Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

Boeing 737 MAX crash and the rejection of ridiculous data

philip.greenspun.com

Post reply on HN
Print view View original

31–40 of 194 posts

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

#31

Post by fixermark »

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

He doesn't seem to be an experienced owner of a home outdoor thermometer though.

Or if he is, I need to know where he found one that goes to 452 degrees and why he even needs that much range. ;)

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

#32

Post by falcolas »

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

A pilot is not a programmer. A minor, but important distinction.

EDIT: Aaaand I'm an idiot. My bad.

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

#33

Post by rhinoceraptor »

cpplinuxdude wrote: post #5
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…

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.

You can't perfectly simulate the pilot though, that's the big danger with human/automation hybrids.

As with what potentially happened to these crashes, the operating procedures might not be fully known by the pilot. The simulation might also allow the simulated pilot to do things that humans can't, for example turn the trim wheel when the jackscrew is under heavy load from the nose being pitched down.

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

#34

Post by Gibbon1 »

chupa-chups wrote: post #7
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…

- 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 elevator is under load. That's dodgy, they got away with it because runaway trim is really rare.

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

#35

Post by ronilan »

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.

Down by 4 at the Super Bowl, 2nd & Goal, 27 seconds on the play clock, Lynch in the back field.

  Run the ball
About 10 characters of code. Sometimes armchair quarterbacking has a point.

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

#36

Post by ninkendo »

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...

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

#37

Post by Xixi »

If you read about Air France Flight 447, at some point instruments data went so ridiculously far from expected values that the stall warning stopped, because the computer did exactly what is suggested here: rejecting ridiculous data. So trying to fight the stall would... trigger the stall warning, adding confusion and cognitive load to an already dire situation (IIRC it is very likely, though we will never know, that the pilots didn't quite understand until the very end that the plane had stalled).

The "solution" described in the article seems to add more complexity and surprises to a system (MCAS) that already behaves in surprising and unexpected ways. Engineering is hard, adding a band aid on top of an ill devised clutch without carefully thinking, writing, and peer reviewing all the possible consequences, is unlikely to help.

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

#39

Post by Zak »

falcolas wrote: post #32
js2 wrote: post #19

Earlier quoted context omitted.

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

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.

Post reply on HN