Boeing 737 MAX crash and the rejection of ridiculous data • Hacker News Board

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

#51

Post by mtnGoat » Wed, Apr 10, 2019, 7:18 PM UTC

js2 wrote: post #19

yongjik wrote: post #8

With all due respect, this sounds too much like the kind of armchair quarterbacking that routinely appears on HN when avionics/politics/astronomy is mentioned, where a lone programmer feels competent enough to criticize an industry for missing "something obvious". I mean, this particular change might have saved the particular 737, but I'd rather hear it from someone who actually knows how 737s fly.

Philip Greenspun is a highly experienced pilot: https://philip.greenspun.com/flying/milestones

I too have hundreds of flights and am also a programmer, but I would not assume having both skills would make you anything more then an armchair quarterback. If you have not written code that runs planes, what knowledge are you basing your idea off of. IMO.

i too also play games, but i wouldn't assume to tell a game developer how to write their code.

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

#52

Post by fixermark » Wed, Apr 10, 2019, 7:18 PM UTC

Zak wrote: post #39

falcolas wrote: post #32

Earlier quoted context omitted.

A pilot is not a programmer. A minor, but important distinction. EDIT: Aaaand I'm an idiot. My bad.

Philip Greenspun is, in fact a programmer. He's probably best known in these circles for the quote "Any sufficiently complicated C or Fortran program contains an ad hoc, informally-specified, bug-ridden, slow implementation of half of Common Lisp." It is possible to be both a pilot and a programmer.

This is referred to in the industry as a "Dr. McNinja scenario."

(... nobody refers to it as that ;) ).

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

#53

Post by oliveshell » Wed, Apr 10, 2019, 7:18 PM UTC

ninkendo wrote: post #36

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…

Fascinating. But it seems like the kind of scenario that would be odd for the pilot, no? You nose down, a stall warning goes off, so you nose back up again? That's exactly the opposite of what a pilot would be trained to do, right? I'd imagine in a car, if my vehicle warned me that I was going too fast when I slowed down, my reaction wouldn't be to speed the vehicle back up to avoid the warning...

I recall sleep deprivation was found to be a factor in the Air France crash. The pilots weren’t thinking clearly.

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

#54

Post by dingaling » Wed, Apr 10, 2019, 7:19 PM UTC

starpilot wrote: post #3

Now, imagine you have 5,000 such checks in millions of lines of flight control system code, many of them interdependent, and you have to fly the ship to test each one. You need to schedule time with the test pilots (who have lives of their own) and get the data dump from IT post-flight. It's aerospace so all this undergoes review, documentation, and signoff, and it all takes time. How do you prevent a single check fr…

> How do you prevent a single check from slipping through?

For a start, by not ramming an essentially new type through all test flying & certfication testing in 17 months before declaring it ready for service.

There is a reason that the Soviet Ministry of Aviation made newly-approved types fly only cargo and mail for a year or more; not just because their techniques were 'unsophisticated' per the Western view but also because they understood that introducing a complex system into a complex environment of weather and human factors couldn't be fully modelled but had to be subjected to prolonged real-World experience.

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

#55

Post by laydn » Wed, Apr 10, 2019, 7:20 PM UTC

Despite all we've read about this subject, are we really still implying that this is a software problem?!

Very basic rules of commercial aircraft design were violated during the development of 737MAX.

No point in listing all the issues here again. Let's just point out one: There should have been more than 2 AoA sensors feeding MCAS. No excuses.

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

#56

Post by chupa-chups » Wed, Apr 10, 2019, 7:20 PM UTC

ninkendo wrote: post #36

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…

Fascinating. But it seems like the kind of scenario that would be odd for the pilot, no? You nose down, a stall warning goes off, so you nose back up again? That's exactly the opposite of what a pilot would be trained to do, right? I'd imagine in a car, if my vehicle warned me that I was going too fast when I slowed down, my reaction wouldn't be to speed the vehicle back up to avoid the warning...

Exactly, and this is what caused the crash. The other pilots simply did not think of one of them trying to pitch up.

This is well sourced, sadly not on wikipedia. But there are countless threads of real pilots.

I guess Air France had some influence.

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

#57

Post by olliej » Wed, Apr 10, 2019, 7:21 PM UTC

I mean of course Arizona does get hot enough to melt street signs: https://www.accuweather.com/en/weather-news/its-so-hot-in-ar...

Maybe those were metal ones a 400+ degrees does happen? I mean survivor bias means we won't have met anyone from Arizona who went out in that heat :D

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

#58

Post by cm2187 » Wed, Apr 10, 2019, 7:21 PM UTC

Diederich wrote: post #26

chupa-chups wrote: post #2

This video from "Mentour pilot" has been deleted (supposedly upon request from boeing): https://www.youtube.com/watch?v=EzgBft-79U8 You can see it here (european version of youtube): https://vimeo.com/329558134 Or more info here: https://news.ycombinator.com/item?id=19627525

Strongly suggest everyone check out the video. It's...kind of shocking.

One thing I don't understand is that if the trim is stuck in a position that pushes the plane down, shouldn't the high speed (and therefore strong air flow) help to put the trim back in a neutral position rather than making it more difficult?

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

#59

Post by fixermark » Wed, Apr 10, 2019, 7:21 PM UTC

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…

Not a pilot yet myself, but I know exactly enough about flight physics and control to suspect that while you are likely correct about the MCAS jack-screw positioning being undesired, the solution is probably not to auto-return it to neutral.

If we assume MCAS is only disabled in an emergency, we don't want the result of cutting out an automated system to be further automatic manipulation of the control surfaces, right? That's one more weird behavior for the pilots to have to keep in their situational awareness while doing the most important step (i.e. flying the plane).

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

#60

Post by oconnor663 » Wed, Apr 10, 2019, 7:21 PM UTC

> Beyond 25 degrees, therefore, it is either sensor error or the plane is stalling/spinning and something more than a slow trim is going to be required.

I am not a pilot, and I'm going to take this pilot at his word. But my first question would be, if we add this additional rule into the system (that runaway trim turns off above 25° AOA), will any pilot ever need to know about this rule?

If the answer is "absolutely not, never" then that's all well and good. But if there's some way-out-there scenario where the plane is wavering between 24° and 26° AOA, and in that scenario the pilot needs to be aware that the computer's trim behavior is switching back and forth between two different laws, then I'd want to ask whether that's presenting pilots with too much complexity.

There's a rule of thumb in software design, when we're thinking about designing a complicated system to solve some messy problem. Will users have to deal with the system getting things wrong? If the answer is pretty much never, then the system can be very complicated if that means doing a good job. But if the answer is that the system won't always be right, and that users will have to step in some of the time, then making the system complicated inevitably means that users will have to learn all that complexity. I wonder if that applies here.

EDIT: Ah yeah, top comment right now is about Air France 447. That's a very good example.