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What is Prist and why is it used?

Posted by on May 30, 2008 13 Comments Category : Flight Instructor Blog

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!)


  1. Djuno on Sep 08, 2008

    Excelent explanation.
    In the LearJet 35, we usually has the requirement to apply the FSII Prist, in flights above FL300, longer than 1H30Min, when the fuel is not premixed, or if we are refuelling the aircraft with a company wich we do not know about its reliability.
    Thank a lot.

  2. instructor on Sep 08, 2008

    Glad you enjoyed it. The Premier 1 that I fly has a requirement that Prist is to always be used regardless of the flight conditions or altitude.

  3. LRJetDriver on Apr 02, 2009

    Excellent explanation. You included a lot of facts, descriptions and key points that make the idea of “Prist” extremely easy to understand. Nicely Done!

  4. John L. Lee on May 15, 2009

    I always use Prist in the outers on the MU 2

  5. Bruno mad on Aug 26, 2009

    Use ICE-5 from Aero-Sense Belgium.
    Much cheaper and same certifications!

  6. Wayne Odegard on Aug 27, 2009

    As discussed, “Prist” is a trade name desribing the chemical used as a fuel system icing inhibitor, or fuel additive.

    The best selling fuel additive in the General Aviation industry is trade named “Dice”, which is manufacturered for and provided around the world by Aviation Laboratories. “Dice” is chemically equivalent to “Prist”, with the same approvals and quality standards. It is available in 5 gallon pails, 55 gallon drums, totes, and bulk.

  7. cheap gas price on Mar 19, 2010

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  8. Jimbo on May 01, 2010

    “According to Tom Burr of Arrow Energy, it does this by actually surrounding the water molecule to keep it from freezing.”

    Tom better get a new job. Unless this “prist” generates heat, you get cols prist with an ice-cube inside of it. Water still freezes at 0ºC.

  9. Jerry Anderson on Oct 14, 2010

    The formulation for Prist Hi-Flash used to be an ethylene glycol compound. Due to a change in EPA regulations in the 1990’s, the additive formulation was changed to diethylene glycol monomethyl ether. This new formulation has no anti-microbial properties.

    The most recent data from the IATA fuel microbiology community indicates that under the very best of circumstances, DiEGME (diethylene glycol monomethyl ether) can only slow the growth of microorganisms by about 40%. Again, that’s best case scenario.

    Regardless of how often you operate, its important to regularly perform fuel system analysis. The test kits we recommend are from Conidia Bioscience. They are by far the most popular, accurate and easy to use. These kits are sold through Satair. Based on what you find, treat with an approved biocide.

    There are only 2 biocides approved for use in aviation globally – Biobor JF and Kathon FP 1.5. Biobor JF has been the standard for many years. Its safe and easy to use. Kathon FP 1.5 is an oxidizer and is highly corrosive. I would discourage anyone from putting this product into an aircraft fuel system.


  10. Vince Latona on Nov 25, 2011

    Nice job….The POH says that Prist is required to be used in my TBM. Both the 700 and the 850.

  11. Kay Smith on Apr 24, 2012

    Thank you for a clear and concise summary. Having flown only piston helis and turbines in Africa, I had never heard of prist until a fuel stop on a flight yesterday in an EC155.
    According to the manufacturer’s website:
    On the molecule encapsulation:
    “When applied correctly, in conjunction with the fuel being maintained correctly, PRIST® Hi-Flash™ will stay in solution indefinitely. PRIST® Hi-Flash™ will not change the freeze point of fuel; it simply encapsulates water droplets that form in fuel, altering their freeze point.”

    On biocidal properties:
    PRIST® Hi-Flash™ is not a “tank cleaner” or a “fuel purifier”, it will not eliminate water nor will it replace the need for good housekeeping of fuel which requires daily sumping and removing of water and particulates. Lastly, PRIST® Hi-Flash™ IS NOT a biocide and it will not “KILL” microorganisms.

  12. GARY GILBERT on Aug 17, 2013

    I wonder if Prist and/or other fuel additives are part of the bee collapse problem..

  13. Fressy Villegas on Dec 29, 2015

    Thank you so much for this information. It was very useful!

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