Boeing 737 Max MCAS System

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A heck of a lot of engineering goes into these systems and I'm sure engineers will say they have thought of all the failure modes and have written procedures for pilots to follow, but when accidents happen we scratch our heads and wonder why the pilots didn't do x or y. Perhaps things are just too complicated?
I think this alludes to the somewhat fuzzy line between these control systems which are computer aided, due to forces required / hydraulics and in turn for force feedback compared to pure fly-by-wire systems. I would suggest the more complex the computer intermediary in the control systems, the closer it is becoming fly-by-wire. Add an autopilot and more complex systems like MCAS (that in this case pilots are not even aware of) and the line is quite blurry indeed.
 
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A very interesting article by a pilot giving good details on not just the 737 MAX MCAS system but the more general subject of automated trim adjustments, how a plane feels to the pilot, certification requirements for systems, and the impact of fly-by-wire systems on all this:

https://airfactsjournal.com/2019/03/can-boeing-trust-pilots/

Note: AFAIK the 737 series as a whole does not have fly-by-wire (FBW). However, MCAS on the 737 MAX introduces some of the issues of FBW by automating the trim adjustment, which makes what is said about FBW in this article relevant to the MCAS discussion.
This was a fascinating article. A pilot in the comment section notes that the articles concise description of MCAS is good: "That pitches the nose down and gives the pilot the stick force to know that he is pulling too close to the stall margin." So it does seem MCAS is closely related to protecting the plane from entering a stall scenario.

The pilot goes on with an interesting comment: "Boeing contends that the standard runaway stabilizer trim procedure is valid; this is not entirely true, since the first step in that procedure is to firmly oppose the control column forces, using the column cutout switches to disable the runaway. However, all of us flying the bird know exactly where the master trim cutout switches are, and I guarantee that at the first indication of an MCAS malfunction, those switches will be shut off in a nanosecond."

It seems here that there is no clear way to clear way to disable the MCAS in the event of a failure. Am I reading this right?
 
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I think this is a very important debate regarding human machine interaction. But I hate conducting it while the actual causes of these accidents are speculative.

Think how foolish all this talk will sound if the final report cites a cause that has nothing to do with MCAS or trim or handling or FBW or autopilot. It reminds me of TWA 800.
 
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I was researching a bit about AI and commercial flight, thinking about the lack of a good interface / verbal / visual feedback to the pilot from MCAS or other systems.
There is an interesting article I came across by wired here: https://www.wired.com/2017/03/ai-wields-power-make-flying-safer-maybe-even-pleasant/

Autopilots ace basic piloting tasks in non-emergency conditions, but outside the straight and level stuff, they suffer.
It seems existing autopilots are fairly limited in the scenarios they can deal with and engineers are looking to AI to build better more robust systems.

My interest however as I alluded to earlier in the thread, is less in making a better autopilot or smarter automated systems, which of course is a necessary development. But instead in AI taking a role in communicating to the pilot the current status of the vehicle, both in a emergency and non-emergency scenarios. This would help ease the information overload burden on the pilot, especially as more and more complex systems are added to the airplane like MCAS. The article seems to suggest this somewhat:

Baomar wants to build an AI-based autopilot that can respond reliably and correctly to whatever's happening, while ensuring the human in the cockpit knows what’s going on.
It closes on this point, which seems relevant to our thread:

Assuming these systems someday clear those regulatory hurdles and roll out to commercial airlines, they could provide a stepping stone between the eras of human pilots and what comes next. The days of stick-and-rudder piloting are rapidly fading as cockpit automation ramps up, and the benefits of flying absent the threat of human fallibility might prove too appealing to resist.

But getting there is half the battle, and the in-between period, with some automation going on and some manual control, will need to be deftly controlled to ensure that pilots can still manage their aircraft well. AI could prove invaluable to plugging that gap.
 
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I think this is a very important debate regarding human machine interaction. But I hate conducting it while the actual causes of these accidents are speculative.

Think how foolish all this talk will sound if the final report cites a cause that has nothing to do with MCAS or trim or handling or FBW or autopilot. It reminds me of TWA 800.
I think no matter how both investigations come out, the discussions we are having in this thread will still be very relevant. The overwhelming thesis of this thread is the dynamic between the automated systems and how those are communicated to the pilot, both during flight in feedback and made aware to the pilot via training or accurate bulletins before flight. Further, the ability for the pilot to circumvent these systems easily and whether or not that is continuing to be feasible as these aircraft become more and more dependent on autonomous or fly-by-wire systems. I think those issues are very relevant even if MCAS wasn't at fault.
 

berkeman

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Is the AMT what looks like flaps on the elevators?
No. The AMT is the whole horizontal tail airfoil. There is an axle that runs down the middle of the airfoil from the plane fuselage to the tip of the tail airfoil, and if you look at the leading edge of the tail airfoil there is a slot in the fuselage for the moving front support. You can see marks painted on the fuselage at the leading edge showing three AMT angles (probably nominal, max up and max down).
 

berkeman

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The AMT is the whole horizontal tail airfoil.
Check out this video. You can see the AMT on these F-14 fighter aircraft during takeoff and landings and other maneuvers. See the land-based takeoff at the beginning of the video, and the carrier launches at 3:00, and the carrier landing at 0:55. Feel free to enjoy the rest of the video as well... :smile:

 
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Check out this video. You can see the AMT on these F-14 fighter aircraft during takeoff and landings and other maneuvers. See the land-based takeoff at the beginning of the video, and the carrier launches at 3:00, and the carrier landing at 0:55. Feel free to enjoy the rest of the video as well... :smile:

Weird, I just watched this movie for the first time a few days ago. A little cheesy and definitely a Navy recruitment video in some ways, but it has some really amazing flight and carrier footage and a neat sci-fi premise.

I see the AMT fluctuating at 0.55. Is that under computer control or is that the pilot or a mixture?
 
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I agree with that. I just prefer to see them in a thread that is not linked to a recent accident.
I understand your sentiment. Let's keep in mind that while the incidents raise these concerns and inquiries, they are not closed cases, and while there is some compelling initial evidence that implies there is something wrong with the plane, it's definitely not conclusive. Whether the aircraft is at fault or not, no one should be prematurely assigning any culpability.
 
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All 737 Max grounded !!

Looks like something nasty has shown up in the flight data...

Truly, advances in safety are too-often bought in blood...
To clarify, I don't think it was the data from the FDR. It seems unclear whether the FAA did this from public pressure and optics or it is in fact from the same data Canada received about the plane altitude during the flight via satellite. The odd thing to me is that I saw the graphs for the erratic rate of decent and climb almost immediately after the crash, which would show the similarity to Lion Air. Perhaps the satellite data confirmed this previous early data or perhaps indeed it was more about optics.
 
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Is that under computer control or is that the pilot or a mixture?
On the F-14 that would be pilot control. The F-14 never got any kind of fly-by-wire. They're all out of service now; I believe the F/A-18, which is the Navy's main jet aircraft now, does have some fly-by-wire controls with computer software involved.
 
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It seems here that there is no clear way to clear way to disable the MCAS in the event of a failure. Am I reading this right?
Not as far as I know. As I understand it, the "master trim cutout switch" referred to disables all automatic trim control, including MCAS. What the pilot is saying is that, now that pilots know about the risks involved with MCAS, at the first sign of any anomaly, instead of going through the steps Boeing says, which they don't trust to actually be effective, they're just going to disable automatic trim control entirely and adjust it by hand, since that way they know what they're dealing with.
 
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All 737 Max grounded !!

Looks like something nasty has shown up in the flight data...

Truly, advances in safety are too-often bought in blood...
It looks like indeed the early information was radar / land based. The new information that lead to Canada and then the FAA grounding the plane was from satellite data from Aireon. Not sure why the early radar data wasn't enough when other authorities felt it was, probably multiple factors involved, but it's safe to say it doesn't look good on Canada or the FAA to not lead on this.

 
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t does seem MCAS is closely related to protecting the plane from entering a stall scenario.
Only in the sense that it is giving the pilots the feedback, through the stick force, about how close the plane is to a stall, that the pilots are used to from previous 737 models. The point of MCAS is that without it, the stick force feedback as a function of angle of attack would be different from what the pilots were used to, so they might misjudge how close to a stall they were.
 
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Only in the sense that it is giving the pilot the feedfback
Is it not also adjusting the pitch in addition to providing feedback?
 
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Is it not also adjusting the pitch in addition to providing feedback?
They aren't two separate things. Giving the feedback means adding more nose down trim, which, if the pilot does not compensate by increasing the force he uses on the yoke, will pitch the nose down. Any adjustment of trim does the same thing: it changes the force the pilot needs to exert on the yoke to maintain a particular pitch attitude.
 
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Not as far as I know. As I understand it, the "master trim cutout switch" referred to disables all automatic trim control, including MCAS. What the pilot is saying is that, now that pilots know about the risks involved with MCAS, at the first sign of any anomaly, instead of going through the steps Boeing says, which they don't trust to actually be effective, they're just going to disable automatic trim control entirely and adjust it by hand, since that way they know what they're dealing with.
This is somewhat confusing. The pilot commenting also says:

Second, for obvious reasons, the control column cutout switches do not disable MCAS, which is different from a runaway stabilizer. In that case, simply opposing the control column force kills the trim motor. There wouldn’t be much point in having the MCAS if the control column switches could disable it.
Is he not implying that MCAS is not effected by the bypass cutouts? When he says "trim motor" is he saying that if the pilot opposes the force by pulling on the yoke it disables MCAS? This seems like it couldn't of been the case looking at what we know from the flight data available. It's pretty clear the pilots were fighting against the pitch down maneuvering by the computer. And we see MCAS continuing to pitch down in spite of continued pilot input.
 
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They aren't two separate things. Giving the feedback means adding more nose down trim, which, if the pilot does not compensate by increasing the force he uses on the yoke, will pitch the nose down. Any adjustment of trim does the same thing: it changes the force the pilot needs to exert on the yoke to maintain a particular pitch attitude.
Wait but then are they not indeed completely correlated and not separate? MCAS adding more nose down trim provides the feedback and also literally is pitching the nose down. It provides feedback, but it also effects the actual pitch / attitude of the plane.
 
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Is he not implying that MCAS is not effected by the bypass cutouts?
Hm, you're right, that is confusing. I think we probably would need to have the specifications of the trim control and adjustment system to know for sure.
 
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MCAS adding more nose down trim provides the feedback regardless of whether the pilot changes the force he's exerting on the yoke or not and also literally is pitching the nose down if the pilot does not change the force he's exerting on the yoke.
See the bolded statements I added. (And note that they apply for any method of adjusting the trim, not just MCAS. The same things would be true if the pilot manually added nose down trim using the trim wheel, or whatever manual trim adjustment system the aircraft has.)
 
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See the bolded statements I added. (And note that they apply for any method of adjusting the trim, not just MCAS. The same things would be true if the pilot manually added nose down trim using the trim wheel, or whatever manual trim adjustment system the aircraft has.)
OK, but isn't it safe to say that in these cases, the pilot would see the plane pitching down and indeed, exerted force on the yoke to correct it? In which case the MCAS should of disabled / trim motor. And perhaps it did, but the faulty sensor or perhaps some other error may have allowed the system to execute the pitch down trim again and again?
 
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isn't it safe to say that in these cases, the pilot would see the plane pitching down and indeed, exerted force on the yoke to correct it?
Not under normal conditions. Under normal conditions, the pilot would feel the feedback and increase the force he exerts on the yoke before there was time for the plane to pitch down. The feedback is a continuous process; it's not a series of discrete events. Under normal conditions, the pilot expects the feedback force to increase as angle of attack increases, so if he wants to pitch the plane up, he will be automatically adjusting the force he exerts on the yoke based on the feedback he expects to receive. The increase in the feedback force as the plane gets near a stall is what he would expect; the fact that it's the MCAS causing it, because he's flying a 737 MAX, instead of the natural pitch moment of an older 737, doesn't make a difference in what he feels or what he does, under normal operation.

This assumes, though, that the system is working properly and has accurate sensor data. The difference with MCAS is what failure modes the system has: if MCAS goes wrong because of faulty sensor data, it could suddenly dial in a large nose down trim while the plane is flying level, or climbing at a constant rate at a stable pitch attitude, when the pilot does not expect any sharp change in the feedback force. Under those conditions, yes, the plane would pitch down, because the pilot would not be expecting the change and wouldn't be adjusting the force he exerts on the yoke to compensate. And "adjusting the feedback force" isn't really a good description of what the MCAS is doing under this failure condition, because the plane's pitch attitude is not actually changing, so the feedback force should not be changing either.
 

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