Prist is the trade name of a mixture containing mainly Diethylene Glycol Monomethyl Ether (DEGMME) and is a common anti-icing additive used in jet fuels. Prist meets MIL-STD MIL-DTL-85470B. Jet fuel itself doesn't begin to congeal until -40 degrees centigrade, and it rarely gets that cold, but there is a certain amount of water suspended in it at all times. As temperatures get colder, the water will propagate out as ice crystals and collect on fuel filter elements eventually blocking them. As little as a tablespoon of water can completely block the flow of fuel through a filter. Prist lowers the freezing point of water to -43 degrees centigrade.

Larger jets do not require Prist as they are equipped with fuel heaters that heat either the fuel itself or the filter elements. The general rule of thumb is that aircraft without fuel heaters require Prist, although it also depends on the expected cruise altitude and type of operation. Some aircraft that do have fuel heaters can also be dispatched with them inoperative as long as Prist is able to be used in the correct concentration.

In aircraft that require it, the use of Prist must be highly precise, the Prist concentration in fuel must be between .10 and .15% for it to be effective. The Prist must also be injected into 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. The Prist supply itself must be kept clean and dry as the Prist is intended to seek out water and any water in it will inhibit this ability. Safe handling of Prist is important as well, Prist is highly toxic so gloves should be worn when handling it undiluted and when fueling wash your hands often.

Correct handling by fuel providers is imperative. In July 2004, a Flight Options Beechjet experienced a double flameout over the Gulf of Mexico. The crew was able to glide to a lower altitude and restart the engines, the aircraft landed safely. FAA Investigators found no mechanical problems with the engines but when fuel samples were taken, Prist concentration was only .02%. It was found that the Prist 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.

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