Fuel System Icing Inhibitor

Fuel System Icing Inhibitor (FSII) is an additive 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.

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Chemically, FSII is an almost pure (99.9%) ethylene glycol monomethyl ether (EGMME, 2-methoxy ethanol, APISOLVE 76, CAS number [109-86-4]), or since 1994 diethylene glycol monomethyl ether (DEGMME, 2-(2-methoxy ethoxy)ethanol, APITOL 120, Methyl Carbitol, CAS number [111-77-3]). [1] Prior to 1994, Prist® was made to conform to MIL-I-27686E standard, which specified use of EGMME, but later it was switched to the MIL-DTL-85470B, specifying use of less hazardous DEGMME with higher flash point. [2] It also retards the growth of bacteria eventually present in the fuel (mostly Cladosporium resinae and Pseudomonas aeruginosa, known as "hydrocarbon utilizing microorganisms" or "H.U.M. bugs", which live in the water-fuel interface of the water droplets, form dark, gel-like mats, and cause microbial corrosion to plastic and rubber parts). EGMME was certified as a pesticide by the EPA, but the requirement changes raised the certification costs meanwhile so DEGMME has no official pesticide certification. DEGMME is a potent solvent, and its high concentrations can damage fuel bladders and filters; long-term storage of FSII-fuel mixtures is therefore suggested against.

Anhydrous isopropyl alcohol is sometimes used for the same purpose.

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 (simply adding 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. FSII dissolves itself in particulate 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 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 FSII is added to the fuel. In aircraft that require it, FSII must be injected into the fuel as it is pumped into the tank; it can't simply be dumped into the tank as the concentration will not be consistent throughout the entire fuel supply.

Storage procedures for FSII are extremely important. Drums containing FSII must be kept clean and dry as the additive is hygroscopic and can absorb water directly from moisture in the air. As well, some brands of FSII are highly toxic so gloves should be worn when handling it undiluted. Many FBOs allow FSII injection to be turned on or off so that one fuel truck can service those planes that do require FSII and those that don't. Line crew must be certain that they are delivering FSII when it is needed though. This was illustrated when a Flight Options Beechjet experienced a double flameout over the Gulf of Mexico in July 2004. The crew was able to glide to a lower altitude and restart the engines, and the aircraft landed safely. FAA investigators found no mechanical problems with the engines but when fuel samples were taken, FSII concentration was only 0.02% It was found that the FSII injector in the refueler at the FBO where the aircraft had taken fuel was either inoperative or the line staff failed to turn it on.

In the video game Portal, the main villain, the artificial intelligence GLaDOS, is said to be a FSII. A presentation found in the game explains that GLaDOS is not only an ice inhibitor but also "probably alive", making it superior to other ice inhibitors.