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Among the three different types of hydraulic fluid types (mineral based/petroleum based, synthetic esters, naturally occurring vegetable oils) which one is used in the aviation industry and why?

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  • $\begingroup$ Define "better". The companies that promote these witches brews generally propose stuff that is as complicated and proprietary as possible, because the more proprietary it is, the more profit everybody makes. Often simple substances are the best choice for the consumer, but nobody makes a profit selling cheap generic materials that anybody can get at a hardware store, like mineral oil. So, are you talking about what is good for the manufacturers or the operators? $\endgroup$ – Tyler Durden Oct 23 '17 at 18:23
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    $\begingroup$ Better for what? Brake systems? Ground trucks? This is way too broad. $\endgroup$ – Ron Beyer Oct 23 '17 at 18:49
  • $\begingroup$ Way too broad? OP mentions aviation industry, is that not within the scope of the Aviation Stack Exchange? $\endgroup$ – Koyovis Oct 24 '17 at 1:04
  • $\begingroup$ @Koyovis it's closed as too broad, not off topic. $\endgroup$ – fooot Oct 24 '17 at 2:03
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    $\begingroup$ Apparently lemonade is a good choice $\endgroup$ – TomMcW Oct 27 '17 at 17:42
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For the aviation industry, flammability is a major concern, and aviation hydraulic systems require Fluids that cannot burn yet don't create corrosion and still lubricate. Skydrol for instance is a phosphate based ester, a nasty substance that irritates the skin, but at least has some flame resistance.

Old hydraulic motion systems on simulators used the stuff, later ones went to mineral oils like Shell Tellus. If there is an oil leak in a pressure hose, a fine mist of fluid droplets appears, flammable if mineral oil but this could be handled in a ground facility with extinguishers everywhere. Not a good circumstance in an aviation accident though, and that is why in aircraft the esters have to be used.

Vegetable oils are not used for hydraulic systems in the aviation industry at this moment as far as I am aware.

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    $\begingroup$ It all comes from Swissair Flight 306. That accident moved the authorities to demand only flame resistant hydraulic fluids. By the way, here is more inside info on that accident. $\endgroup$ – Peter Kämpf Jan 17 '18 at 21:46

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