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I am quite interested in the firefighting fluid tank choices in turbofan-equipped aircraft. As seen in this picture:

aircraft apu fire extinguishing agent

Although this is only the APU, but the fire extinguishing agent bottle looks quite small. would it be enough to extinguish a fire? How much extinguishing gas is used up in one firefighting session for one turbofan (small jet sized turbofan) anyways?

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2 Answers 2

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enter image description here
— A320 technical training manual

Even the bottles for a turbofan engine (shown above) aren't big. After all the agent is typically a gas that is stored under very high pressure.

Part 25 regulations state:

The capacity of each required built-in fire extinguishing system must be adequate for any fire likely to occur in the compartment where used, considering the volume of the compartment and the ventilation rate.

As an example, this model for a Boeing on ebay.com holds 28 lbs (12 kg) of agent at 360 psi:

enter image description here

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    $\begingroup$ Also it's Halon 1301. If the bottle was filled with CO2, it would have to be much larger and still would be nowhere near as effective. That's why the cutout for aviation when Halon was banned. Now though there is a switch to a new Halon derivative extinguishant called Halotron that is about as effective without the problematic ozone thing. $\endgroup$
    – John K
    Nov 4, 2021 at 18:57
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    $\begingroup$ Ooops fixed thanks. As it happens I have a halon 1211 3 lb bottle on my desk I took out of my plane because the 12 years was up. I can't get it recertified anywhere. In Canada extinguishers are mandatory in GA aircraft. $\endgroup$
    – John K
    Nov 4, 2021 at 19:00
  • $\begingroup$ Such bottle contains about 12 kg of Halon 1301, with a volumetric mass of 1.54 kg/m3. The bottle then expands to 7 m3. Fire suppression requires a concentration of about 5% in volume. So the bottle can protect about 140 m3. The APU compartment is smaller, this allows to maintain the concentration several minutes. $\endgroup$
    – mins
    Nov 4, 2021 at 19:50
  • $\begingroup$ @FaitoDayo: The material/alloy should be a separate question, and it's probably proprietary to the manufacturer, i.e. there is no regulation specifying what the bottle should be made of. $\endgroup$
    – user14897
    Nov 7, 2021 at 23:35
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To specifically address the small size of the bottle: these fire extinguishing systems use gas to suppress the chemical reaction of the fire. The amount of the extinguishing agent needed to suppress fire is not particularly high. Halon is one example of a common agent used and only needs to reach a concentration of less than 7% of the volume to be effective. As noted in ymb1's answer, the FAR requires the gas to reach a sufficient percentage of the compartment's volume and must remain above that percentage for some time, considering ventilation.

A fire bottle is typically emptied when used to extinguish a fire, so it's sized based on the volume of the compartment it serves. There may be multiple bottles to allow for additional attempts at extinguishing if needed.

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