[dmckeon]

Tragedy from a Swiss cheese failure - several small holes/failures all line up. The issue of halon extinguishers versus oxygen-powered combustion producing many toxins while failing to extinguish is interesting - good that:

> Halon fire extinguishers are scheduled to be phased out of most commercial aircraft by the end of 2025.

[wk_end]

In commercial aviation the vast majority of tragedies are Swiss cheese failures, at this point. The system has worked very hard to optimize out the possibility of any one individual failure leading to catastrophe. But the explosion of potential combinations - along with the extremely low odds of them ever occurring - makes preparing for one of them much less feasible.

[FridayoLeary]

I'm surprised that a catastrophic runaway oxygen leak is possible on an aircraft in todays day and age. Aircraft design is dominated by strict safety regimes that take into account even far fetched scenarios. Putting one valve in the rubber hose sounds sub optimal. Gas station pumps, for example have a valve that closes if the hose gets torn away. (which does happen when people forget about the pump and drive off). From the article it sounds like a button closes the oxygen tank but a pilot sitting next to a flamethrower might not remember to press it. One obvious solution is that the oxygen tank should be activated only in an emergency instead of being on by default during the flight but i assume the current procedure exists for a reason.

[wat10000]

The article says that a risk analysis was done for the system and the risk was found to be “extremely improbable,” meaning between 1 in 100 million to 1 in a billion flight hours.

This flight may have been extremely unlucky, or the risk analysis may have been wrong. This is why the behavior of the Egyptian authorities is so frustrating; the purpose of the accident investigation is to see if there are problems that should be addressed.

[CamperBob2]

Also, even if the risk analysis was right, it didn't justify an "extremely improbable" conclusion. If the global airline industry operates a total of about 50 million flights per year, and the average duration is about 2 hours, then we stand a good chance of seeing an accident like this every few years.

[wat10000]

Reading up a little on the regulations, the FAA defines “extremely improbable” as less than one in a billion per hour, with the goal that a given type of airplane should be unlikely to ever experience a catastrophic failure during its service life.

Of course, there’s more than one type of airplane in the world, so you do have to wonder if that standard is adequate. I didn’t see how they quantify “unlikely,” but if it’s, say, 1 in 10 then the wide range of aircraft types means many of them will experience a catastrophic failure.

I’d expect this stuff to be gradually tightened. The current standard would have been ridiculous and unobtainable some decades ago. As technology and experience advances, there should be room to improve it further.

[masklinn]

The rarity of such events (as outlined by TFA) is probably a major reason, even more so as they seem to generally be caused more by maintenance and from places with less than stellar incident reporting.

While there is some amount of proactivity in aircraft safety I'm not sure there are people with enough free time that they can make up failure modes or trawl through every minor incident report until (again as in the case of TFA) prompted by an actual failure, unless one of the minor incidents is itself proactively raised as a major risk avoided by blind luck.

[userbinator]

instead of being on by default during the flight but i assume the current procedure exists for a reason.

Suppose it fails the other way --- pilot needs oxygen but the valve refuses to open. I think they definitely did a risk analysis and came up with the current design, reasoning that the increased risk of an oxygen fire would be less than the risk of a pilot suffocating if the system failed the other way.

[cyberax]

This really is a non-issue. If you're at the point where toxins from halon pyrolysis are a problem, then you're likely already dead from other factors (heat, smoke, etc.).

Halon can works even at concentrations of just 2-5% by volume. This is entirely safe to breathe for humans. There's a video of a person discharging a halon extinguisher in a room, and then proceeding to try to light a cigarette. The matches go out immediately after striking the matchbox and the lighter can't even ignite.

CO2 extinguishers are really worse, they need to displace most of the oxygen to be effective. Unfortunately, humans also need this oxygen.

In addition, CO2 stream can cool the burning material, but it can also spread it (so be careful if you use it on burning liquids).

[buildsjets]

In testing aircraft installations, the FAA requires you to demonstrate 6% concentration of Halon 1301 at 12 FAA chosen locations in a compartment for 0.5 seconds, simultaneously, when the test is run at sea level and standard temperature. This is to allow for the fact that the halon bottles may be cold soaked down to -60F and that the system needs to be effective at density altitudes down to -1000 feet.

[jabl]

Engine rooms in ships typically have CO2 systems. There's been several fatalities when crew have reentered the space before the CO2 has been ventilated away. Made worse by CO2 being heavier than air so it can remain in some crawlspace under the engine frame or such.

[cyberax]

Yep. Found that safety video: https://youtu.be/NrP5-E9jmas?t=344 - I was misremembering it a bit.

It's really unfortunate that halon is so dangerous for the ozone layer, none of the replacements are as good as it was.

[jabl]

High-pressure water mist systems seem pretty good, including being usable in spaces with lots of electrical stuff like machine rooms and data centers.

https://www.marioff.com/en/

[cyberax]

Second 292 for the demo.

[buildsjets]

No, they are not. Source: me. Also here’s the applicable EU regulation. https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=CELEX%3A... Only handheld portable extinguishers are required to be phased out in 2025, and only in locations that are regulated by EASA. The FAA does not give a rip, and neither to the many regulatory bodies around he world who defer tho the FAA. I don’t know what the CAAC is doing. As far as the FAA concerned, compliance with the Montreal Treaty is the Department of State’s problem. Btw, since there is only one company that has certified a non-halon (2-BTP) handheld, they have jacked the retail/list price up to $2630 compared to an equivalent sized Halon 1211 handheld for $475.

Did you know that in the wrong circumstance a 2-BTP extinguisher will feed a fire rather than extinguish it? It’s a phenomenon called subinerting. One manufacturer blew up an FAA lab pretty badly while testing 2-BTP. Here’s a report on the earth-shattering kaboom. I only got to see he wreckage a few weeks later. https://www.nist.gov/publications/chemical-kinetic-mechanism...

Permanently installed Halon firex systems in commercial aircraft will not be phased out until 2040. I have been working, as a part of larger team, to certify a non-halon based firex in cargo and engine compartments for many many years now. It's been slow going. All commercial aircraft from all manufacturers still use Halon for their permanently installed firex and will continue to do so for the near future. We have put non-halon systems into some military aircraft that go thru a commercial certification, for example the KC46 tanker, but there are some good reasons it would not be the best choice for an actual commercial aircraft. https://www.af.mil/News/Article-Display/Article/740629/kc-46...

If anyone actually gave a rip about ozone depletion, they would ground the F-16 fighter. The F-16 inerts it’s fuel tank ullage space with Halon. Every F-16 flight is a direct injection of pure Halon straight into the stratosphere. Mainline that stuff, feels so good. https://arc.aiaa.org/doi/abs/10.2514/6.1981-1638

[thedrbrian]

>The F-16 inerts it’s fuel tank ullage space with Halon.

why can't they use nitrogen ?

[buildsjets]

Weight, and the small size of the aircraft. F-16s are tiny. It would take a LOT more stored nitrogen gas to dilute the O2 level below the upper explosive limit, than it takes of CF3Br to chemically interfere with combustion. Modern aircraft use continuous nitrogen gas separators that run off engine bleed air to accomplish the task, at the cost of some weight and a ~3-5% increase in fuel burn. There was a study to replace the CF3Br with CF3I back in the 1980s but it was shelved due to technical challenges, which could have been overcome with additional investment.

https://www.nist.gov/system/files/documents/el/fire_research...

[sgerenser]

Thanks for the real world information! Username checks out.

[wkat4242]

That's more because of the ozone layer.

While it's not good to use halon in a semi contained space like an airplane, it is incredibly effective at killing fire. It really sucks the heat out of it. Thus in most cases the fire is killed quickly and not much toxins are produced. This is important too because fire itself produces a lot of lethal toxins too, most people in a fire don't burn to death but get poisoned by the smoke.

So it's a big loss imo. I understand why because it's one of the most potent ozone layer killers. But still.. you're not using the stuff unless you have no other choice. If you're not using it it doesn't end up in the environment.

In this case it didn't work because the cargo bay in question was not fitted with extinguishers if I remember correctly.

[bitwalker]

> It really sucks the heat out of it.

One of the problems with halon, and the write-up mentions this, is that it is super effective at starving the fire of oxygen, but has zero effect on the heat of the fuel that was burning. So the fire goes out, but if oxygen is reintroduced before the fuel has a chance to cool sufficiently, it reignites - and now not only are you back where you started, but you have all the toxic byproducts that burning halon produces, which will kill you in a hurry if you breathe them in.

[cyberax]

> One of the problems with halon, and the write-up mentions this, is that it is super effective at starving the fire of oxygen

That's not actually quite how it works. But yes, the end result is the same. I'll copy-paste my comment from the Medium:

That's NOT how halon works! It's a common misconception, but it's incorrect. In fact, halon doesn't react with pretty much anything, it's very chemically stable. You can mix halon with pure oxygen and it'll just sit there, doing nothing.

This stability is exactly why it works so well. You need only a few _percent_ of halon by volume to stop the fires, not even close to consuming even a fraction of the 21% of oxygen.

Normal oxygen consists of two atoms bonded together (thus "O2"). And fire is spread by oxygen radicals, lone oxygen atoms that have an unpaired electron, eager to make bonds. In a fire, an oxygen radical reacts with a molecule of fuel, and this reaction produces enough energy to create at least one more oxygen radical, sustaining the chain reaction.

But halon has these chlorine and bromine atoms, they are bound tightly to carbon, but not as tightly as oxygen would be. So oxygen radicals have enough energy to displace them and bind to the central carbon atom. But the resulting energy release is not enough to create _more_ radicals, so the chain reaction is stopped.

Moreover, the chlorine radical can then snap back onto another carbon atom (from the fuel source), releasing a bit of energy, but not enough to create a new oxygen radical. And the cycle can repeat again.

[kergonath]

> That's not actually quite how it works.

What you wrote is not contradicting the parent, who just said that it was “super effective at starving the fire of oxygen”. You just described the mechanism. You also contradict yourself by first saying that halon is inert, and then that it neutralises oxide ions by swapping halogens, which is the opposite of non-reactive. The effect of that is that it immobilises reactive oxygen before it oxidises the fuel. And it indeed does nothing to decrease the temperature, which does mean that the fire restarts as soon as oxygen is re-introduced. I know you’re not wrong, but the delivery could be improved.

Anyway, you can elaborate and provide information without disagreeing with the comment you’re replying to. It’s fine, and often informative.

[cyberax]

Typically, "starving of oxygen" means that there's not enough oxygen around anymore. That's how CO2 extinguishers work, for example. They literally remove enough of the oxygen to make the combustion stop.

Halon does NOT remove the oxygen, there's always plenty of it available. Instead, it stops the chain reaction.

> You also contradict yourself by first saying that halon is inert, and then that it neutralises oxide ions by swapping halogens, which is the opposite of non-reactive.

As I said, you can mix halon and oxygen, and they won't react (even if you try to ignite them). Halon is very unreactive, but it's obviously not _totally_ inert like helium.

[userbinator]

As I said, you can mix halon and oxygen, and they won't react (even if you try to ignite them).

That makes me wonder if any of the original designers of the oxygen system considered whether a halon-oxygen mix would've been better than pure oxygen.

[wkat4242]

As far as I understood it reduces temperature also because it boils so easily (very low boiling point). That pulls energy from the fuel. As well as capturing oxygen.

This is why it was used as a refrigerant also.

Also if the fuel is below the auto ignition temperature but above flashpoint it would need another spark to re-ignite.