Boeing 737 MAX crash and the rejection of ridiculous data
philip.greenspun.com
Boeing 737 MAX crash and the rejection of ridiculous data
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Re: Boeing 737 MAX crash and the rejection of ridiculous data
#2This 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
Re: Boeing 737 MAX crash and the rejection of ridiculous data
#3Now, 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 from slipping through? It's not easy. These software engineers and others in the process screwed up, but it's a failure of processes and not just forgetting a conditional statement.
Re: Boeing 737 MAX crash and the rejection of ridiculous data
#4Do we actually have the raw data from the sensor in the flight recorder, or do we have the flight computer's account of that sensor's data?
> IF AOA > 15 AND AOA And if the AOA is frozen at 16 due to some fault? What is a loss of signal interpreted as? Does it use last-known value? 0? 100?
How often does the AOA get sampled? Is there any attempt to smooth the data? Was the data corrupted (bit errors) during transmission/reception? Perhaps there's a flaw in the hardware (or microcode) protocol implementation of the data bus (or whatever it would be in this case) doesn't disregard packets with parity errors? Or perhaps it only checks for 1 bit of parity and it needs to check for 3? Or perhaps it's sending int64 and the flight computer is expecting int32.
It's all so simple to make completely uneducated guesses.
When Boeing made such obvious mistakes such as "don't re-command full-nose down after reset" that should have been caught be any competent user acceptance / QA testing, I have to call the entire engineering effort into question. I cannot assume they did even 1 thing correct in this system, and it really needs to have an independent review of the entire system hardware and software implementation.
Re: Boeing 737 MAX crash and the rejection of ridiculous data
#5Now, 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.
Re: Boeing 737 MAX crash and the rejection of ridiculous data
#6I refuse to take any US domestic flights on 737 MAX until it is decomm and engines are replaced with the ones that were intended for its body.
Boeing is not going to fly its execs on domestic flights for certain number of months to regain consumers confidence.
Re: Boeing 737 MAX crash and the rejection of ridiculous data
#7Now, 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't believe me, see pprune.org (real pilots discussing the issue) and https://www.satcom.guru/2018/11/stabilizer-trim.html
This is not a software fault.
Re: Boeing 737 MAX crash and the rejection of ridiculous data
#8With 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.
Re: Boeing 737 MAX crash and the rejection of ridiculous data
#9The problem with this is the idea that this kind of software would ideally be written or specified with "if" statements.
I'm not sure about the argument that AOA values of more that 25° constitute an error, but a more plausible design would be a module that monitors the signal from the AOA sensor and classifies it into various advisory categories. That module could get complex internally but still have a clean interface, providing data of the form (AOA, Advice) where "Advice" would be an indication of the modules' conclusion or recommended action.
Rejecting ridiculous data is potentially a very subtle ML problem. To suggest that it should be taken care of in such a simplistic ad hoc way really doesn't do justice to the problem.
Re: Boeing 737 MAX crash and the rejection of ridiculous data
#10There must be all kinds of neat things one could do with access to the raw sensor data. Perhaps manufacturers should be forced to make it available, so owners can hook up a laptop and run independent analysis software in real-time.
I imagine a frozen Angle Of Attack sensor would stand out like a sore thumb to a neural net that has access to all the sensors. In fact, I imagine it would return a suspiciously constant value even considered on its own.
This doesn't seem all that difficult, really. Teams at engineering universities do more challenging stuff for their master's project.