Fuel System Icing Inhibitor FSII
Fuel System Icing Inhibitor (FSII) is a spec
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.
Fuel System Icing Inhibitor (FSII) is a specially formulated additive designed to prevent the formation of ice in aircraft and diesel fuel systems caused by water contamination. It ensures safe operation in extreme cold conditions by reducing the freezing point of water and dispersing small water droplets in fuel.
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.
- FSII is primarily used in turbine engine fuels to prevent icing in fuel lines and filters at high altitudes and sub-zero temperatures.
- Recommended concentration: 0.10–0.15% v/v.
- Add FSII directly into the fuel tank or during fuel transfer to ensure proper blending.
- Particularly important for aircraft operating in cold and humid climates where water condensation in fuel is common.
- Used in critical diesel generator sets, marine engines, and military vehicles operating in cold regions.
- Prevents ice crystals in fuel filters and lines during cold starts.
- Helps in maintaining reliable fuel flow and combustion efficiency.
- FSII protects against icing during fuel transfer, storage, and pumping operations.
- Can be dosed in bulk fuel storage tanks, ensuring protection across distribution chains.
- FSII also exhibits biocidal activity, minimizing microbial growth at the fuel-water interface, thus reducing fuel contamination and filter plugging.
- Ensures reliable fuel delivery in offshore drilling rigs, ships, and naval vessels operating in cold waters.
- Prevents operational downtime due to frozen fuel lines.
Dosage
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.
NOTE: Dosage varies on various conditions and is subject to change
- FSII must be blended uniformly; partial dosing or poor mixing can lead to localized icing.
- It is not a substitute for proper fuel maintenance; water should still be drained regularly from tanks.
- FSII is consumable – once mixed, it may evaporate or degrade slightly over time, so fuel should not be stored too long without rotation.
Useful Area
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.