I'm trying to understand prist.

If you run a turbine and you aren't staying low, and I'm betting that's the vast majority of turbines, then you have very cold temps to deal with. Given that jet fuel without prist stops flowing and gels when it gets cold enough, couldn't you flame out without prist if it's cold enough?

Add to the mix that most places I go premix the prist anyway and charge you based on whether or not you asked for it (I'm guessing most do, I always do) rather than whether or not you got it (everyone does).

Don't you kind of have to have prist if you're going where it's -40C?

Somewhat airframe dependant.

Does your AFM mention it?

I don't think my airframe heats the fuel, but even when I had a Cheyenne that did, I was always told to use the prist in case the heater couldn't keep up with the cold. I'm fairly certain I have to have it, but even if I didn't, wouldn't I be crazy not to?

The MU2 needs Prist. I don't have my AFM handy, but I believe it is referred to as Mil-Spec xxxxx.

The Gulfstream is an airframe that doesn't require it. The fuel is heated through a oil/fuel heat exchanger. Or, the oiled is cooled through this heat exchanger. The system works very well.

Prist is corrosive and costs money, so airplanes that don't need it, try to get negative Prist. But, like you say, most Jet-A these days is pre-mixed.

Here's a good one:
viewtopic.php?f=4&t=80333&hilit=prist

Thanks fellas.

All MU2s (and I think all TPE331 engines) have oil/fuel heat exchangers, thus your fuel is heated when it first reaches the engine. There are no other heaters prior to that, so the fuel has to flow out of the tank and through the boost pumps without being warmed.

Only reasons not to use a fuel system icing inhibitor (FSII), what we commonly call Prist, would be extra cost and/or inconvenience if you have to add it personally. Typical extra charge is 6 to 8 cents per gallon.

The FSII prevents water in the fuel from forming crystals and precipitating out, clogging the fuel system. It does not prevent the fuel from freezing directly. Jet-A (typically US) freezes at -40C/-40F, Jet-A1 (typically international) freezes at -47C/-53F. If a plane flies for too long in those temperatures, you can get the fuel to gel. The FSII doesn't help this problem, it only deals with the water content in the fuel.

Whether an MU2 requires FSII or not is a matter of some debate. Early models under A2PC type certificate (generally 3 blade) have AFMs that says FSII "may" be added. Later models under A10SW type certificate (generally 4 blade) have AFMs that say FSII "must" be added.

I've never heard of an MU2 having any fuel system icing problems, but that doesn't mean it doesn't happen.

Mike C.

Great. One more thing to fret about at altitude. I'm not in -40C all the time, but I am sometimes, and sometimes for an hour or more. I thought the downward limit for operation in flight on the MU2 was lower, like -50's or something...

Fuel doesn't chill that fast, so it takes a long time to get it down to the air temperature.


Engine operational envelope goes down to -54C.

Mike C.

I just got off the phone with Reece Howell. The coldest Mr. Howell has operated an MU2 continuously is -52C coming home from Russia at high altitude. He reported no issues with his operation at that temp. The book says -54C is the limit.

I'll never have to worry about that, lol. It just doesn't get that cold where I go.

The other advantage to the additive is that it contains a Biocide that prevents the algae that grows in Jet Fuel from multiplying. The Algae creates a black sludge in the bottom of the fuel tanks, and can clog the fuel filters. The algae will only grow in the presence of water, and is more of a problem with aircraft that don't fly regularly.

King Air's have a required inspection of the nacelle tank (the low point) for the sludge. We are a Phillips 66 dealer, so, our fuel comes to us premixed, and our tanks are always very clean.

Actually they are careful to state that Prist is not a biocide.

My personal guess is that it is somewhat biocidal but does not comply with some Mil-spec fully so they are required to say it is not. But that is a guess.

Hi Craig, FSII (Prist) is required by the AFM in our Solitaires.

It turns out to be a real hassle if you have to add it via cans, so that might be a reason not to want it if you don't require it.

The antimicrobial properties of Prist are somewhat debatable.

From the Prist web site:

What biocidal/antifungal properties does the PRIST® Hi-Flash™ Fuel Additive have?

PRIST® Hi-Flash™ Fuel Additive meets ASTM D4171 and MIL-DTL-85470 (Current Revision Issue) specification for DiEthylene Glycol Monomethyl Ether (DiEGME). The primary purpose of this chemical is to prevent freezing of water in the fuel system of aircraft. It is this property that is covered in ASTM and/or Military specifications. Neither of these specifications contains any biocide requirements.

Prior to 1994, PRIST® was made to a Military Standard MIL-I-27686, which specified Ethylene Glycol Monomethyl Ether (EGME). Provisions were made to have this product certified as a pesticide with the EPA. This allowed the product to be advertised as having the ability to retard growth of microbes in aviation fuels. The correct term is for the additive is microbiostat (-stat means it controls or retards growth by putting the microbes in a static condition), it is not a true microbiocide, (-cide means it kills microbes).

With all of the new EPA requirements, it became economically prohibitive to certify the new DiEthylene Glycol Monomethyl Ether, DiEGME based additive as a pesticide. Summarizing the above, it is widely believed that DiEGME does have a retarding effect on microbial growth; however, we no longer officially claim this property for the PRIST® Hi-Flash™ Fuel Additive.

So officially, Prist is not antimicrobial, but in actual usage, it has some antimicrobial properties.

The military did a study of DiEGME to see if it was, in fact, antimicrobial like EGME was. Here is the study:

http://oai.dtic.mil/oai/oai?verb=getRec ... =ADA517851
http://www.dtic.mil/get-tr-doc/pdf?Loca ... =ADA517851

It tended to show DiEGME was reasonably effective at being an antimicrobial agent at concentrations below what GA typically uses (0.10% to 0.15%).

The study did find microbes that would survive DiEGME, though, so it isn't an assured thing.

There's a small technicality with using modern Prist on older airplanes. In the AFMs of older airplanes, they call for FSII under MIL-I-27686 which was EGME. Modern Prist is under MIL-DTL-85470 using DiEGME. So you can't technically use what the AFM calls for since it doesn't exist any more. Nobody seems to be concerned about this (nor am I), but goes to show you how the rules sometimes get messed up even when things are reasonable.

Note that Biobor JF is NOT DiEGME, NOT Prist. It isn't an FSII at all.

Mike C.

Ethylene glycol is very toxic. With the Di in front, don't know but might have a clue comparing the two MSDS ?