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.