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Preliminary conclusion MCAS misfired in Ethiopian's 737 max crash

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Re: Preliminary conclusion MCAS misfired in Ethiopian's 737 max crash

#111

Post by Rooster61 »

in_cahoots wrote: post #55
tapland wrote: post #11

Earlier quoted context omitted.

Maybe time was an issue? Saw in an earlier post that the issue had to be mitigated within 40 seconds of it occurring.

That 40 second quote is a bit misleading. As I understand it, if the pilots do absolutely nothing then they only have 40 seconds until MCAS has adjusted the trim the maximum amount. But the Lion Air flight flew for several minutes with the MCAS firing; the pilot manually counter-intervened each time. It was only when he handed control to a co-pilot that the plane nosedived. Theoretically they could have flown indefin…

> Theoretically they could have flown indefinitely while they debugged the problem.

Not true. From what I have read, MCAS runs in 10 second intervals, with the amount of trim added increasing with each successive run. Once the trim has reached a certain point, it over-powers the amount of control the pilot has by putting pressure on the yoke (this controls elevators, which have less overall influence on the aircraft than the trim at full bore).

We know that the pilots on the previous flight with the 3rd deadheading pilot were attempting to right the plane using yoke pressure, and at one point the first officer mentioned that the yoke was "too heavy to hold back". That indicates that MCAS had pushed the trim past the point at which the pilots could physically overcome it's influence using the elevators. (source here: https://www.reuters.com/article/us-ethiopia-airplane-regulat...)

Re: Preliminary conclusion MCAS misfired in Ethiopian's 737 max crash

#112

Post by prolepunk »

just_steve_h wrote: post #92

Here's the design decision that puzzles me the most: MCAS commands nose-down trim. The pilot uses the trim button on the yoke to command nose-up trim. Why isn't that enough to disable MCAS and cause an audible alert? Think of an analogy: when the autopilot commands a nose-down elevator input, and the pilot pulls back on the control column, that's enough to disengage the autopilot. Or think of cruise control in a car:…

From what I understand is that the comparison between cruise control and MCAS breaks down when it comes to how quickly the potential catastrophic situation may happen.

When driver steps on the brakes -- the system disengages and car slows down.

When pilot disables MCAS -- the system disengages and the plane is more likely to enter aerodynamic stall and crash.

So with cruise control there is no potential for catastrophic positive feedback loop when the system disengages, whereas MCAS needs to be pretty aggressive in what it does.

Re: Preliminary conclusion MCAS misfired in Ethiopian's 737 max crash

#113

Post by rubinelli »

salex89 wrote: post #90
fopen64 wrote: post #83

Earlier quoted context omitted.

If I pack air, sell it as panacea, and employ 150k people doing that, do I deserve to be be in business?

If you can actually pay 150k people, maybe you do, since you're bloody good at it! Joke aside, let's be fair, Boeing did not sell air. Their greed got the better of them, and the institutions set to prevent that did not do their job. They should take responsibility for the things they did (or in this case, did not). There is no penalty high enough to bring back lives.

I wish there was a law to nationalize a company that was found guilty of gross negligence, to subsequently pay damages and re-sell them in the market. Make shareholders accountable, instead of sending a couple of middle managers to jail and keeping the same incentives that caused the first tragedy in place.

Re: Preliminary conclusion MCAS misfired in Ethiopian's 737 max crash

#114

Post by briandear »

wp381640 wrote: post #96
just_steve_h wrote: post #92

Here's the design decision that puzzles me the most: MCAS commands nose-down trim. The pilot uses the trim button on the yoke to command nose-up trim. Why isn't that enough to disable MCAS and cause an audible alert? Think of an analogy: when the autopilot commands a nose-down elevator input, and the pilot pulls back on the control column, that's enough to disengage the autopilot. Or think of cruise control in a car:…

> Why isn't that enough to disable MCAS From what I understand it did but it would reengage again. It's the reason why the Lion Air flight lasted longer in the air than Ethiopian, because the pilots in that case kept hitting the trim and buying themselves 40-60 seconds each time

They could have disabled the whole thing if they simply followed the standard runaway trim procedure. A system malfunction should appear to a pilot the same way a runaway trim wheel appears. The result is that there is runaway trim checklist—and a procedure to work around it. You turn off the electric trim and go to a manual reversion. It’s something every 737 pilot trains for.

Here’s some discussion about this from 2009: https://www.airliners.net/forum/viewtopic.php?t=761011

Even if the cause is an MCAS failure, the way that failure presents in the cockpit follows the same procedures as runaway trim: which means to me that the Ethiopian and Lion Air pilots were simply under-trained. The 200 total hours of the Ethiopian first officer supports that assertion. In the US, a first officer has a 1500 hour requirement: having a 200 hour pilot in the cockpit can’t be ignored as a contributing cause especially since following the standard checklists would have saved the plane— if the first officer would have known to consult those checklists. In the US, you can’t even fly Cessnas commercially with 200 hours and this first officer was copiloting a modern 737.

Re: Preliminary conclusion MCAS misfired in Ethiopian's 737 max crash

#115

Post by losvedir »

just_steve_h wrote: post #92

Here's the design decision that puzzles me the most: MCAS commands nose-down trim. The pilot uses the trim button on the yoke to command nose-up trim. Why isn't that enough to disable MCAS and cause an audible alert? Think of an analogy: when the autopilot commands a nose-down elevator input, and the pilot pulls back on the control column, that's enough to disengage the autopilot. Or think of cruise control in a car:…

> MCAS commands nose-down trim. The pilot uses the trim button on the yoke to command nose-up trim. Why isn't that enough to disable MCAS and cause an audible alert?

It does. It's just that MCAS tries again in 5 seconds. That's why the experienced pilots in both the Lion Air and Ethiopian crashes kept the MCAS in check - trimming to neutral column force is second nature to them - and it only crashed when the first officers took over, who allowed it to get to full nose down trim.

A constantly incorrectly trimming system is supposed to trigger the runaway stabilizer trim checklist, so Boeing thought a failure in this system was covered by existing training. Unfortunately, a periodic every-5-seconds trim wasn't recognized as such. I'm not a 737 pilot, so I don't know what runaway stab trim normally looks like; maybe it's usually constant?

Re: Preliminary conclusion MCAS misfired in Ethiopian's 737 max crash

#116

Post by Rooster61 »

rubinelli wrote: post #113
salex89 wrote: post #90

Earlier quoted context omitted.

If you can actually pay 150k people, maybe you do, since you're bloody good at it! Joke aside, let's be fair, Boeing did not sell air. Their greed got the better of them, and the institutions set to prevent that did not do their job. They should take responsibility for the things they did (or in this case, did not). There is no penalty high enough to bring back lives.

I wish there was a law to nationalize a company that was found guilty of gross negligence, to subsequently pay damages and re-sell them in the market. Make shareholders accountable, instead of sending a couple of middle managers to jail and keeping the same incentives that caused the first tragedy in place.

> instead of sending a couple of middle managers to jail

So you want to turn the company over to the same governmental body that lets companies get off by firing a few middle managers? The same body that possibly allowed this lapse in safety to get through regulations in the first place?

Re: Preliminary conclusion MCAS misfired in Ethiopian's 737 max crash

#117

Post by linuxftw »

ams6110 wrote: post #73
linuxftw wrote: post #68

Still doesn't answer the question of, why if MCAS is required during critical portions of the flight to prevent stalling is disabling of MCAS an acceptable solution for this aircraft? Introducing a new condition of "We think MCAS is on, but we're not quite sure / it's cutting in and out due to censor disagreements" is preposterous.

This might be splitting hairs, but MCAS is not there to prevent stalling. It's there to meet a specific certification requirement on control stick forces at high angles of attack. Yes the intent of the requirement is to reduce the tendency of the pilot to pitch up too much near the stall. But MCAS doesn't prevent stalling. Modern aircraft have many automatic systems that are there to make the pilot's job easier. That…

Every source I've found refers to it as an anti stall system, and the system is designed to limit angle of attack (eg, prevent the system from stalling).

It's definitely a safety feature that's required and not an assistive device. It was required for certification, and in fact as originally specified was unsuitable to prevent stalls, so Boeing had to increase the authority of the system to make it functional.

Re: Preliminary conclusion MCAS misfired in Ethiopian's 737 max crash

#118

Post by VBprogrammer »

Rooster61 wrote: post #111
in_cahoots wrote: post #55

Earlier quoted context omitted.

That 40 second quote is a bit misleading. As I understand it, if the pilots do absolutely nothing then they only have 40 seconds until MCAS has adjusted the trim the maximum amount. But the Lion Air flight flew for several minutes with the MCAS firing; the pilot manually counter-intervened each time. It was only when he handed control to a co-pilot that the plane nosedived. Theoretically they could have flown indefin…

> Theoretically they could have flown indefinitely while they debugged the problem. Not true. From what I have read, MCAS runs in 10 second intervals, with the amount of trim added increasing with each successive run. Once the trim has reached a certain point, it over-powers the amount of control the pilot has by putting pressure on the yoke (this controls elevators, which have less overall influence on the aircraft…

The pilot can run the trim in the other direction using the normal switches on the yoke. You can see that if you look at the trim position graph in the Lion Air accident.

For some reason before the crash the pattern of repeatedly correcting changes, possibly because he handed control to the first officer.

Re: Preliminary conclusion MCAS misfired in Ethiopian's 737 max crash

#119

Post by amanaplanacanal »

briandear wrote: post #114
wp381640 wrote: post #96

Earlier quoted context omitted.

> Why isn't that enough to disable MCAS From what I understand it did but it would reengage again. It's the reason why the Lion Air flight lasted longer in the air than Ethiopian, because the pilots in that case kept hitting the trim and buying themselves 40-60 seconds each time

They could have disabled the whole thing if they simply followed the standard runaway trim procedure. A system malfunction should appear to a pilot the same way a runaway trim wheel appears. The result is that there is runaway trim checklist—and a procedure to work around it. You turn off the electric trim and go to a manual reversion. It’s something every 737 pilot trains for. Here’s some discussion about this from…

> A system malfunction should appear to a pilot the same way a runaway trim wheel appears.

I thought the problem here was that it doesn't appear the same.

Re: Preliminary conclusion MCAS misfired in Ethiopian's 737 max crash

#120

Post by lisper »

losvedir wrote: post #115
just_steve_h wrote: post #92

Here's the design decision that puzzles me the most: MCAS commands nose-down trim. The pilot uses the trim button on the yoke to command nose-up trim. Why isn't that enough to disable MCAS and cause an audible alert? Think of an analogy: when the autopilot commands a nose-down elevator input, and the pilot pulls back on the control column, that's enough to disengage the autopilot. Or think of cruise control in a car:…

> MCAS commands nose-down trim. The pilot uses the trim button on the yoke to command nose-up trim. Why isn't that enough to disable MCAS and cause an audible alert? It does. It's just that MCAS tries again in 5 seconds. That's why the experienced pilots in both the Lion Air and Ethiopian crashes kept the MCAS in check - trimming to neutral column force is second nature to them - and it only crashed when the first of…

> I'm not a 737 pilot, so I don't know what runaway stab trim normally looks like; maybe it's usually constant?

Yes, exactly.

The problem (well, one of the problems) is that during normal operations the trim wheels are constantly moving intermittently, so additional intermittent motion isn't immediately seen as unusual.

(Disclaimer: I am not a 737 pilot either, but I am a pilot, I fly a plane with an electric trim (Cirrus SR22) and the info above came from a very experienced 737 pilot named Juan Browne who hosts an excellent channel on YouTube called Blancolirio.)

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