Boeing 737 Max MCAS System

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It does seem that MCAS pulls the nose down if the AOA is too high, thereby preventing a stall.
Not really. What the MCAS does is add nose down trim to compensate for the pitch up moment due to the new engines. No compensation is needed at low AoA; it's only needed at higher AoA. But adding nose down trim is not the same as pulling the nose down; the pilot can still push the nose up with the yoke, he just has to push harder if the MCAS is adding nose down trim. So, for example, if the pilot really wanted to stall the aircraft, he could keep pulling harder on the yoke to push the nose up, despite MCAS--and then, if automatic stall prevention were active, it would eventually kick in and force the nose down regardless of the pilot's input. Which is something different from what the MCAS was doing. (All this assumes accurate AoA sensor data.)
 
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Regarding earlier posts about cockpit indications, it looks like Southwest took steps after the Lion Air crash to add an optional avionics package to their 737 MAX aircraft that includes an AoA display in the cockpit to help pilots identify possible AoA sensor errors:

https://theaircurrent.com/aviation-safety/southwest-airlines-is-adding-new-angle-of-attack-indicators-to-its-737-max-fleet/
That seems like a good implementation to the GUI. Sort of surprised it's not there by default.
 
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Not really. What the MCAS does is add nose down trim to compensate for the pitch up moment due to the new engines. No compensation is needed at low AoA; it's only needed at higher AoA. But adding nose down trim is not the same as pulling the nose down; the pilot can still push the nose up with the yoke, he just has to push harder if the MCAS is adding nose down trim. So, for example, if the pilot really wanted to stall the aircraft, he could keep pushing harder on the yoke, despite MCAS--and then, if automatic stall prevention were active, it would eventually kick in and force the nose down regardless of the pilot's input. Which is something different from what the MCAS was doing. (All this assumes accurate AoA sensor data.)
Again, thanks for clarifying.
 
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Sort of surprised it's not there by default.
Yes, this would seem to be exactly the sort of thing that should have been in the base aircraft, not an optional package.
 
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Yes, this would seem to be exactly the sort of thing that should have been in the base aircraft, not an optional package.
I guess that's why "Boeing did not immediately respond to a request for comment."
 

CWatters

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To confirm, are you saying there is a mechanical device, AMT, that controls pitch which the pilot has no control over?
Not quite.

On a typical light aircraft, glider or model aircraft you have a fixed horizontal tail plane with hinged movable elevators behind.

On a supersonic jet fighter you don't have separate tail and elevators, instead its all one surface called an All Moving Tail.

On many passenger jets you have a combination of both. There is a tail plane and separate elevators but both can move. I might be wrong but as I understand it typically the pilot controls the elevators and the electronic flight systems control the angle of the tail plane. The pilot can also control the tail plane via the manual trim wheel.

I have difficulty articulating my concern about this. The AMT part of a passenger aircraft has to have quite a large movement to cope with the wide speed range of modern aircraft. Likewise the range of movement of the elevator has to vary depending on speed and configuration so as to provide enough control at low speed but not too much at higher speeds. There are quite complicated laws/equations that determines how much elevator travel is produced for any given input by the pilot. It's no longer a simple relationship.

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?
 
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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...
 
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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.
 

berkeman

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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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