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The half a millimeter error that nearly cost 469 lives
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NiHaoMike:
https://admiralcloudberg.medium.com/a-matter-of-millimeters-the-story-of-qantas-flight-32-bdaa62dc98e7
I have heard of the near disaster before but the analysis of how the defect causing it happened was new to me. The most unbelievable part was how several loosening of tolerances were just accepted in the aerospace industry. I once worked at a place where any change to regression test code must be justified or it won't be accepted, none of which had to do with safety or security.
EPAIII:
Fantastic read!

A half mm is slightly less than 20 thousandths of an inch for those of you in the US or who otherwise aren't real familiar with metric measure. Or about the thickness of five sheets of ordinary copier paper. Not much to stand between life and death.

In addition to working in electronics I have been a long time, amateur machinist. Even from my first forays in that realm and using amateur quality machines, i was able to easily make parts with a tolerance of a thousandth of an inch or 0.025mm. And can/could measure them to a ten thousandth of an inch or 0.0025mm. An error of half a mm may sound small but, in the reality of a modern machine shop, is actually enormous. And RR was using the very best available machines to make these assemblies and then to measure/inspect them. And that error was made by some of what are supposedly the best engineers and machinists in the business.

I am not trying to sound like a pompous ass here. I too make mistakes, but I usually work alone with no back-up. It is just difficult to understand how so much can go wrong with so many experts involved. People who know far more about designing and building aircraft than I ever will. Perhaps when you work alone you know it is all on you. So you double and triple check everything. You ask questions and aren't afraid of the answers you may get either from yourself or from others. But when working as part of a large organization, there are so many who can be assumed to be backing you up that you get complacent. Is that it? Or are you afraid of admitting you are wrong? Or are afraid of being shouted down if you question things? I don't know.

I can tell you one thing. The fact that no one was hurt or killed. And the fact that the plane survived and was restored to service tells you something about the process. It can't all be wrong. Perhaps it is a wonder that so little does go wrong with these monstrous projects. I am presently struggling with a project that has only about two dozen parts and a circuit as simple as a doorbell.

porter:
Here is an interesting article about the safety of the 737 max. Another look into design decisions and quality control.

HOW THE BOEING 737 MAX DISASTER LOOKS TO A SOFTWARE DEVELOPER


--- Quote ---The flight management computer is a computer. What that means is that it's not full of aluminum bits, cables, fuel lines,
 or all the other accoutrements of aviation. It's full of lines of code. And that's where things get dangerous.

Those lines of code were no doubt created by people at the direction of managers. Neither such coders nor their managers are as in touch with the particular culture
 and mores of the aviation world as much as the people who are down on the factory floor, riveting wings on, designing control yokes, and fitting landing gears.
 Those people have decades of institutional memory about what has worked in the past and what has not worked. Software people do not.
--- End quote ---

https://spectrum.ieee.org/how-the-boeing-737-max-disaster-looks-to-a-software-developer
niconiconi:

--- Quote from: EPAIII on December 15, 2023, 08:56:37 am ---A half mm is slightly less than 20 thousandths of an inch for those of you in the US or who otherwise aren't real familiar with metric measure. Or about the thickness of five sheets of ordinary copier paper. Not much to stand between life and death.

[...] In addition to working in electronics I have been a long time, amateur machinist. Even from my first forays in that realm and using amateur quality machines, i was able to easily make parts with a tolerance of a thousandth of an inch or 0.025mm.

--- End quote ---

The cheapest 2-layer circuit board process get you down to 10 mils traces, a reasonable one can make 6 mils, also that the dielectric layer between the power and ground in a modern 4-layer board is ~4 mils... So yeah, 20 mils is also a bad tolerance in the eyes of electronics workers - it's roughly the width of a power or ground trace to a logic chip on a circuit board.
johansen:
there are millions of other equally interesting failures waiting to happen.

can you find them?
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