that the purpose of MCAS was to make the plane feel like previous 737s was made when they went to the FAA to get the 737 MAX grandfathered under the certification of previous 737s. That seems like pretty strong evidence to me that that was Boeing's intent, particularly in light of the fact that they did not give pilots any details about MCAS or how it worked. They must have believed that MCAS would, in fact, make the 737 MAX feel similar enough to previous 737s that pilots wouldn't notice, or at least wouldn't have any issues. And the FAA must have agreed, or it wouldn't have approved the certification.

In light of what's happened, any statements regarding their systems are going to be very carefully worded and I'm going to guess their going to keep the details pretty close to their chest as much as they can.

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.

 

[PeterDonis]

which however you want to describe it, acts to increase negative pitch attitude

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.

 

[cyboman]

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.

 

[cyboman]

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.

 

[PeterDonis]

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.

 

[cyboman]

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.

 

[PeterDonis]

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.

 

[cyboman]

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 canceled 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.

 

[PeterDonis]

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.

 

[cyboman]

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.

 

[cyboman]

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]

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.

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

 

[PeterDonis]

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]

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]

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.

 

[PeterDonis]

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.

 

[cyboman]

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]

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.

 

[PeterDonis]

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]

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..

 

[cyboman]

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.

 

[cyboman]

unless ,maybe you're pulling out of a steep dive..

Right, I see. I'm wondering how probable that is with these 737s though.

 

[cyboman]

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.

 

[PeterDonis]

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.

 

[cyboman]

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".

 

[PeterDonis]

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.

 

[cyboman]

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?