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

[cyboman]

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.

 

[cyboman]

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.

 

[PeterDonis]

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.

 

[PeterDonis]

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.

 

[cyboman]

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.

 

[PeterDonis]

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.

 

[cyboman]

Only in the sense that it is giving the pilot the feedfback

Is it not also adjusting the pitch in addition to providing feedback?

 

[PeterDonis]

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.

 

[cyboman]

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.

 

[cyboman]

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.

 

[PeterDonis]

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.

 

[PeterDonis]

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.)

 

[cyboman]

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?

 

[cyboman]

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.

Is this helpful? http://www.b737.org.uk/mcas.htm

 

[PeterDonis]

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.

 

[berkeman]

or a mixture?

Fly-by-wire plus some enhancement by the computer probably.

For example, as I understand it, in launch mode off a carrier, fighter jets are put into an automatic launch mode, where the computer flys the jet for the first few seconds. That's because the g-forces from the launch can possibly compromise the pilot's ability to control the takeoff well. In-cockpit videos often show the pilot grabbing onto handles near the baseline of the canopy for the launches.

So in general, fly-by-wire computers take the pilots' inputs and process them to best control the aircraft. Some advanced aircraft are too unstable to control with just human control (not commercial aircraft).

 

[PeterDonis]

Is this helpful?

This says "Using electric pitch trim will only pause MCAS, to deactivate it you need to switch off the STAB TRIM SUTOUT switches." This seems to indicate that the cutout switch the pilot comment we were discussing referred to does disable MCAS.

 

[berkeman]

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?

"See" it how? What if they are climbing out through clouds, and there is no visible horizon? What do pilots use to keep their situational awareness (SA or Sierra Alpha) with no visible horizon?

https://upload.wikimedia.org/wikipe..._Horizon.jpg/184px-VMS_Artificial_Horizon.jpg

 

[cyboman]

This says "Using electric pitch trim will only pause MCAS, to deactivate it you need to switch off the STAB TRIM SUTOUT switches." This seems to indicate that the cutout switch the pilot comment we were discussing referred to does disable MCAS.

Right, it also has a note underneath that that says, "High control forces may be needed to correct any stabilizer nose down trim already applied." Then it says to use electronic trim control to correct it first, then cutout, then manual can be used after.

This sounds ridiculously cumbersome in the event of the MCAS operating in error, where the plane can literally be in a dive at fast airspeed. Really I think what we're going to see is a complete revamp of the MCAS system if it can even be redesigned effectively.

In fact this is alluded to in the March 11, Continued Airworthiness Notification to the International Community (CANIC):

Reviewed Boeing’s production processes related to the AOA vane and Maneuvering Characteristics Augmentation System (MCAS)

Ongoing oversight activities by the FAA include:

- Boeing’s completion of the flight control system enhancements, which provide reduced reliance on procedures associated with required pilot memory items. The FAA anticipates mandating these design changes by AD no later than April 2019.

- Design changes include:

 MCAS Activation Enhancements
 MCAS AOA Signal Enhancements
 MCAS Maximum Command Limit

The last design change is something I suggested along with the minimum altitude threshold, basically that if the pilot is continually pitching upward and MCAS keeps sending a pitch down command it should be smart enough to know something is wrong. Hence "MCAS Maximum Command Limit"

It may be the design of the MAX 8 was so aggressive to save on fuel that software or MCAS can't reliably create safe dependable flight controls in all scenarios. For Boeing sake, hopefully that's not the case. But given their revenue yearly, it's a cost they could afford and recover from.

 

[cyboman]

"See" it how? What if they are climbing out through clouds, and there is no visible horizon? What do pilots use to keep their situational awareness (SA or Sierra Alpha) with no visible horizon?

https://upload.wikimedia.org/wikipe..._Horizon.jpg/184px-VMS_Artificial_Horizon.jpg

View attachment 240271

Right, that's the wrong verbiage. A better term would be "determine".

Yes, I imagine that is a very mission critical instrument to a pilot. Not many failure modes for that sucker I imagine.

 

[Tom.G]

Here are a couple excerpts from the Los Angeles Times newspaper published Thur. Mar 14, 2019, FAA relents, grounds 737Max planes, pg1 continued on pg5. These excerpts are from pg5. I couldn't find the article online, hence no link given.

Referring to the grounding of the 737 Max.
Daniel K. Elwell, acting administrator of the FAA, told CNBC that the decision was made based on enhanced satellite data that showed the track of the Ethiopian Airlines flight was "very close" to that of the Lion Air 737 Max flight that crashed off Indonesia in October.
"We don't make decisions about grounding aircraft... without actionable data. In this case the actionable data did not arrive until today."

Initial data about the Ethiopian Airlines flight were "very incomplete" and "raw" he said. The flight took place in an area that is not served by radar, so the FAA used satellite data that had to be enhanced.
...
The FAA is now awaiting data from the black box of the Ethiopian Airlines plane as the agency decides how long to keep the 737 Max airliners grounded.

 

[cyboman]

The flight took place in an area that is not served by radar, so the FAA used satellite data that had to be enhanced.

Interesting, I don't think this statement is accurate. As I posted earlier with the youtube link from CBC, they said initial flight data was from ground radar.

 

[Tom.G]

And excerpts from the New york Times. https://www.nytimes.com/2019/03/14/world/boeing-737-max-ethiopian-airlines.html


Pilots were abuzz over publicly available radar data that showed the aircraft had accelerated far beyond what is considered standard practice, for reasons that remain unclear.

“The thing that is most abnormal is the speed,” said John Cox, an aviation safety consultant and former 737 pilot.

“The speed is very high,” said Mr. Cox, a former executive air safety chairman of the Air Line Pilots Association in the United States. “The question is why. The plane accelerates far faster than it should.”
...
Flight 302 was just three minutes into its flight, the person said, and appeared to have accelerated to even higher speeds, well beyond its safety limits.


The article also shows a map indicating that radar visibility was available only for the first half of the flight, about three minutes.

 

[cyboman]

And excerpts from the New york Times. https://www.nytimes.com/2019/03/14/world/boeing-737-max-ethiopian-airlines.html


Pilots were abuzz over publicly available radar data that showed the aircraft had accelerated far beyond what is considered standard practice, for reasons that remain unclear.

“The thing that is most abnormal is the speed,” said John Cox, an aviation safety consultant and former 737 pilot.

“The speed is very high,” said Mr. Cox, a former executive air safety chairman of the Air Line Pilots Association in the United States. “The question is why. The plane accelerates far faster than it should.”
...
Flight 302 was just three minutes into its flight, the person said, and appeared to have accelerated to even higher speeds, well beyond its safety limits.


The article also shows a map indicating that radar visibility was available only for the first half of the flight, about three minutes.

Ahh I see so the satellite looks like it filled in the 3 minute gap. Still it is inaccurate for the FAA to say the flight took place where there was no radar available, it just wasn't for the final 3 minutes of the flight.

This is from 4 days ago: https://www.theguardian.com/world/2...ght-et302-visual-guide-to-what-we-know-so-far

The graph shows unstable vertical airspeed, much like Lion air. This is likely what had other authorities ground the planes before the FAA.

 

[cyboman]

Reading here: https://www.washingtonpost.com/worl...-common/?noredirect=on&utm_term=.10ad37d7d68b

The initial findings from the preliminary Aircraft Accident Investigation Report on the Lion Airflight 610 crash states that the AoA sensor was replaced and tested on October 28th. That's a day before the fatal flight. That means the sensor failed a day after being replaced. That seems like a pretty high failure rate, especially if we trust that it was indeed tested as stated. Is that not extremely unlikely for the AoA sensor to fail after one day of use? Unfortunately, they didn't test the sensor the next day. Likely because it was just replaced.

“Black-box data released by Indonesian investigators showed that the pilots were pulling back on the control column, attempting to raise the plane’s nose, with almost 100 pounds of pressure before they crashed,” The Post reported.

That sounds incredible, I suppose it was both pilots pulling on their yokes together as hard as they could. I read elsewhere that in the Lion Air flight the MCAS (allegedly) pitched the nose down 26 times during the climb.

 

[cyboman]

And excerpts from the New york Times. https://www.nytimes.com/2019/03/14/world/boeing-737-max-ethiopian-airlines.html


Pilots were abuzz over publicly available radar data that showed the aircraft had accelerated far beyond what is considered standard practice, for reasons that remain unclear.

“The thing that is most abnormal is the speed,” said John Cox, an aviation safety consultant and former 737 pilot.

“The speed is very high,” said Mr. Cox, a former executive air safety chairman of the Air Line Pilots Association in the United States. “The question is why. The plane accelerates far faster than it should.”
...
Flight 302 was just three minutes into its flight, the person said, and appeared to have accelerated to even higher speeds, well beyond its safety limits.


The article also shows a map indicating that radar visibility was available only for the first half of the flight, about three minutes.

I'm going to hypothesize that the high air speed was due to repeated pitching down maneuvers. As the pilot struggled to regain control of the plane I would guess he was more concerned with the negative pitch attitude pointing the plane into the ground rather than correcting the air speed. Keep in mind this is all happening within a few minutes.

 

[cyboman]

On another note, why would the MCAS only look at the AoA sensor when performing such a flight critical control correction?

Should it not be comparing attitude pitch or artificial horizon data as well? If it did, it surely would be able to calculate that the AoA sensor is in error.

 

[jim hardy]

reading those pilot's reports at ycombinator that Peter Donis linked in post# 2

i note randmoness in just when this occurs
and pilots report it happens with autopilot ON , and to my understanding MACS is supposed to be OFF when on autopilot.

https://news.ycombinator.com/item?id=19373707

ACN: 1597286

"Day 3 of 3 departing in a MAX 8 after a long overnight. I was well rested and had discussed the recent MAX 8 MCAS guidance with the Captain. On departure, we had strong crosswinds (gusts > 30 knots) directly off the right wing, however, no LLWS or Micro-burst activity was reported at the field. After verifying LNAV, selecting gear and flaps up, I set "UP" speed. The aircraft accelerated normally and the Captain engaged the "A" autopilot after reaching set speed. Within two to three seconds the aircraft pitched nose down bringing the VSI to approximately 1,200 to 1,500 FPM. I called "descending" just prior to the GPWS sounding "don't sink, don't sink." The Captain immediately disconnected the autopilot and pitched into a climb. The remainder of the flight was uneventful. We discussed the departure at length and I reviewed in my mind our automation setup and flight profile but can't think of any reason the aircraft would pitch nose down so aggressively."

ACN: 1597380

"It was day three of six for me and day three with very good FO (First Officer). Well rested, great rapport and above average Crew coordination. Knew we had a MAX. It was my leg, normal Ops Brief, plus I briefed our concerns with the MAX issues, bulletin, MCAS, stab trim cutout response etc. I mentioned I would engage autopilot sooner than usual (I generally hand fly to at least above 10,000 ft.) to remove the possible MCAS threat.

Weather was about 1000 OVC drizzle, temperature dropping and an occasional snow flake. I double checked with an additional personal walkaround just prior to push; a few drops of water on the aircraft but clean aircraft, no deice required. Strong crosswind and I asked Tug Driver to push a little more tail east so as not to have slow/hung start gusts 30+.

Wind and mechanical turbulence was noted. Careful engine warm times, normal flaps 5 takeoff in strong (appeared almost direct) crosswind. Departure was normal. Takeoff and climb in light to moderate turbulence. After flaps 1 to "up" and above clean "MASI up speed" with LNAV engaged I looked at and engaged A Autopilot. As I was returning to my PFD (Primary Flight Display) PM (Pilot Monitoring) called "DESCENDING" followed by almost an immediate: "DONT SINK DONT SINK!"

I immediately disconnected AP (Autopilot) (it WAS engaged as we got full horn etc.) and resumed climb. Now, I would generally assume it was my automation error, i.e., aircraft was trying to acquire a miss-commanded speed/no autothrottles, crossing restriction etc., but frankly neither of us could find an inappropriate setup error (not to say there wasn't one).

With the concerns with the MAX 8 nose down stuff, we both thought it appropriate to bring it to your attention. We discussed issue at length over the course of the return to ZZZ. Best guess from me is airspeed fluctuation due to mechanical shear/frontal passage that overwhelmed automation temporarily or something incorrectly setup in MCP (Mode Control Panel). PM's callout on "descending" was particularly quick and welcome as I was just coming back to my display after looking away. System and procedures coupled with CRM (Resource Management) trapped and mitigated issue."

my guess is some little software routine is not re-entrant and occasionally corrupts a global variable someplace
that's only a guess... my only basis is the randomness of the occurrence. It just looks like software to me.

Time will tell.

old jim

 

[jrmichler]

Here's some more information: http://knkt.dephub.go.id/knkt/ntsc_aviation/baru/pre/2018/2018 - 035 - PK-LQP Preliminary Report.pdf.

 

[jedishrfu]

Example:
Air France 447 crashed because the flying pilot held full back-pressure on the control stick and stalled the plane from cruise until it hit the ocean about 4 minutes later. The flight control system had a stall-prevention system, but it was receiving faulty airspeed indication, so it disconnected that feature. It's difficult to know what the pilot was thinking, but it is possible he didn't realize it was possible to stall the plane.

NOVA did an excellent documentary on this crash. It shows how terribly wrong things can go with extreme weather conditions, failed sensors and reliance on too much automation.