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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.
At high altitude and/or low temperatures, free water in aviation fuels can crystallize and block fuel filters. Despite the stringent measures which have been put in place to eliminate water from aviation fuels, the possibility of free water contamination still exists.
In civilian aircraft, fuel heaters can help to minimize the formation of ice in fuels. However, military aircraft do not generally use these heaters, due to restrictive weight requirements.
Fuel System Icing Inhibitor (FSII) acts to prevent the formation of ice crystals, providing additional security in the event of free water contamination in aviation fuel. It is a mandatory additive in many military aviation fuel specifications.
Fuel System Icing Inhibitor FSII dissolves sparingly in fuel, but more easily in water. Any water present will extract FSII from the fuel; the additive then acts to reduce the freeze point of the free water, preventing the formation of solid ice crystals which could block the fuel supply system.