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Some aircraft fuel tanks are fitted or retro-fitted with internal bladders, to prevent fuel evaporating inside the tank. I've heard of this being a requirement for at least one type, perhaps because of a poor fuel tank design, but how commonplace is it? How many aircraft have these bladders?

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    $\begingroup$ In Australia there were a string of fatal accidents involving Robinson R44 helicopters. The crashes were survivable but the old rigid fuel tanks burst and the fire caused the fatalities. As a result the authorities mandated that all R44s be retrofitted with the new bladder-like fuel tanks. atsb.gov.au/newsroom/news-items/2015/… $\endgroup$ – Ben Jun 21 '15 at 12:14
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According to this wikipedia article :

Most jet fighters and all US military rotary wing aircraft have some type of self-sealing tanks. Military rotary wing fuel tanks have the additional feature of being crashworthy. High altitudes require the tanks to be pressurized, making self-sealing difficult. Newer technologies have brought advances like inert foam-filled tanks to prevent detonation. This foam is an open cell foam that effectively divides the gas space above the remaining fuel into thousands of small spaces, none of which contain sufficient vapour to support combustion. This foam also serves to reduce fuel slosh. Major manufacturers of this technology include Hutchinson, Amfuel (Zodiac) (formerly Firestone), Meggitt (formerly Goodyear), Robertson Fuel Systems, GKN USA and FPT Industries. FPT is now part of GKN. For military use, tanks are qualified to MIL-DTL-27422 (includes crashworthiness requirements) or MIL-DTL-5578 (non-crashworthy). An aircraft fuel tank sometimes consists of several interconnected fuel cells. The interconnecting hoses are typically also self-sealing.

In addition to fighter aircraft some military patrol vehicles and armoured VIP limousines feature self-sealing fuel tanks.

Self-sealing fuel tanks using military technology are also required in some motorsport categories.

Given the fact anti explosion tanks were invented in 1917, and are in use since then, and have undergone many technological advances since then, they are being used quite commonly and in most of the planes where pressurization is required.

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