Boeing 737 Max MCAS System

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@fresh_42, re post #22, these look like the same group of incidents that were being discussed in the Hacker News thread I linked to. The pilots' comments are very good information.
 

CWatters

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Some thoughts and questions for experts. Please correct me if I have anything wrong. I only ever flew gliders..

1) Inherent stability: Most cambered airfoils have a negative pitching moment (a tendency to pitch the nose down). This is usually countered by a combination of a rearward centre of gravity position and down force on the tail. Problem is down force on the tail creates drag and the wing has to create more lift to compensate which creates more drag. So to improve efficiency and reduce fuel consumption, you ideally want as little downforce on the tail as possible. You can reduce this by moving the centre of gravity further back but that can create stability issues. These stability issues can be fixed using computers.

Q. How far down this path have we gone in passenger aircraft?

2) "The balance of power": On something like a light aircraft pitch trim is effected by a small trim tab, typically much smaller than the elevator. As I understand it most large passenger aircraft have both elevators and a "all moving tail plane" or AMT. Typically the pilot controls pitch via the elevators, trim and flight systems control pitch via the AMT. I understand the AMT has and needs to have quite a large range of movement to cope with different flight configurations (with/without flaps extended for example). In some aircraft if the pilot holds in up elevator flight systems interpret this as an out of trim condition and move the tail to reduce the stick forces the pilot has to apply. I believe this may have been a factor in AF447? In the Lion Air accident MCAS appears to have applied repeated nose down inputs (trim runaway) presumably also via the powerful AMT.

Q. Is anyone else a little uneasy that electronic flight systems have a more powerful control surface (AMT) under their command than the human pilot (Elevator)? I'm sure aircraft designers will say there are systems in place to ensure pilots have full control at all times - but it just feels wrong.
 

Klystron

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[snip] Automation such that it is, if it exceeds the limits of a pilots ability to take over with manual control, leaves us at the whims of software that is at this point not as dependable and rigorous as human experience.[snip]
Crew factors have been a focus for research since the dawn of aviation, indeed one of the founding reasons for NACA, now NASA. Airline flight crews have shrunk over time. For instance Flight Engineers have been replaced by automation and advanced engine designs. Navigators, almost always trained pilots, have been mostly if not entirely replaced on commercial flights by enhanced navigation and communication. Relief pilots and Training pilots ("third seaters") are specific to regulations and airlines but have proved vital in alleviating problems with advice, also available if a pilot becomes incapacitated.

No matter what the cause of the accident, consider the composition of the Ethiopian Airlines crew. Airliner.net reports a 29 year old captain supervising a first officer with ~200 hours flight experience. While a common on-the-job training (OJT) scenario, what happens if the captain is unable to perform?

Managing crew costs are a major economic factor for successful airlines. While analogies with ground transportation are common, even long-haul trucks only have one driver. Transportation unions fight for reasonable hours and relief operators. Autonomous ground vehicles soon followed by surface vessels are rapidly becoming reality. Transport crew sizes have reduced while information to pilots and drivers has increased. As airliner automation improves, two trained pilots may seem economically redundant. Triple redundancy, an important engineering principle, may not have an economical counterpart.
 
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No. No. No. When the plane stalls you must put the nose down to increase airspeed, not get level. As a glider pilot, I'm used to flying at the edge of stall speed for prolonged periods. Adding an engine changes the parameters, but it does not change the basic physics of flight.

It is counter-intuitive at first. If you stall close to the ground, you must immediately push the stick forward to put the nose down. But after training, the counter-intuitive becomes intuitive.
Ahh, right. That makes sense, so you can increase airspeed by pitching down and then increase lift as well.
 
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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.
Right, to clarify, when I say engage, what I mean is the when the MCAS activates erroneously. Such that as we see in the Lionair case, it repeatably tries to pitch down and the pilot fights it trying to climb. My point is that when this system is in error, say during take-off, there very likely is not time to bypass it before the system has pitched the aircraft into an irrecoverable dive. I sounds to me that the MCAS is basically a sub system of the fly-by-wire system, since as you said under manual control it is always active. If that's the case then it looks like we're moving too far from "direct law" with sub systems that make the plane "easier" to fly or more like previous aircraft. It seems that was a very expensive mistake and training for the different flight characteristics would of been a way better direction, irregardless of cost or convenience. I also find it hard to believe that with all the reports we see (including those that didn't result in crashes) it's always a faulty sensor causing MCAS to fail. I think the code should take into account pilot input and altitude. If it detects the pilot is constantly pitching up and the altitude is falling to a dangerously low level, it should automatically shut down without the need for the pilot to deliberately bypass.

I agree this would be a good idea. I don't know what information Boeing's cockpit communicates currently, but I know that in several previous Airbus incidents, one of the findings of the investigation was that the cockpit information system was not communicating information well to the pilots.
It almost sounds like a good fit for an AI. A CGI human face that is reporting to the pilot what the computer is doing at the time. Almost like another member of the flight crew. I was watching some coverage and they said in the one of the reports of the problem the computer was saying "Don't Sink, Don't Sink". If that's true that seems like a pretty ridiculous verbal feedback to a pilot. But maybe I'm wrong and that's following best practices? I mean I would expect that sort of verbiage in a videogame sim. An actual airliner I would expect to say something more specific like, Stall risk, or MCAS pitching down, push to deactivate...etc...

No, it isn't. If a total sensor failure happens, the odds of all three sensors agreeing with each other to within the required tolerance are too remote to worry about. The system I've described would see a total sensor failure as all three sensors disagreeing, and would stop believing any of them and light up a big red light in the cockpit that says "automatic systems disabled because of bad sensors", and the pilot would take over.
Right, I'm still not convinced with all the reports it's always a sensor failure. It could be in the code itself and how it's interpreting that input. Also, "too remote to worry about" sounds like something an engineer might regret stating, but I get your point.

It's not a matter of "use cases" not being known; they are known. Remember we're not talking about aerobatics or military flying or recreation; we're talking about commercial airliners flying between known airports on known routes with known flight profiles. That's a much narrower requirement than "be able to deal with anything that could ever be done with an airplane, better than a human does". But the algorithms do require accurate sensor data, so making sure all sensor data is checked for accuracy before acting on it seems like an obvious design requirement.
I see your point. I do still think you're underestimating the variables and complexity of landing and take-off, weather, unsecured cargo load, possible mechanical failure all with aerodynamically unstable vehicles that need MCAS like systems.
 
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Note: Here I am talking about the incidents I've described where we know what happened, and where MCAS issues were observed, or where uncommanded pitch down events happened in autopilot. We don't know enough yet about the Ethiopian Airlines incident to know what caused it. With Lion Air I think we know more, but I'm not sure MCAS has been definitely fingered as the only root cause--but we know from the flight profile that "the plane stalled and the automatic system prevented the pilot from recovering properly" is not what happened; the plane wasn't stalled at any point.
Right, I'm getting confused with how the MCAS is trying to prevent a stall and what's actually happening is that there is no risk of stall and instead there's basically a battle between the MCAS input and the pilot input, where unfortunately the MCAS prevailed.
 
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Q. Is anyone else a little uneasy that electronic flight systems have a more powerful control surface (AMT) under their command than the human pilot (Elevator)? I'm sure aircraft designers will say there are systems in place to ensure pilots have full control at all times - but it just feels wrong.
To confirm, are you saying there is a mechanical device, AMT, that controls pitch which the pilot has no control over?
 
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Crew factors have been a focus for research since the dawn of aviation, indeed one of the founding reasons for NACA, now NASA. Airline flight crews have shrunk over time. For instance Flight Engineers have been replaced by automation and advanced engine designs. Navigators, almost always trained pilots, have been mostly if not entirely replaced on commercial flights by enhanced navigation and communication. Relief pilots and Training pilots ("third seaters") are specific to regulations and airlines but have proved vital in alleviating problems with advice, also available if a pilot becomes incapacitated.

No matter what the cause of the accident, consider the composition of the Ethiopian Airlines crew. Airliner.net reports a 29 year old captain supervising a first officer with ~200 hours flight experience. While a common on-the-job training (OJT) scenario, what happens if the captain is unable to perform?

Managing crew costs are a major economic factor for successful airlines. While analogies with ground transportation are common, even long-haul trucks only have one driver. Transportation unions fight for reasonable hours and relief operators. Autonomous ground vehicles soon followed by surface vessels are rapidly becoming reality. Transport crew sizes have reduced while information to pilots and drivers has increased. As airliner automation improves, two trained pilots may seem economically redundant. Triple redundancy, an important engineering principle, may not have an economical counterpart.
Very interesting. I for one would pay more for a ticket from an airline that embraces human redundancy and a little over-engineering in the name of robustness and safety.
 
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In this thread alone there seems to be different opinions about failures of AOA sensors in the Lion Air Crash.

Had anybody seen claims of sensor failure on Lion Air that are based on the FDR data rather than speculation by journalists?
 
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In this thread alone there seems to be different opinions about failures of AOA sensors in the Lion Air Crash.

Had anybody seen claims of sensor failure on Lion Air that are based on the FDR data rather than speculation by journalists?
I have not seen the data but on a press conference a ministry of transportation safety or something stated that in the Lion Air case, the MCAS was in error due to a faulty AOA sensor. As I understand the investigation may still be ongoing however.
 
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In this thread alone there seems to be different opinions about failures of AOA sensors in the Lion Air Crash.

Had anybody seen claims of sensor failure on Lion Air that are based on the FDR data rather than speculation by journalists?
Even if the AOA sensor was the cause in the Lion air case, it seems hard to believe that all reported problems with the MCAS is due to sensor failure. For example like the article referenced earlier:
See heading: Pilots on U.S. routes had reported concerns about the Max 8
 
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The more I read about these reports the more I'm less inclined to believe it's always a faulty sensor. I think there is a much deeper flaw in the design of the MCAS system.
You have to be careful to distinguish MCAS issues from autopilot issues. The reports referenced in the NY Times article were about uncommanded pitch down events with autopilot engaged. That's not an MCAS issue. Based on the similarity with the Airbus incidents of uncommanded pitch down, I suspect those autopilot incidents were due to faulty AoA sensor data not being spotted as faulty. But it's true that, since those pilot reports have not been investigated to pin down a root cause, we don't know that for sure. It's possible that your suspicions about other flaws somewhere in the automated systems are correct.
 
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I'm getting confused with how the MCAS is trying to prevent a stall
It's not. Again, there are different automated functions involved that it's important to keep distinguished. We've talked about three of them in this thread:

(1) Autopilot, which basically means getting the plane to a desired altitude, airspeed, and heading and keeping it there. This includes climb to cruise altitude, cruise, and descend to landing.

(2) Automatic stall prevention, which means detecting when the angle of attack is too high and automatically pitching the nose down.

(3) MCAS, which is a manual system designed to constantly input some amount of nose down trim to make the 737 MAX feel to the pilot like previous 737 models.

All three of these functions rely on accurate AoA sensor data, but the functions themselves are separate and are there for different purposes.
 
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It's not. Again, there are different automated functions involved that it's important to keep distinguished. We've talked about three of them in this thread:

(1) Autopilot, which basically means getting the plane to a desired altitude, airspeed, and heading and keeping it there. This includes climb to cruise altitude, cruise, and descend to landing.

(2) Automatic stall prevention, which means detecting when the angle of attack is too high and automatically pitching the nose down.

(3) MCAS, which is a manual system designed to constantly input some amount of nose down trim to make the 737 MAX feel to the pilot like previous 737 models.

All three of these functions rely on accurate AoA sensor data, but the functions themselves are separate and are there for different purposes.
Right, that helps. Thank you for that clarification. I was conflating 2 and 3.

It does seem that MCAS pulls the nose down if the AOA is too high, thereby preventing a stall. So doesn't MCAS overlap with stall prevention in that way?
 
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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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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.
 

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