Boeing Boeing 737 Max MCAS System

FactChecker

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mcas is not there to prevent a stall; the plane does have pilots on board that are there to fly the plane. The pilots prevent stalls, implicitly.
The mcas was to maintain the flight characteristics of the plane as not to require a new type certificate. Simple as that.
Apparently the plane flies "just fine" without mcas;
That is not the impression I had, but I do not have an authoritative source.
 
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That is not the impression I had, but I do not have an authoritative source.
it'll be my new source of aircraft news...seems we have that much choice of source...no need for impressions.

note this "source" did not say,
mcas is not there to prevent a stall; the plane does have pilots on board that are there to fly the plane. The pilots prevent stalls, implicitly.

That's just my impression ;)
 
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The mcas was to maintain the flight characteristics of the plane as not to require a new type certificate. Simple as that.
It's really not so simple. That's one way to summarize the system and situation. And somewhat accurate from a high level. But when describing MCAS itself it's way more complex. MCAS is qualified differently at different times by Boeing themselves and their motives for it's implementation are complex, not just certification. Also, I've heard an airliner pilot on youtube describe the MCAS as a type of stall prevention system. That's likely because, when you look at a system that commands nose down trim in high AoA scenarios, one of the main threats that system is trying to mitigate is the pilot entering a stall. It doesn't mean it handles the stall without pilot intervention. It means it modifies the aerodynamics of the airplane to assist in preventing a stall scenario.

One issue is in these discussions, everyone tries to summarize the problem it their own way, typically over simplifying the failure modes and all the various motivations and involved actors. In general, if you read this entire thread you'd see we've been over a lot of this ground. And certainly, the contention that MCAS was a sort of "hack" or jerry-rigged approach has certainly been suggested more than once.
 
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Note in that vid, he mentions a system that may operate the stabilizer trim even with the stab trim cut-off in the off position. Regarding a "mach trim". It was recorded that while the stab trim cut off was in off position that the stab trim move a bit (with no explanation on what moved it), Juan posits it could be a mach trim, given the speed of the craft at the time.
Interesting, that's not entirely surprising as I've suggested the speed trim system is very tightly coupled functionally to MCAS it seems, the mach trim is another automated system commanding trim. I'm pretty sure speed trim and or mach trim have been associated with other crashes as well. Again, it's about the pitch authority these systems have.

So such "behind the scenes" automation can work, it just needs to have an acceptable logic to it.
Sure they can. But it's how they are implemented not necessarily the robustness of the logic. The logic and the testing always need to be robust, that's a given - and it's certainly an issue with MCAS. But even with the best logic, accidents can happen. A system can fail. And a big part of what's gone wrong with MCAS, I would contend, is that the system is not communicating with adequate feedback to the pilot. And the training and maneuvers to disable the system is lacking and very problematic. Remember, this isn't something that adjusts automatically the speed of windshield wiper blades based on a water volume sensor. It's a system that has tremendous pitch authority. If the pilot doesn't even know it exists (which they didn't initially, it wasn't even in the manual), then there is very bad evidence of a dangerous paradigm shift in how we approach implementations of flight critical automated systems.

One good thing that has come out of these horrible tragedies is that those implementations are going to be ruthlessly reexamined and all actors and motives will be under question and further, as we've seen, under investigation.
 
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The relevancy may not be so obvious. Strong note; mcas is not there to prevent a stall; the plane does have pilots on board that are there to fly the plane. The pilots prevent stalls, implicitly.
What I think you're missing is that @FactChecker is saying that moving the CG back has big efficiency gains but also sacrifices aerodynamic stability. Making the engines larger is the same thing. More efficiency but it came at a big aerodynamic cost, CG was affected. MCAS was implemented to solve that problem. The issue is, how far do you push an air-frame before it's not stable? Is it safe to implement automation like MCAS to deal with those changes (and do so without telling the pilot)? Many pilots are of the opinion the air-frame was not adequate for those engines. I suggested earlier and I had read similar elsewhere, that the stab needed to be bigger / redesigned.
 
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After hearing the report from the video provided (which was good analysis):

Why does the stab cutout switches not disable all automated systems commanding the stab? How could it disable MCAS but not speed or mach trim? (I don't believe elevator feel is involved here as he suggests as that's only commanding elevator not the stab trim from my understanding). In fact the procedure for a runaway stab trim is to use the cutout switches, that exists before MCAS. He explains being trained for that scenario.

He's right, it's gut wrenching and aggravating.

Also, much of what was hypothesized here earlier in the thread has borne out. Some very smart people participating on this forum.
 
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In those airplanes, does "manually" mean that there is no hydraulic assistance?
To further clarify: I think that with trim, manual control may in fact not be hydraulically assisted. So, if I'm not mistaken when they use the stab cutout switches that may result in no hydraulic or "electric" assistance to control the trim and the trim wheels in effect work mechanically. I'm not sure of this however. It may be the case that the stab cutout only bypasses automated systems and the trim switches on the yoke still operate using electric control of the trim. Given the recent tragic scenarios, I think that's preferable.

For other systems, like elevator, aileron and rudder, manual control would still mean hydraulically assisted since as previously explained direct mechanical control is not a practical flight scenario and more of a redundant backup architecture. Manual control in this sense, is disabling autopilot.
 
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Airliners crash very infrequently these days. It can certainly be said that the Boeing design philosophy contributed to these crashes, but it can also be said about the Airbus design philosophy. Neither is perfect.
Ya this is a great point. It really dives into the heart of the issue. We've been dancing around this a lot. More automation etc... I do think the Boeing model has some merits. I do like the idea of the pilot still being a central "CPU", that's the case for airbus model too, but with Boeing I think that's a larger focus. I also like the idea of mechanical redundancy. It's a challenge perhaps with these large crafts but as a non expert, I do lean towards the Boeing flight controls philosophy.

In fact, perhaps part of what Boeing did wrong with MCAS is deviate from their flight controls philosophy.

These days crashes are so infrequent that it seems to me that most are caused by poor relationships/communication between the pilot, the computer and the plane -- because everything else has become so close to perfect.
Yes, HCI is critical and seems to be lagging behind the growth of automation and AI.
 
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What I think you're missing is that @FactChecker is saying that moving the CG back has big efficiency gains but also sacrifices aerodynamic stability. Making the engines larger is the same thing. More efficiency but it came at a big aerodynamic cost, CG was affected. MCAS was implemented to solve that problem. The issue is, how far do you push an air-frame before it's not stable? Is it safe to implement automation like MCAS to deal with those changes (and do so without telling the pilot)? Many pilots are of the opinion the air-frame was not adequate for those engines. I suggested earlier and I had read similar elsewhere, that the stab needed to be bigger / redesigned.
I don't believe I missed that point. Not sure about the aerodynamic changes. The point of thrust is more forward on the wing, as well as the weight. Apparently such a situation causes a "natural" tendency to pitch up. Not to some crazy degree that pilots cannot control without automation. But enough to say it flies differently from the previous model and requires a new type certificate. enter mcas to bring those flight characteristics back to being the same as the previous model, for which the pilot is already trained.

To your rhetorical question/point regarding "how far do you push an air-frame" via changes. Only to have automation adjust the resulting flight characteristics so the plane "flies" the same (as previous 737's) from the pilots perspective.

Is FAA at fault for allowing such a system circumvent the requirement to have pilot training in order to fly this essentially new aircraft? Is boeing also culpable because they did a poor job of implementation?

At first I thought it was economic reasons that the FAA held out grounding the plane for so long (obvious economic implications for Boeing)

I imagine the pilots of the failed flights would say that mcas specifically was the issue.

Am not sure pilot's opining on engineering of the plane is a good basis; maybe best left to aerospace engineers.

That guy in the vid Juan, he talked about this particular airforce jet he flew that was all about AoA. Had a big AoA reading display right in the center of the panel. Had a crazy high landing speed too. He liked how it flew...I imagine many would not, particularly someone not trained in it's use.

In other words, maybe all 737 max needed was pilot training....a new type certificate.
 
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For who whould like to deepen knowledge on 737 all version and find exaustive technical description of all systems there's this site: The Boeing 737 Technical Site.

Site author, Chris Brady, is a pilot of 737 from 1994, in this case I find very interisting the analisys on MCAS system here: 737 MAX - MCAS.
 
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In other words, maybe all 737 max needed was pilot training....a new type certificate.
I don't think that's "all". If you read through this entire thread and the preliminary reports of the two crashes (which I'd recommend, but it's tough read - as in disturbing), I find it hard to believe you'd feel that way.

Also, it's pretty clear that's not all the max needed by the software updates rolling out. Many of which were suggested early on in this thread. MCAS was a very bad design with an even worse implementation and that's just the tip of this complicated iceberg.
 
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I learned after finally reading the reports that the stab cutout disables all electric and automated control of the trim. Only the trim wheel can be used at that point. So there is no manual control of the trim that is hydraulically or electrically assisted without reactivating the automated pitch trim systems. This is a flawed design approach imo.

EDIT: This turns out is only the case in the Max and not the NG.

Also, I watched a c-span where a FAA official qualifies MCAS as a sub-device of the speed trim system. This was part of the excuse he was giving to a question as to why training wasn't provided.
 
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To your rhetorical question/point regarding "how far do you push an air-frame" via changes. Only to have automation adjust the resulting flight characteristics so the plane "flies" the same (as previous 737's) from the pilots perspective.
I have read many opinions that the air-frame has been pushed way too far with the max and the motivations for Boeing and their clients are obvious. The question is what is the line (EDIT: regarding efficiency / automation vs safety / increased RSS) and does the FAA actually even have one defined.

Not sure about the aerodynamic changes.

From Boeing: "Flight Control Computers and Stability Augmentation
The trend in the design of modern airplanes is to have less static longitudinal stability--frequently referred to as relaxed static stability (RSS)--to capture the benefit of improved fuel efficiency. Simply stated, some airplanes are now designed to be aerodynamically efficient, and stability is augmented electronically so that stick force gradients will meet certification requirements. Many methods exist for augmenting stability. For example, the Boeing 777 and MD-11 use flight control computers that adjust the elevator actuator positions to give the appearance of more longitudinal stability than the airplane actually has. In other words, computers absorb the extra workload caused by flying with RSS. -https://www.boeing.com/commercial/aeromagazine/aero_02/textonly/fo01txt.html

All good and fine until HAL doesn't work as expected, or encounters a scenario it's not programmed for. Or you don't provide feedback of HAL's status to the pilot. Or you don't tell anyone HAL exists and therefore don't train or design for easily disabling HAL. And then what happens when HAL's disabled. Is the plane still airworthy? In all scenarios?

One of the main functions of MCAS was to make the plane "simpler to fly". To make it simpler to certify and get up in the air. It's pretty evident increased RSS and automation add incredible complexity and new failure modes in their effort to keep things simple for the pilot.
 
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In those airplanes, does "manually" mean that there is no hydraulic assistance?
To clarify with my latest understanding, and so @FactChecker perhaps gets a notification: As I posted above

I learned after finally reading the reports that the stab cutout disables all electric and automated control of the trim. Only the trim wheel can be used at that point. So there is no manual control of the trim that is hydraulically or electrically assisted without reactivating the automated pitch trim systems. This is a flawed design approach imo.
 

nsaspook

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I learned after finally reading the reports that the stab cutout disables all electric and automated control of the trim. Only the trim wheel can be used at that point. So there is no manual control of the trim that is hydraulically or electrically assisted without reactivating the automated pitch trim systems. This is a flawed design approach imo.
I don't think so. The stab cutout should be like an EMO switch in most life-safety systems. When you hit that button it needs to be off, period, as you don't know where the problem is, you just need it to stop moving. The root cause flawed design is MCAS overpowering possible manual control by moving trim too far out while going too fast.
 
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I don't think so. The stab cutout should be like an EMO switch in most life-safety systems. When you hit that button it needs to be off, period, as you don't know where the problem is, you just need it to stop moving. The flawed design is MCAS overpowering possible manual control.
The issue I see is that there is no true manual electric control of the trim without having automated trim systems active (EDIT: this is only the case in the Max and not the NG). It's akin in a way to disabling autopilot and all the sudden all hydraulic control is lost (not a perfect comparison). Given the size of the stab and thus the control forces required to move it under a variety of flight scenarios, I think there should be a pathway for the pilot to control it electrically without any automated trim systems. So in an automated sys failure, there is still electric control. Take a look at the jackscrew on the 737, it's huge:

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Listen to that motor. Not something you'd want to be adjusting mechanically under heavy load conditions.

If there was a switch that just disabled automated trim systems. Then in the Ethiopia air case, the pilots potentially wouldn't of been stuck dealing with an immovable trim wheel.

EDIT: And if you look at the preliminary Ethiopian flight report, before the final dive it appears they disengaged the cutouts to try to command electric trim as a last ditch effort (since they could not trim via the trim wheel). It worked, but then MCAS was able to activate again, commanding more nose down trim, effectively making the dive irrecoverable.

Boeing solution is to use the electric trim and then quickly use the stab cut out before any automated systems can command trim. That sounds a lot like a hack to me.
 
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nsaspook

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The issue I see is that there is no true manual electric control of the trim without having automated trim systems active. It's akin in a way to disabling autopilot and all the sudden all hydraulic control is lost (not a perfect comparison). Given the size of the stab and thus the control forces required to move it under a variety of flight scenarios, I think there should be a pathway for the pilot to control it electrically without any automated trim systems. So in an automated sys failure, there is still electric control. Take a look at the jackscrew on the 737, it's huge:

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Not something you'd want to be adjusting mechanically under heavy load conditions.

If there was a switch that just disabled automated trim systems. Then in the Ethiopia air case, the pilots potentially wouldn't of been stuck dealing with an immovable trim wheel.

Boeing solution is to use the electric trim and then quickly use the stab cut out before any automated systems can command trim. That sounds a lot like a hack to me.
The problem is you just don't know what's causing the problem. The electrical assist drive sub-system could be the problem (stuck activator, relay, shorted wire, etc ...) while the automation front-end(s) is/are totally operational. I've seem many videos of manual trim being used for decades on the 737 when it's far out of trim but not at the MCAS driven extremes that seems to have caused the loss of so many lives.
 
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The problem is you just don't know what's causing the problem. The electrical assist drive sub-system could be the problem (stuck activator, relay, shorted wire, etc ...) while the automation front-end(s) is/are totally operational.
Right, so you still need a way to disable electric trim. But I still contend there should be a way to disable automated pitch trim systems without giving up the electrical assist. That would of been really invaluable in these latest tragedies I think. (EDIT: This is exactly how it operated on the NG model, which has speed trim / AP trim systems but not MCAS.) It's really just one more switch. In fact, with the Boeing flight control philosophy being what it is. I'm surprised they take this approach with their automated systems.

This functionality, looked at another way, is also solved by an override that allows direct law control.

I've seem many videos of manual trim being used for decades on the 737 when it's far out of trim but not at the MCAS driven extremes that seems to have caused the loss of so many lives.
Well in the extremes it's going to take a ton of rotations to correct the trim and that's if the load forces are manageable. Which requires time and at least one hand off the yoke if there is no co-pilot to help.
 
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Tom.G

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A three position switch Off-Manual-MCAS.
 
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A three position switch Off-Manual-MCAS.
Sounds simple enough. They may have to label it different than MCAS. Since there is speed trim and mach trim that also command pitch trim. Perhaps "OFF-MAN-AUTO".

Looking at the existing switches:

P9e43.jpg


It looks like the first one cuts out the main electrical assist control and the second cuts out auto pilot commanded trim. To disable MCAS Boeing's instructions are to cutout both. It looks like they tied MCAS into those in such a way that both need to be cut out. My guess is it's tied into MAIN ELECT and it's procedure to cutout AP too just in case.

So either a third switch could be labeled AUTO SYS for the auto trim systems. Or a three position switch on the MAIN ELEC as you've suggested. However the three position switch may not follow best practices with these guarded switches.

EDIT: Or, perhaps they could tie all the auto trim systems into the AP cutout and change the label to just AUTO. Cutting it out would take out all auto to the trim. No new switch required. Could even just black out PILOT. However, maybe there are scenarios where you'd want to cutout the AP and not the speed, mach or MCAS trim systems. (EDIT: After reading Peter Lemme's material, in the NG the AP switch here cuts out the speed trim sys and AP is disabled. Tying MCAS into this AP cutout switch may of been the best direction from a pilot pov for consistency with the NG.)

EDIT 2: The original photo here is from the NG. They are labelled differently in the MAX as PRI and B/U (attached below) and as suggested with the NG you could still command electric trim and disable auto systems. Not so in the Max.

D3ST6ThUUAEurh3.jpg
 
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I think a big part of the issue is concern for pilot information overload and of course additional training. Boeing may view these auto pitch trim systems as bulletproof enough to be seen as augmentation to the feel systems like the elevator feel sys. Given how we've seen they can fail and their pitch authority, that's perhaps not a good standpoint.

I know if I was a pilot of one of these crafts I'd say, "What do the systems do? How much pitch authority do they have? OK. Ya, give me the extra switches please. I'd like to be able to cutout those systems."

One of the changes to MCAS rolling out is to decrease it's pitch authority. I think I'd still want the cutout however.
 
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Found this from Peter Lemme at satcom:
The autopilot trim command on 737NG is either the trim while the autopilot is engaged, or the trim command from the Speed Trim System (STS). MCAS issues commands on the same autopilot interface and would be inhibited by this cutout switch if it were on 737MAX.

For reasons I have yet to understand, Boeing changed the cutout switches on 737MAX to PRI and B/U. In this configuration, both cutout switches are thrown in any runaway situation,

With the 737NG cutout switches, MCAS runaway is stopped by just throwing the autopilot cutout switch, leaving electric trim fully operable.

With the 737MAX cutout switches, MCAS runaway is stopped by throwing both switches, losing electric trim altogether. In this case, the flight crew must rely on manual trim via turning the trim wheel/crank. As discussed above, the manual crank can bind up, making flying much more difficult.
https://www.satcom.guru/2019/04/stabilizer-trim-loads-and-range.html

Some really great information on his site regarding MCAS. Some of his opinions seem to echo much of what's been said here. His credentials are very impressive in avionics engineering so that's a very good thing.
 
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I have a forum related question. Does it make sense to edit previous posts noting an edit has been made. So that that post is stand alone more accurate? Or does it make more sense to just make another post clarifying? It seems the former is fairly tedious and maybe not sustainable in very active discussions.
 
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I don't think that's "all". If you read through this entire thread and the preliminary reports of the two crashes (which I'd recommend, but it's tough read - as in disturbing), I find it hard to believe you'd feel that way.

Also, it's pretty clear that's not all the max needed by the software updates rolling out. Many of which were suggested early on in this thread. MCAS was a very bad design with an even worse implementation and that's just the tip of this complicated iceberg.
A new type certificate would make mcas moot; not required. Am surprised after being told that mcas was installed to avoid the need for a new type certificate that you would not see my point. What's more my comment is qualified with a "maybe". I find it hard to believe you would attempt to dispute a "maybe".
 
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A new type certificate would make mcas moot; not required. Am surprised after being told that mcas was installed to avoid the need for a new type certificate that you would not see my point. What's more my comment is qualified with a "maybe". I find it hard to believe you would attempt to dispute a "maybe".
MCAS is intrinsically faulty as a flight control augmentation system. I think I get that you're saying that it's not required if it fell into a new type. But that's not what happened.

It's in how you frame your argument. When you say maybe, it's not so benign. In this context its refuting my more complex perspective and position. And in effect simplifying the scenario such that if one, more training was provided and two, a new type certification was attained, all would be good. Well, both of those things needed to happen potentially but that doesn't remedy the totality of the problems and challenges that automated systems like MCAS create. And it wouldn't be moot or unnecessary I would argue. I think with those huge LEAP engines, you needed MCAS. Training could potentially supplant it but you'd still be left with an unstable aircraft and instead of MCAS you'd be putting that instability on the pilot to correct for.
 

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