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Watching air crash investigations, and aware of problems with batteries, the issue of fire and sparking is recurrent. As a noble gas, pure helium would not burn, and if I understand correctly would reduce though not necessarily eliminate sparks. While any additional lift would be marginal, it would likely be sufficient to compensate for the materials necessary to ensure a sufficiently sealed envelope.

So... Has anyone considered filling aircraft voids with helium?

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    $\begingroup$ Helium is expensive and limited. As the 2nd smallest element, it is very good at leaking out of any space that isn't 100% sealed (air-tight is not good enough, "helium-tight" is much tighter). $\endgroup$ – abelenky Jun 14 '18 at 14:02
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    $\begingroup$ Also the dielectric strength of helium is less than that of air (if I'm reading the table right) so sparks would be even more likely $\endgroup$ – ratchet freak Jun 14 '18 at 14:13
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    $\begingroup$ Consider replacing "helium" with "helium or nitrogen" in your question. Nitrogen is much more abundant than helium, so nitrogen is usable in a lot of cases where helium is not. $\endgroup$ – Timber Swett Jun 14 '18 at 14:16
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    $\begingroup$ @TannerSwett's right, also N2 will escape much more slowly (a much bigger molecule, much smaller difference in partial pressures) $\endgroup$ – Chris H Jun 14 '18 at 15:41
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    $\begingroup$ Do battery fires even depend on oxygen from the air? Or are they just the battery chemicals reacting with each other? $\endgroup$ – jamesqf Jun 14 '18 at 18:44
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Yes, it has been considered, but with some differences.

First, the inerting gas used is not helium - it's nitrogen. Nitrogen also doesn't support combustion, and is for practical purposes inert. It also has major advantages over helium in this role. For one, nitrogen is very close to air in density, and doesn't leak as easily as helium, so it's easier to retain.

Nitrogen is also much easier to obtain by separating air. It can be done on the ground with nitrogen carried in cylinders, or generated onboard with OBIGGS. If you were to use a more exotic gas, it would be halon, which also breaks the chemical reaction sustaining most fires.

In practice, inerting is only applied to fuel tanks, not to avionics compartments (which house the batteries). One reason is that inerting won't prevent lithium fires: the batteries already contain all the reagents they need.

Nitrogen would prevent a secondary fire, but another reason comes in play: avionics compartments aren't sealed. Fires in these bays, like in the cabin, can be addressed by the crew. This requires insulation for the lithium cells (as has been installed) and fire detection in the compartment to allow response before the secondary fire is out of control.

But, as the first linked paper states, it's certainly possible to improve on that with quick automated fire suppression. Aircraft safety design is a set of tradeoffs and it's an evolving field. It's possible that in some years, as the amount and importance of avionics inevitably increase, more weight will be spent on their fire protection.

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    $\begingroup$ Military fighters do use Halon. Also notable are developmental systems to break down the fuel vapors into CO2. $\endgroup$ – user71659 Jun 14 '18 at 18:11
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    $\begingroup$ @user71659: that link has lots of great info! F16 uses halon, but more modern planes like F35 and some civilian jets use air-separation modules to split bleed air into nitrogen-rich vs. oxygen-rich streams. (Suppressing fire in fuel tanks doesn't require zero oxygen, just low enough that fuel vapour won't combust or detonate.) A tank of Halon replaced a tank of LN2 that had to be refilled before every flight. But ASM replaced both of those by generating it on the fly. $\endgroup$ – Peter Cordes Jun 15 '18 at 4:07
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    $\begingroup$ Also note that depriving crew accessible areas of oxygen is a hazard. Sooner or later, there will be an error and someone will make their way into such a compartment before normal atmosphere has been restored. Even worse if the low-oxygen atmosphere starts to leak into and replace the passenger cabin air... $\endgroup$ – MichaelK Jun 15 '18 at 7:43
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    $\begingroup$ AIUI the cabin floor of airliners is NOT a pressure bulkhead. So there have to be vents between cargo hold and passenger cabin to protect the floor in the case of sudden decompression.. $\endgroup$ – Peter Green Jun 15 '18 at 15:20
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    $\begingroup$ @PeterGreen Yes, those vents are in the side walls behind the side wall liners. At least on Boeing aircraft, the floor does not extend all the way to the skin of the airplane, it stops on the inside edge of the bulkhead, leaving about a 3 inch gap. Some, but not all, of the sidewall liners themselves may have a visible grid of holes near floor level, which improves the air path through this gap. $\endgroup$ – Rayanth Jun 16 '18 at 3:54

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