Fuel System Icing Inhibitor (FSII)
Fuel System Icing Inhibitor (FSII)
FSII
Prist®
D ice®
Description
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. In many localities, FSII is referred to by the registered, generic trademark "Prist", in others as "D ice".
Discussion
Stringent measures have been put into place, during production, transport, storage and aircraft uplift, to eliminate water from aviation fuel. Despite these measures, the possibility of free water contamination of aviation fuel still exists. In conditions of low temperature and/or at high altitude, any free water in the fuel can crystallize and block fuel filters, fuel pump screens, fuel lines or other fuel system components. These blockages have the potential to cause loss of power, engine rollback or flameout.
In most large commercial aircraft and newer general aviation (GA) business jets, components have been added to the fuel system to help minimize the formation of ice. These components might include electric fuel heaters, fuel system/oil system heat exchangers or fuel system/hydraulic system heat exchangers. However, many older commercial and business aircraft, small GA aircraft and many military aircraft are not fitted with additional components designed to eliminate ice by raising the fuel temperature, often due to cost and/or restrictive weight requirements.
In the event of free water contamination, an FSII acts to prevent the formation of ice crystals. FSII is sparingly fuel soluble, but it dissolves much more easily in water. In the event that the fuel is, or becomes, water contaminated, the water will extract the FSII from the fuel. The icing inhibitor acts as a freezing point depressant, reducing the freeze point of the free water and thus prevents the formation of solid ice crystals which could block components of the fuel supply system. FSII is a mandatory additive in many military aviation fuel specifications. For commercial grade fuels, the addition of FSII is at the discretion of the operator.
Uplift Proccess
To work effectively, the amount, by volume, of FSII added to the fuel should be between 0.10% and 0.15% and it must be evenly mixed with the fuel. As mentioned in the preceding section, although FSII is fuel soluble, it does not readily mix with fuel. Consequently, simply topping of the fuel tank with an appropriate amount of icing inhibitor will not have the desired effect.
FSII must be injected into the fuel as it is uplifted into the aircraft. To accomplish this, most fuel bowsers are fitted with a tank specifically purposed to selectively inject FSII into the fuel stream as it is pumped from the bowser into the aircraft tanks. Refuelling personnel will often ask the crew the question "positive or negative" as they connect the bowser to the aircraft, meaning "do you want fuel with or without FSII".
Safety Issues
Fuel System Icing Inhibitor is a clear, colourless and practically odourless fluid, properties that it shares with Diesel Exhaust Fluid (DEF), another fluid which is becoming more and more prevalent within the airport environment. Whilst FSII is intended as a fuel additive, DEF is NOT. On more than one occasion, the icing inhibitor tank on a fuel bowser has inadvertently been serviced with DEF. The resulting fuel contamination during fuel uplift has resulted in serious in-flight engine problems. For more details, refer to the article Diesel Exhaust Fluid (DEF) as a Fuel Contaminant.
Accidents and Incidents
- B772, London Heathrow UK, 2008 On 17 January 2008, a British Airways Boeing 777‑200ER crash-landed 330 metres short of the intended landing runway, 27L, at London Heathrow after a loss of engine thrust on short final. This un-commanded reduction of thrust was found to have been the result of ice causing a restriction in the fuel feed system. Prompt crew response minimized the extent of the inevitable undershoot so that it occurred within the airport perimeter.
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