I’ve had the unfortunate opportunity lately to learn a lot about Prist. I say unfortunate because my learning opportunity revolved around a misfueling incident with the corporate aircraft that I fly. When I say “Prist” I am actually referring to “fuel system icing inhibitor” or FSII. Prist is one of the most commonly used brands so FSII is commonly referred to just as “Prist”. Kind of in the same way that everyone refers to common bandages as “Band-aids”.
Why do we use FSII?
Jet aircraft fly at very high altitudes where it can get very very cold. I routinely, even in the summer, can see -54 deg. Celsiuses, that’s cold. Jet fuel naturally has a small amount of suspended water. To help protect the water in the fuel from freezing, aircraft manufacturers usually will add some kind of fuel / oil heat exchanger to cool the engine oil and heat the jet fuel. They also will sometimes add heater elements in the wings to help keep fuel warm. As an added protection, some manufacturers also require that FSII is to be added to the fuel. There is a specific ratio for this mixture which is usually .10% – .15% by volume.
So exactly is FSII and how does it work?
The brand “Prist” is manufactured by Prist Aerospace Products of Conroe, TX. Besides making anti-icing additves, Prist Aerospace also makes a window cleaner that a lot of people use. So when you hear someone say that they need “Prist” on their windows and “Prist” in their gas, it is the same company, just a different product. Prist is
manufactured to meet military specifications, or MIL-J-85470.
The main ingrident in the Prist brand of FSII is DEGMME or diethylene glycol monomethyl ether. When this stuff is mixed in with jet fuel it migrates to the small amount of water in jet fuel and lowers the water’s freezing point to -46 deg F. According to Tom Burr of Arrow Energy, it does this by actually surrounding the water molecule to keep it from freezing. It is important to note that FSII has to be added to the fuel while fueling and cannot be added afterwards. The reason for this is that FSII is heavier than fuel and will sink to the bottom of the tank and not mix in with the fuel you are trying to treat.
The brand Prist also has some biocidal and pesitcide properities. That means it can help kill bacterial growth in your fuel tanks. We all know that bacteria likes to grow in damp dark places. And nothing is as damp and dark as a fuel tank. DEGMME is a pesiticide, although the manufacturers of Prist say on their website that it has not be certified by the EPA as such (just a guess, but it is probably cost inhibitive).
How and when can Prist be added?
As mentioned before, Prist must be added at the time of fueling. When you buy JetA from Phillips66 it is pre-mixed. That means, the fuel already has the FSII in it. This is usually done either at the loading dock when the fuel truck in uploading the fuel or done when the fuel truck is dumping the fuel to the storage tank. Prist can also be added at the time of fueling or even by spraying it into the fuel stream with an aerosol can.
Most FBOs or fueling centers are required to check for the proper mixture of fuel to Prist daily. This is done using a B2 Test Ki which contains a refractometer. A refractometer is a instrument by which you can take a small amount of fuel, place it on the end of the refractometer and then hold it up to the light. You can then read the specific gravity and the salinty which looking at a corresponding scale will tell you how much FSII is in the fuel.
As always, be sure to consult your POH or AFM for more information on your aircraft requirements for FSII. This can usually be found in your limitations section (section 2) under fuel limitations.
Fly Safe (and if you need it..with FSII!)