Another (2nd) 737 Max 8 crash

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# 2211919 6-Apr-2019 20:30

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Technofreak:

dafman:
The article states the findings are that the pilots correctly followed the emergency procedures provided by Boeing. It can't be much clearer than that.

Yes the article does say that, in fact it says, and I quote;

The country’s transport minister, Dagmawit Moges, did not cite the aircraft’s controversial anti-stall system by name, but said: “The crew performed all the procedures repeatedly provided by the manufacturer but was not able to control the aircraft.

The final part of Boeing's Operating Instructions state the TRIM CUTOUT switches are to moved to the CUTOUT position and stay there for the rest of the flight.

The only way for the pilots could have repeatedly performed the instructions was to have repeatedly moved the TRIM CUTOUT switches back to the operating position in contravention of the Boeing instructions. No doubt you see the incongruity of what is in that article to what is in the Boeing Instructions? Therefore the pilots cannot have performed the Boeing instructions repeatedly and also done them correctly.

That brings me back to what I said in my other post, the pilots either didn't understand the instructions or didn't follow the instructions correctly.

With all due respect, as you clearly know quite a bit about planes and piloting, I'll rely on the official finding as the authoritative word.

As a general observation, from day one, your posts appear to be consistently anti pilot and pro Boeing over the two disasters. Yet, from what I read, the general consensus seems to be coming down on anti Boeing and pro pilots (but, clearly, more time is required before we will know for sure).

These two crashes have resulted in a tragic loss of life. I just think we need to wait to fall back on facts before attributing blame.

Technofreak

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# 2211993 6-Apr-2019 22:49

dafman:With all due respect, as you clearly know quite a bit about planes and piloting, I'll rely on the official finding as the authoritative word.

As a general observation, from day one, your posts appear to be consistently anti pilot and pro Boeing over the two disasters. Yet, from what I read, the general consensus seems to be coming down on anti Boeing and pro pilots (but, clearly, more time is required before we will know for sure).

These two crashes have resulted in a tragic loss of life. I just think we need to wait to fall back on facts before attributing blame.

The link you provided doesn't go to the official finding. With all due respect, it goes to a news item which isn't authoritative by any means.

Yes, many of my posts may appear anti pilot and pro Boeing, I am quite aware of that. I just want to see some balance. Part of the basis of my posts is to counter some of that general consensus you mention. Unfortunately much of that general consensus has been built up by incorrect news reports.

I am not anti pilot or pro Boeing. Boeing do have shoulder some of the blame. However since there are reports on the FAA database of similar MCAS failures in the US but none that resulted in fatalities, the question has to be asked why have some aircraft crashed but others didn't. One obvious difference is the crew. Was it ability, experience or training?

You are correct we will need to wait for the facts before attributing blame. As I have already said, my bet is there will be fault found with Boeing and with the airlines and possibly the pilots. In the case if the airlines it may be maintenance practices and/or pilot training practices.

If you were to believe general consensus, the only blame to be laid will be laid at Boeing's door. I don't believe that is the case.

P.S

I've just read a news report from an aviation news source with the headline "Report At Odds With Claim That Ethiopian Pilots Followed Boeing Guidance"

From that article

....But the report, which draws no final conclusions, is silent on whether the crew simply didn’t know how to use mechanical manual trim or if trim input was inhibited because the airplane was flying at such high speed. The pilots retained takeoff/climb power throughout the accident sequence. Ethiopian Airlines said that following the crash of a Lion Air MAX 8 last October in Indonesia, the crew was briefed on Boeing-provided information on how to disable MCAS. And although some mainstream news organizations have reported that the Ethiopian pilots followed Boeing’s checklist, the report suggests they departed from it in one key detail: After initially disabling electric trim to isolate MCAS, they reengaged it later, allowing the malfunctioning system to trim the airplane nearly full nose-down.....

....Following Lion Air, Boeing’s guidance for this situation—published in the Ethiopian preliminary report--called for several steps that combine its existing standard runaway trim with the MCAS’s peculiarities. Specifically, the checklist calls for disengaging the autopilot and autothrottles and, if the runaway continues, setting electric trim to the cutout position, disabling electric trim. Boeing said it should remain off for the remainder of the flight. The checklist advises to trim manually with the mechanical wheel and to “anticipate trim requirements.” Following Lion Air, Boeing also said that a significant out-of-trim condition caused by a runaway could first be corrected with electric trim before the cutouts are used. Flight data appears to show that the Ethiopian crew didn’t do this.

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alasta

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# 2212115 7-Apr-2019 08:17

Technofreak:

I've just read a news report from an aviation news source with the headline "Report At Odds With Claim That Ethiopian Pilots Followed Boeing Guidance"

From that article

Following Lion Air, Boeing also said that a significant out-of-trim condition caused by a runaway could first be corrected with electric trim before the cutouts are used. Flight data appears to show that the Ethiopian crew didn’t do this.

This part seems to me to be really important. It may well be a legitimate criticism if they failed to trim the nose up using the buttons on the yoke before activating the cut-out switch.

However if they failed to do this initially, but subsequently re-engaged the electric trim system, then why was it not possible to trim the nose up using the yoke buttons at that point? If the trim wheel wasn't working and Boeing's guidance was that the yoke buttons could overrule MCAS then you would think that temporarily reactivating the electric trim for this purpose would be effective.

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# 2212154 7-Apr-2019 10:14

alasta:

However if they failed to do this initially, but subsequently re-engaged the electric trim system, then why was it not possible to trim the nose up using the yoke buttons at that point? If the trim wheel wasn't working and Boeing's guidance was that the yoke buttons could overrule MCAS then you would think that temporarily reactivating the electric trim for this purpose would be effective.

Good questions.

I'm not sure there is evidence they couldn't trim the nose use using the yoke switches? I'm not sure there is evidence the manual trim wheel wasn't working at least in the early stages of the incident.

When aerodynamic loads on the tailplane increase as can happen with a nose down trim situation the forces required to operate the manual trim wheel can be very high. However I think this situation occurs gradually. I very much doubt the manual trim wheel would have been inoperative in the early stages of this incident.

You would think it could be effective to reactivate the electric trim. But if they needed to reactivate the electric trim then they have misunderstood the process they should have been following. Unless they had realised their mistake they were doomed to repeat the same error and not resolve the situation.

Reactivating the electric trim also by default also re-enables the MCAS system outputs to the trim. It's my understanding the yoke switches will override the MCAS but with the fault they had as soon as the yoke switches are released the MCAS will drive the trim again. This may explain why the aircraft pitched up and down, all the time the speed was increasing as they still had take off power set. Thus increasing the aerodynamic forces on the tailplane.

While the Boeing instructions don't specifically say this, if you understood the trim system properly it should be fairly obvious the CUT OUT switches would need to be operated when the yoke switches were released, or in fact operated prior to releasing the yoke switches. I suspect this didn't happen.

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gzt

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# 2212155 7-Apr-2019 10:25

I've changed my mind about flying Max 8. After reading about the FAA approval process for Max 8 I do not have confidence in Boeing or FAA. The fact that independent experts rejected Boeing's first attempt at software fix does not instill much confidence either.

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# 2212159 7-Apr-2019 10:31

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gzt: I've changed my mind about flying Max 8. After reading about the FAA approval process for Max 8 I do not have confidence in Boeing or FAA. The fact that independent experts rejected Boeing's first attempt at software fix does not instill much confidence either.

One assumes you will also not be flying on any Airbus aircraft either? They have had the exact same issues with their AoA sensors, and software of the plane doing the exact same thing (pointing the nost down) because they believed the plane was in a stall. Airbus replaced faulty sensors on hundreds of aircraft.

Lets not forget here that the plane or the plane software was not the root cause of the issue - that was the AoA sensor on the outside of the aircraft.

dafman

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# 2212274 7-Apr-2019 12:59

sbiddle:

gzt: I've changed my mind about flying Max 8. After reading about the FAA approval process for Max 8 I do not have confidence in Boeing or FAA. The fact that independent experts rejected Boeing's first attempt at software fix does not instill much confidence either.

One assumes you will also not be flying on any Airbus aircraft either? They have had the exact same issues with their AoA sensors, and software of the plane doing the exact same thing (pointing the nost down) because they believed the plane was in a stall. Airbus replaced faulty sensors on hundreds of aircraft.

Lets not forget here that the plane or the plane software was not the root cause of the issue - that was the AoA sensor on the outside of the aircraft.

Except Airbus 320 Neos have, to date, managed to stay in the air.

+1 for not flying in a Max 8 for the foreseeable.

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# 2212298 7-Apr-2019 15:40

sbiddle:

One assumes you will also not be flying on any Airbus aircraft either? They have had the exact same issues with their AoA sensors, and software of the plane doing the exact same thing (pointing the nose down) because they believed the plane was in a stall. Airbus replaced faulty sensors on hundreds of aircraft.

Lets not forget here that the plane or the plane software was not the root cause of the issue - that was the AoA sensor on the outside of the aircraft.

Yes and no

According to what I have read, the AoA sensor on the B737MAX is exactly the same item as has been used for many years on the B737NG: it is the same part number.

But on the B737NG, there have been very, very few occasions when the AoA sensor failed and almost none of causing a fatal crash (one I can think of at Amsterdam), despite over 6,900 air-frames being delivered over twelve years. Yet on the B737MAX, there have been two fatal crashes in five months when only about three hundred and fifty air-frames have been delivered, both crashes apparently caused by MCAS operation in response to "AoA sensor failure".

How can it be that a fairly simple (by modern aircraft standards) device installed in thousands and thousands of air-frames and with a very, very low incidence of incorrect operation for over a decade, suddenly kills two plane-loads of passengers in five months within less than two years of service (first MAX delivery was a MAX 8 on May 6, 2017) and a fleet of just a few hundred air-frames?

There must be some vital difference in the AoA measuring system between the B737NG and the B737MAX, that causes the AoA sensor as being seen to fail relatively extremely frequently on the MAX.
Whether this difference is possibly aerodynamic, or wiring harness, or computer software, or computer hardware I have no idea. What I do know is that it is extraordinarily unlikely to be the AoA sensor itself.

If Boeing just changes the MCAS software so it doesn't kill planeloads of passengers every few tens of thousand of flights, then presumably by the end of this year we will be seeing numbers of MAX aircraft being snagged with "AoA Disagree" every month

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# 2212465 7-Apr-2019 20:59

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How many people can recall AF 447 and QF 72 both A330's that either crashed and killed all on board due to failures of airspeed sensors in one case, or severely injured several passengers due to software issues in the other case. Both incidents were 8 months apart. The A330 is still a popular aircraft providing good service with many airlines.

Those that won't fly on the 737 Max should also consider whether or not they would fly on aircraft like the A330.

With all the attention the 737 Max design is getting right now from the authorities I'd say it will be a very good bet to fly on once it is back in service.

Depending on who the operator was, I would fly on a 737 Max tomorrow, just as I wouldn't fly on any aircraft, no matter the brand or model, flown by some operators.

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Technofreak

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# 2212478 7-Apr-2019 21:46

PolicyGuy:

Yes and no

According to what I have read, the AoA sensor on the B737MAX is exactly the same item as has been used for many years on the B737NG: it is the same part number.

But on the B737NG, there have been very, very few occasions when the AoA sensor failed and almost none of causing a fatal crash (one I can think of at Amsterdam), despite over 6,900 air-frames being delivered over twelve years. Yet on the B737MAX, there have been two fatal crashes in five months when only about three hundred and fifty air-frames have been delivered, both crashes apparently caused by MCAS operation in response to "AoA sensor failure".

How can it be that a fairly simple (by modern aircraft standards) device installed in thousands and thousands of air-frames and with a very, very low incidence of incorrect operation for over a decade, suddenly kills two plane-loads of passengers in five months within less than two years of service (first MAX delivery was a MAX 8 on May 6, 2017) and a fleet of just a few hundred air-frames?

There must be some vital difference in the AoA measuring system between the B737NG and the B737MAX, that causes the AoA sensor as being seen to fail relatively extremely frequently on the MAX.
Whether this difference is possibly aerodynamic, or wiring harness, or computer software, or computer hardware I have no idea. What I do know is that it is extraordinarily unlikely to be the AoA sensor itself.

If Boeing just changes the MCAS software so it doesn't kill planeloads of passengers every few tens of thousand of flights, then presumably by the end of this year we will be seeing numbers of MAX aircraft being snagged with "AoA Disagree" every month

There has been suggestions there had been maintenance performed on the Lion Air aircraft which may have impacted on the accuracy of the AoA vanes and a bird strike damaging an AoA vane on the Ethiopian Airlines aircraft. So there may not be an issue with the AoA sensors.

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# 2212517 8-Apr-2019 07:16

Technofreak:

How many people can recall AF 447 and QF 72 both A330's that either crashed and killed all on board due to failures of airspeed sensors in one case, or severely injured several passengers due to software issues in the other case. Both incidents were 8 months apart. The A330 is still a popular aircraft providing good service with many airlines.

Those that won't fly on the 737 Max should also consider whether or not they would fly on aircraft like the A330.

With all the attention the 737 Max design is getting right now from the authorities I'd say it will be a very good bet to fly on once it is back in service.

Depending on who the operator was, I would fly on a 737 Max tomorrow, just as I wouldn't fly on any aircraft, no matter the brand or model, flown by some operators.

Don't forget the Lufthansa A321 issue a few years ago as well https://avherald.com/h?article=47d74074

What happened was a tragedy, but there is still a lot to suggest the pilots didn't do everything they should have done. I too would happily fly on a MAX tomorrow, but there are certainly airlines I would never fly on regardless of the aircraft type they flew.

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# 2213653 9-Apr-2019 14:27

Technofreak:
Reactivating the electric trim also by default also re-enables the MCAS system outputs to the trim. It's my understanding the yoke switches will override the MCAS but with the fault they had as soon as the yoke switches are released the MCAS will drive the trim again. This may explain why the aircraft pitched up and down, all the time the speed was increasing as they still had take off power set.

Yes. I wonder how an MCAS fault differs from a "conventional" trim runaway. If MCAS applies trim at the same (or lower) rate, then it is less severe, because it does it intermittently. So I'm inferring that MCAS must apply trim faster than a "normal" trim runaway caused by e.g. a stuck yoke switch, i.e. faster than the yoke switches? So holding down the nose-up yoke switch would reduce the rate of nose-down trimming (but not eliminate it) while MCAS was in the nose-down part of its cycle. In between, you would be able to reduce the nose-down trim.

Another possibility I can imagine as a one-time embedded systems programmer is that MCAS made its outputs based on the initial stab position, or some absolute position. So after one cycle it positions the stab 2.5deg down from the initial position, and after another cycle 5deg down from initial. So, even if the pilots moved the stab back up 2.5 deg to neutral after the first cycle, the second cycle would move the stab down to 5deg. And, whether pilots did nothing or got it back to neutral, the third cycle would put it 7.5deg nose-down. And these cycles continue, even when the output to the stab is disabled via the switches. So, re-enabling the switches after 3 cycles would result in 7.5deg nosedown, no matter what position they had got it back to.

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# 2213704 9-Apr-2019 14:56

That's almost exactly what some articles have said is the case plus saying the possibility of being at the max screw position to start with and then going beyond via the automated system and jamming the screw, the end.

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# 2213753 9-Apr-2019 18:11

frankv: Yes. I wonder how an MCAS fault differs from a "conventional" trim runaway. If MCAS applies trim at the same (or lower) rate, then it is less severe, because it does it intermittently. So I'm inferring that MCAS must apply trim faster than a "normal" trim runaway caused by e.g. a stuck yoke switch, i.e. faster than the yoke switches? So holding down the nose-up yoke switch would reduce the rate of nose-down trimming (but not eliminate it) while MCAS was in the nose-down part of its cycle. In between, you would be able to reduce the nose-down trim.

My understanding of how the systems works is the trim jack screw can be powered/operated/commanded by various systems as well as being operated mechanically by the manual trim wheel on the control pedestal. The various systems are things like the trim switches on the control column, the auto pilot, MCAS. These systems control power to a motor that drives the jackscrew.

All systems make the jack screw move at the same speed since they all control the same motor. The MCAS only provides a short burst of trim to pitch the nose down a set number of degrees to increase control column pressure. Operation of the control wheel switches over rides the MCAS for the time the control wheel switches are operated. The MCAS cannot drive the trim in one direction while the control wheel switches are driving it the other way. The jack screw can only turn in one direction at one time. So there is no way the MCAS can move the jackscrew further than the control switches can for the time the control wheel switches are operated.

frankv: Another possibility I can imagine as a one-time embedded systems programmer is that MCAS made its outputs based on the initial stab position, or some absolute position. So after one cycle it positions the stab 2.5deg down from the initial position, and after another cycle 5deg down from initial. So, even if the pilots moved the stab back up 2.5 deg to neutral after the first cycle, the second cycle would move the stab down to 5deg. And, whether pilots did nothing or got it back to neutral, the third cycle would put it 7.5deg nose-down. And these cycles continue, even when the output to the stab is disabled via the switches. So, re-enabling the switches after 3 cycles would result in 7.5deg nosedown, no matter what position they had got it back to.

No, this isn't my understanding of how the MCAS is designed to work. I don't remember the number of the degree the MCAS is designed to pitch the nose down but it's not my understanding that it becomes cumulative as in your hypothesis.

What can happen though, is each time the control wheel switches are operated the MCAS is reset and it will drive again, say another 2.5 degrees as in your example, after the control wheel switches are released.. So if the control wheel switches weren't operated long enough to remove the amount of trim initially provided by the MCAS then progressively the nose could be trimmed further and further nose down.

The Boeing Instructions explain what needs to be done, and if followed this shouldn't happen. I do wonder if some of what they put in their instructions may have been lost in translation, bearing in mind in both cases the accident aircraft were flown by pilots where English isn't their first language.

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# 2214034 10-Apr-2019 03:04

Technofreak:

All systems make the jack screw move at the same speed since they all control the same motor.

All driving the same motor doesn't imply all driving it at the same speed. You can change the speed of a DC motor by PWM, or it could be an AC motor or even a stepper motor.

No, this isn't my understanding of how the MCAS is designed to work. I don't remember the number of the degree the MCAS is designed to pitch the nose down but it's not my understanding that it becomes cumulative as in your hypothesis.

I don't clearly remember the amount of trim at each cycle either. But I do recall that the description of MCAS said that if the first nose-down trim didn't correct the AoA reading after a few seconds, it would command another bunch of nose-down trim. And then a third cycle. And that description could match with my description, especially to a software engineer who wasn't particularly aware of the consequences of that much nose-down trim. But of course I'm just speculating.

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