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Boeing 737 MAX crash and the rejection of ridiculous data

philip.greenspun.com

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Re: Boeing 737 MAX crash and the rejection of ridiculous data

#171

Post by quanticle »

fixermark wrote: post #46
quanticle wrote: post #29

On the other hand, one of the contributing factors to the Three Mile Island nuclear disaster was the fact that the temperature sensors were showing their maximum programmed value of 280 degrees Centigrade. The actual core temperature was far far higher, but because the engineers designing the reactor never thought of the meltdown scenario, they programmed the temperature gauge to cut off at 280C, rejecting higher rea…

While this is true, the point that it's worth thinking through the nature and consequences of "obviously erroneous" sensor data is still a valuable exercise holds. If the temperature is past 280, generally speaking the same steps to diminish temperature can be taken (... I'm speaking broadly; this may not actually be true of nuclear reactors and if it's not, additional sensors with larger ranges were definitely warra…

I agree. In the case of the temperature sensor, my opinion is that the correct behavior should have been to show no reading, or an error value. At least that way the operators would have known that they didn't know the true temperature inside the reactor. As it stood, they saw high, but constant temperature, combined with rising pressure. That indicated that the water level was rising inside the reactor, so they opened drain valves to let water out. That was exactly the wrong thing to do, and it contributed directly to the severity of the meltdown.

Re: Boeing 737 MAX crash and the rejection of ridiculous data

#172

Post by cjbprime »

Gibbon1 wrote: post #34
chupa-chups wrote: post #7

Earlier quoted context omitted.

- The MCAS system has more trim authority than the usual (assisted) system - It was originally intended to have an authority of +/- 0.6 degrees of trim - Later it was discovered that this it not enough due to the aerodynamic effects of the engines becoming apparent also at lower speeds, which led boeing to increase authority to +/- 2.5 degrees per iteration with unlimited iterations until the maximum range If you don…

Unless I'm wrong disabling the MCAS system leaves the elevator at the current trim as set by the jack screw. That's really not the behavior you want. You want the control to return to neutral. Which probably isn't the way the trim mechanism is designed. This whole thing smells like a bunch of dodgy decisions. Some of them recent and others historical. For instance not being able to manually adjust trim while the elev…

There's no way to "disable MCAS". There's just "disable the stabilizer trim motor". At which point the motor can't help you restore safe trim.

Re: Boeing 737 MAX crash and the rejection of ridiculous data

#173

Post by cjbprime »

yaur wrote: post #94
cpplinuxdude wrote: post #5

Earlier quoted context omitted.

I'm sorry i'll have to mention that the software can be thoroughly tested in simulation flights. Funnily enough i was involved with some virtualisation software used to test booking systems for Airports. If you can virtualise a booking systems, trust me you can virtualise the on-board flights systems.

Is this really true though? From what I have read it sounds like part of the problem is that manually adjusting the trim wheel requires more strength than at least some pilots possess due to the mechanical forces on the plane. I don't think it's reasonable to expect simulators to replicate those types of forces.

But they do! See the Vimeo link elsewhere in these comments.

Re: Boeing 737 MAX crash and the rejection of ridiculous data

#174

Post by anonu »

Ataraxic wrote: post #21

It should be noted in the doomed Air France 447 flight, the plane activated the stall warning because of a high angle of attack that was leading to stall. (thanks pdx for the corrected info) At some point the system rejected the data and stopped the stall warning because the angle of attack was so severe that it considered the data erroneous. This is speculated to have caused the co-pilot to keep pulling back on the…

https://www.vanityfair.com/news/business/2014/10/air-france-...

This is a fantastic article. Many parallels can be drawn with the recent Boeing crashes.

Re: Boeing 737 MAX crash and the rejection of ridiculous data

#175

Post by tuco86 »

weaksauce wrote: post #116
ChuckMcM wrote: post #90

Earlier quoted context omitted.

Your basic point is spot on, it isn't simple. The challenge I think is to keep two things separated, one is the flight control laws that the system is implementing to keep the plane in the air (to the best of its ability), and the other is the situational awareness indicators for the pilots so that they can tell what what the plane is "thinking" about how it is flying (or not). The closest analogy I can come up with…

> It has been reported that this was an "extra price" option for the jet, and it is that choice that makes it feel negligent to me. not quite. the things they offered for extra were the AoA sensor readouts and an annunciator for AoA disagreement. the mcas was not part of that and the mcas was not briefed in the difference training from the other 737 models. the mcas taking input from a single sensor was to blame. it…

That's what I wonder since I heard of the two sensor thing. Why wheren't there 3 sensors or even more. We omit those kinds of failures for the simplest blog style websites. Feed that info into 3 flight controllers for that matter and fall back to Manual flight in case less than two agree.

Re: Boeing 737 MAX crash and the rejection of ridiculous data

#176

Post by tilt_error »

Ataraxic wrote: post #21

It should be noted in the doomed Air France 447 flight, the plane activated the stall warning because of a high angle of attack that was leading to stall. (thanks pdx for the corrected info) At some point the system rejected the data and stopped the stall warning because the angle of attack was so severe that it considered the data erroneous. This is speculated to have caused the co-pilot to keep pulling back on the…

Interesting. So the system under consideration consists of both the airplane and the two pilots. Is this part of the design effort — I guess it is — and how do we even model the combination of technology and humans?

Re: Boeing 737 MAX crash and the rejection of ridiculous data

#177

Post by dramm »

cjbprime wrote: post #167
dramm wrote: post #163

Earlier quoted context omitted.

AoA disagree does not imply airspeed disagree. Indicated airspeed is purely a pitot and static pressure driven measurement. AoA is there largely as part of the anti-stall warning system, e.g. driving the stick shaker/pusher. An important point of having AoA sensors is to provide a stall warning systems that is independent of IAS, you don't want these systems crossed with each other. B737 airspeed disagree I believe j…

I think the Ethiopian preliminary report says they were in IAS disagree for the whole flight too. I don't know what caused the IAS disagree. It played a role in the crash, since its checklist disallowed lowering flaps (which would have disabled MCAS) or throttling back (which would have avoided surpassing Vmo, which likely made manual retrim impossible).

Ah now my addled brain finally wakes up, well maybe. What may be happening here is the ADIRU is doing AoA based static port compensation. It will normally be a small correction, and I was trying to keep that out of the discussion because I thought there there have been reports of erratic airspeed in previous Lion Air flights and that was not jiving with small changes of airspeed I was expecting (and now I'm going to reread the Lion Air preliminary report to check what was reported). But to start with you only need 5 knots to trip the IAS disagree, so sure I'll expect it could trigger that (and also possibly an altitude disagree). But potentially more intersting is what does that usually small static compensation change do when the ADIRU thinks the AoA is really out of whack, does it keep applying the "correction" or does it ignore it? So that question circles back to be similar to what Greenspun is asking in the original article.

Re: Boeing 737 MAX crash and the rejection of ridiculous data

#178

Post by cpplinuxdude »

hw wrote: post #25
cpplinuxdude wrote: post #5

Earlier quoted context omitted.

I'm sorry i'll have to mention that the software can be thoroughly tested in simulation flights. Funnily enough i was involved with some virtualisation software used to test booking systems for Airports. If you can virtualise a booking systems, trust me you can virtualise the on-board flights systems.

No doubt you can test on-board flight systems, but you can't really compare virtualizing a booking system with flight systems, which are much more complicated and delicate. Now if you worked at SpaceX and worked on testing their flight systems, then I'd be more inclined to trust you.

> Now if you worked at SpaceX and worked on testing their flight systems, then I'd be more inclined to trust you

Wow that's harsh. You may want to add a bit of sugar to that; it's not nice addressing fellow developers like that. Just a thought.

Re: Boeing 737 MAX crash and the rejection of ridiculous data

#179

Post by et2o »

chupa-chups wrote: post #93
ChuckMcM wrote: post #90

Earlier quoted context omitted.

Your basic point is spot on, it isn't simple. The challenge I think is to keep two things separated, one is the flight control laws that the system is implementing to keep the plane in the air (to the best of its ability), and the other is the situational awareness indicators for the pilots so that they can tell what what the plane is "thinking" about how it is flying (or not). The closest analogy I can come up with…

This is not the problem. MCAS in itself sadly is the problem. They had to solve the issue that the pitch-up moment shouldn't be accelerating on its own just depending on AoA, which was (aerodynamically) inevitable without automated adjustments due to the placement of the engines. They had to resort to the worst possible cludge, since they weren't even allowed to add new electrical systems (which would have caused a r…

Thank you for sharing that video! Incredible.

Re: Boeing 737 MAX crash and the rejection of ridiculous data

#180

Post by et2o »

VBprogrammer wrote: post #106
sk5t wrote: post #85

Earlier quoted context omitted.

Honest question, is his experience in small aircraft applicable at all to jetliners? How much carryover, beyond core theory of flight dynamics, is there from piloting a single prop to an Airbus or even a regional jet?

He part owns a Pilatus PC-12, which is pretty much a single engine regional jet. It isn't your grandfather's Cessna.

Turboprop
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