Bit flips: How cosmic rays grounded a fleet of aircraft

[chris_va]

I highly recommend finding a cloud chamber (various science museums have them) to visualize just how much radiation is flying around.

Part of my work touches high power switches. I am going to do a bad job relating this story, but one of the power engineers was talking about how electric train switches in EU (Switzerland?) were having triggering issues. These were big MW scale IGBTs, not something you want to false trigger. Anyway, they eventually traced the problem to cosmic rays, and just turned the entire package vertical so the die was end-on to space (the mountains around were shielding the horizontal direction), and the problem went away.

[Neywiny]

Always good to support the IGBT community

[SkiFire13]

> just turned the entire package vertical so the die was end-on to space (the mountains around were shielding the horizontal direction), and the problem went away.

That's a pretty cool solution! For some reason I was expecting something a lot more elaborated

[actionfromafar]

That's very P. K. Dick. Or maybe more Heinlein.

[RankingMember]

It's important to note that this is just Airbus's best guess as to the cause, as there's no smoking gun: they simply exhausted their troubleshooting and were left scratching their heads so this was the "least unlikely" cause they could come up with given the circumstances.

[RealityVoid]

I thought the same, but in a deeper dive into the postmortem, I think it's not a cop out from their side. The report is actually really well done ( I personally was impressed). The reasons it probably was a bit flip is that the CPU did not have edac on it in this instance so bit flips are expected. The consensus mechanism failed in this case and that is what they are updating, because even though the module gave wrong data because of presumably bit flips, the consensus should have prevented the dive.

[DecentShoes]

Just like that Mario 64 speedrunner! People say it's like it's gospel, but it's really just a bunch of peoples best guess. No proof.

[serial_dev]

…but if I respond with this to a user’s bug report, I’m “not taking this seriously”…

[avazhi]

“ The increasing reliance of computers in fly-by-wire systems in aircraft, which use electronics rather than mechanical systems to control the plane in the air, also mean the risk posed by bit flips when they do occur is higher.”

Bit of an understatement. I don’t think there any active passenger airliners in the first world today that aren’t fly-by-wire. The MD-80 was the last of its kind and it’s been out of passenger operation for what, 10 years now?

[Stevvo]

Any Boeing other than 777/787 does not use fly-by-wire.

However, that doesn't illuminate the possibility of these errors. Whilst the flight-controls are mechanically linked, the autopilot/trim is electric, so is still suspectable to bit-flips.

[SoftTalker]

Boeing 717 is still in service and it's essentially an MD-80. Many 737s are in service and flight controls are hydraulic-boosted cable-and-pulley operated; the type design dates to the 1960s.

[BurningFrog]

Don't passenger aircrafts have redundant systems, so if one computer flips, the backup takes over?

[RealityVoid]

Not to mention, the system affected by the bit flips were designed in the 90's AND newer designed systems have EDAC so they are not susceptible to the same kind of issue. Honestly, if you look into the thing, the press coverage of the event is atrocious.

[neko_ranger]

I swear to god I've been got by cosmic rays modifying a bit before when my boot order changes for random reasons

[charcircuit]

I feel like using "Cosmic Rays" as a reason is equivalent to "Aliens". It makes for good clickbait so everyone is fast to point at it as the reason even if there is no reason to actually believe that the bitflip was due to cosmic rays.

[0manrho]

> even if there is no reason to actually believe that the bitflip was due to cosmic rays.

What if there is reason to consider it as it is actually a known, proven, observable phenomenon, especially one with greater likelihood/intensity as you climb in altitude, like planes do, and that likelihood/intensity also scales with solar cycle intensity, which we are currently experiencing the peak of?

Or perhaps you think the Aurora Borealis are because of Aliens too?

[XorNot]

When you do Raman spectroscopy in a lab the software literally has an automatic cosmic ray rejection mode because for autonomy you are very likely to get cosmic ray initiated return signals over the course of a couple of hours.

"If the signal looks amazingly strong but unexpected and sharp, it's probably a cosmic ray" was what I was trained for.

[on_the_train]

Thank you for bringing reason to this topic where everyone is losing their mind when it comes up. Cosmic rays are sexy, They're sciency, but they're not a good explanation when you actually run the math.

Random but flips are pretty much always bad hardware. That's what the literature says when you actually study it. And that's also what we find at work: we wrote a program that occupied most of the free ram and checked it for bit flips. Deployed on a sizeable fleet of machines. We found exactly that: yes there were bit flips, but they were highly concentrated on specific machines and disappeared after changing hardware.

[ExoticPearTree]

> I feel like using "Cosmic Rays" as a reason is equivalent to "Aliens".

This is actually a thing. Cisco had issues with cosmic radiation in some of their equipment a few years back. Same symptoms: random memory corruption, and when they would test the memory everything would check out, but once in a blue moon, the routers would behave erratically.

[financetechbro]

What evidence do you have that this wasn’t due to solar radiation?

[burnt-resistor]

PSA:

0. Always use a) SECDED hardware ECC and b) checksums on network links and I/O everywhere.

1. When unable to 1.a), add (72,64) 8-bits Hamming code per 64-bits (or) N>2 redundancy copies on physically-separate silicon for critical data and code. This is a significant performance hit, but safety is more important in some uses. (Don't neglect the integrity and reliability of code storage, loading, and execution paths either.)

2. Consider using Space Shuttle high-availability, high-reliability "voting" of N identically-designed behavior, possibly different manufacturer system control elements.

[Borrible]

That reminds me of how the manufacturer's customer service department for my car some thirty years ago tried to convince me that the problems with the ignition electronics could also be caused by solar flares. Which could have been the case, of course, but then it would surely have affected other vehicle owners as well. Though, maybe the sun did shine just for me back then, you can never be sure, can't you. I briefly considered consulting an astronomer.

[djmips]

There was a funny story about how sun shining on a UV sensitive electronic component was the root cause of a mysterious failure that was time and day dependent.

[who-shot-jr]

The Universe is Hostile to Computers - https://www.youtube.com/watch?v=AaZ_RSt0KP8

[SwiftyBug]

I thought planes had insane redundancy exactly so stuff like that don´t happen. How can a bit flip cause the system that controls altitude to malfunction like that?

[procflora]

From what I've heard (FWIW), Airbus released a version of the software for one of the flight computers that removed SEU protections (hence grounding affected models until they could be downgraded to the previous version).

There was still hardware redundancy though. Operation of the plane's elevator switched to a secondary computer. Presumably it was also running the same vulnerable software, but they diverted and landed early in part to minimize this risk.

So not just redundancy but layers of redundancy.

[willis936]

Why would you ever expect one bit flip? You have a flip rate and you design your system to tolerate a certain bit flip rate. Assumptions made during requirements establishment were wrong and nature eventually let them know they had negative margin.

[p_l]

Possibility of bit flips from cosmic radiation only really came to fore in 1990s, and some aircraft and parts predate that.

[bdangubic]

  if (cosmic_ray) {
     do_not_flip_bits()
  } else {
     flip_away()
  }

[air7]

Naive question, but can't this be solved with device-level error correction?

[djmips]

It could be like ECC RAM but you'd have to make new hardware.

[MarkusQ]

This is silly. Rapidly refreshing the data that was (presumably) flipped by a cosmic ray last time won't do anything to prevent an error in whatever it hits next time. Unless the theory is that cosmic rays are somehow more likely to hit these particular bits compared to all the millions (billions?) of others in the system...in which case I have a different objection.

[RealityVoid]

What is silly is media coverage of this. The error was in the ADIRU. They are updating the ELAC. The ELAC takes the decision based on multiple data streams from 3 ADIRU units and the issue being fixed is that it took the wrong decision. The ADIRU will probably continue having SEU but it will be fine.

[AlotOfReading]

Not all circuits are equally sensitive. The parts that are known to be sensitive or critical are protected by redundancies and error checking, which are probabilistic protection. You haven't completely eliminated the possibility of corruption, just made it incredibly unlikely. Refreshing your inputs is another form of probabilistic protection focused on mitigating the consequences.

[preommr]

I had no idea this was a real thing - I always thought that xkcd comic[0] was just a random joke.

[0]https://xkcd.com/378/

[nomel]

It's literally one of the reasons ECC RAM exists.

[aruametello]

To dial up the weirdness, sometimes the solar flare activity has spikes (https://www.spaceweatherlive.com/en/solar-activity/solar-fla...) and these have a mild relationship with the odds of having "bitflips" in that timeframe.

we had a "historic bad solarweather" a bunch of years ago and i talked with a cyber cafe operator that "you could have more computers bluescreen on this week than usual".

to me it got really weird when he said later he really did, but honestly its 50/50 that could had been just incidental.

in another note there are some "rather intense" discussions when someone speedrunning a game gets a "unreproducible glitch" in their favor, some claim its a flaw from ageing dram hardware, but some always point that it could be a cosmic ray bitfliping the right bit. (https://tildes.net/~games/1eqq/the_biggest_myth_in_speedrunn...)

[mikestew]

I had no idea this was a real thing

Oooh, in that case I have another xkcd you might like, involving mint candies and soft drinks…

[jessriedel]

I thought some combination of error correction and redundant systems was already widespread in airplanes to prevent cosmic-ray induced errors. (GPT agrees.) What am I missing? I've read multiple articles on this, and none of them address the fact that the problem, at the level of detail described in the article, should have been prevented by technology available and widely deployed for decades.

[pengaru]

> GPT agrees

What do you think this adds? These things are sycophant confident idiots; they will agree and agree they're incorrect at the slightest challenge in the same interaction.

[RealityVoid]

You're missing that the systems were designed in the 90's and they had no edac on them but instead relied on redundancy and a consensus system. The fact bit flips happened is not why they grounded the things and updated sw, they grounded them to address the consensus algorithm in the other CPU that did not get the bit flips.