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A Fuel System Icing Inhibitor (FSII) is a manufactured fluid which, when added to aviation fuels, helps to prevent the formation of ice crystals in filters, fuel lines and other fuel system components.
Simply adding Fuel System Icing Inhibitor FSII after the fuel has been pumped is therefore not sufficient. As aircraft climbs after takeoff, the temperature drops, and any dissolved water will separate out from the fuel. Fuel System Icing Inhibitor FSII dissolves itself in water preferentially over the jet fuel, where it then serves to depress the freezing point of water to -43 °C. Since the freezing point of jet fuel itself is usually in this region, the formation of ice is now a minimal concern. Large aircraft do not require FSII as they are usually equipped with electric fuel line heaters or fuel/ oil intercoolers that keep the fuel at an appropriate temperature to prevent icing. However, if the fuel heaters are inoperable, the aircraft may be still be declared fit to fly, if Fuel System Icing Inhibitor FSII is added to the fuel.
Fuel System Icing Inhibitor FSII is an agent that is mixed with jet fuel as it is pumped into the aircraft. The mixture of FSII must be between 0.10% and 0.15% by volume for the additive to work correctly, and the FSII must be distributed evenly throughout the fuel.
It is extremely important to store Fuel System Icing Inhibitor FSII properly. Drums containing FSII must be kept clean and dry, since the additive is hygroscopic and can absorb water directly from moisture in the air. Since FSII is toxic, a crew member must wear gloves when handling it undiluted. Many FBOs allow Fuel System Icing Inhibitor FSII injection to be turned on or off so that one fuel truck can service planes that do require Fuel System Icing Inhibitor FSII as well as planes that don't. Line crew, however, must be able to deliver FSII when it is needed.