Fuel system icing inhibitor

Fuel system icing inhibitor (FSII) is an additive to aviation fuels that prevents the formation of ice in fuel lines. FSII is sometimes referred to by the registered, genericized trademark Prist. Jet fuel can contain a small amount of dissolved water that does not appear in droplet form. As an aircraft gains altitude, the temperature drops and Jet fuel's capacity to hold water is diminished. Dissolved water can separate out and could become a serious problem if it freezes in fuel lines or filters, blocking the flow of fuel and shutting down an engine.

Chemically, FSII is an almost pure (99.9%) ethylene glycol monomethyl ether (EGMME, 2-methoxy ethanol, APISOLVE 76, CAS number 109-86-4), or glycol monomethyl ether since 1994 diethylene glycol monomethyl ether (DEGMME, 2- (2-methoxy ethoxy) ethanol, APITOL 120, methyl carbitol, CAS number 111-77-3). [1] Prior to 1994, Prist was regulated under the MIL-I-27686E standard, which specified use of EGMME, but subsequently came under the MIL-DTL-85470B, with use of less hazardous DEGMME with higher flash point. [2]

FSII was thought to retard the growth of microorganisms eventually present in the fuel, mostly Cladosporium resinae fungi and Pseudomonas aeruginosa bacteria, known as "hydrocarbon utilizing microorganisms" or "HUM bugs", which live in the water-fuel interface of the water droplets, form dark, gel-like mats, and cause microbial corrosion to plastic and rubber parts. But has since been removed from labeling.

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