Fuel System Icing Inhibitor (FSII) is an aditive to aviation fuels that prevents the formation of ice in fuel lines. FSII is sometimes referred to by the genericized trademark Prist. Jet fuel can contain a small amount of dissolved water that does not appear in particulate form. As an aircraft gains altitude, the temperature drops and Jet fuel's capacity to hold water is diminished. Particulate 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.