Phillips 66 Suspends Unleaded Avgas Testing


Phillips 66 has “paused” evaluation of its unleaded avgas alternative after what sources have told AVweb was a major test failure. Phillips 66 confirmed to AVweb that evaluation of the fuel it is developing with Afton Chemicals through the Piston Engine Aviation Fuels Initiative has been suspended, at least temporarily. “We can confirm that PAFI evaluation has been paused on the Phillips 66/Afton Chemical 100M unleaded fuel,” the oil giant said in a statement to AVweb. “Phillips 66 is committed to its vision of developing an unleaded aviation fuel offering and is currently evaluating this product’s development and all viable alternative options.”

The FAA also confirmed that “PAFI evaluation of the Phillips 66/Afton Chemical 100M unleaded fuel has been paused due to issues encountered during durability testing.” It did not elaborate on the nature of the durability issues. According to sources with direct knowledge of the issue, the Phillips/Afton fuel was powering a Lycoming engine in a test cell on a 150-hour endurance test and the engine failed due to a buildup of manganese deposits that fouled the spark plugs and/or caused pre-ignition. Phillips did not confirm that as the cause of the evaluation pause.

The departure of the Phillips/Afton entry leaves three contenders in the unleaded avgas evaluation. LyondellBasell/VP Racing Fuels is also going through the PAFI process, which involves the evaluation of the fuel by the FAA with the goal of earning “fleet authorization” as a universally acceptable fuel for spark ignition piston engines. Swift Fuels is seeking Supplemental Type Certificate (STC) approval for its 100R fuel and General Aviation Modifications Inc. has already received STC approval for its G100UL fuel for virtually all gasoline engines currently in use by aircraft in the U.S. To use the GAMI (and eventually the Swift) fuel, individual operators must obtain the STC for the aircraft that will use the fuel. There is some paperwork involved and a nominal fee to get the STC and install a placard. The GAMI fuel has been in limited use for more than a year.

Russ Niles
Russ Niles is Editor-in-Chief of AVweb. He has been a pilot for 30 years and joined AVweb 22 years ago. He and his wife Marni live in southern British Columbia where they also operate a small winery.


  1. Manganese? Where is that coming from? If it’s in the fuel, WHY? why would a fuel contain a relatively rare and expensive metal? Trading one undesirable metal for another?

    I don’t understand the science but all this hoopla for the last 40 years seems more like seance than science.


    • Methylcyclopentadienylmanganes tricarbonyl (MMT) is the octane additive used in these fuels. It can be used at lower concentrations that tetraethyllead (TEL), but MMT is also a very difficult to handle, toxic material.

    • If there ever is 100UL fuel it is going to be so expensive that everybody is going to say why could we not have gone with the obvious solution of 94UL especially when 70% didn’t need anything more than the 94UL. Almost all of the current engines that need 100 octane fuel could be modified to run on 94UL without any loss in power, but it would cost some money, and that group is willing to hold the other planes that don’t need it hostage.

          • So, there’s one STC available for one model with little information on ongoing cost and maintenance issues? So, we are what, a decade away from solutions for the other popular models at best?
            And do you really think adding more WW2 tech to WW2 tech engines is the way we ought to go?
            Unless you have a LOT more this makes your statement above hyperbole at best. Please stop spreading BS.

          • Replying to Eric: As I read it, the company listed a handful of engines and airframes, not just one STC – but if your engine/airframe isn’t on the list, you’re SOL. From their FAQs:

            “My aircraft is not listed, will it be approved in future?

            There are no plans to flight test any more aircraft in future.”

            The FAQs also mention the need for running leaded fuel during the break-in period and on a recurring basis afterwards.

            Yeah, more hyperbole than answers.

      • There will be a significant loss of power when moving to a lower compression engine (if allowed), as well as a loss of fuel efficiency. There is a 100UL fuel ready to go, but it is bound up in unnecessary red tape.

      • That’s a great offering that you are giving…. no loss of power whilst operating at much lower BMEP necessary with 94 vs 100. I’m all ears!!

  2. My guess is that they’re playing with the fuel additives to see what will live and work in an engine, as opposed to what the automobile industry has done with their engines to get power and performance out of lesser grades of fuel.

    I for one don’t want to see variable valve timing and cylinder deactivation under the hood of a general aviation aircraft. Those items are complicated enough in automotive applications. As well as the costs. But what are the alternatives?

    • Cylinder deactivation is for fuel economy at highway speeds. Not applicable to an airplane engine, unless you want to cruise at 50% power.
      Variable valve timing keeps the valves open longer, alerting more fuel/air to enter the cylinder each cycle. That could allow for a ‘boost’ power setting during takeoff or go-around.
      Neither is meant to prevent knock.

    • Variable valve timing does what the name implies: it varies valve timing to attain optimum power and efficiency at different RPMs. This is appropriate for an automobile engine that must constantly accelerate and decelerate, thus operating across the RPM range. Aircraft engines spend the majority of their lives operating within a narrow RPM range, so varying valve timing won’t yield the same benefits.

      Electronic fuel injection and ignition offers MANY benefits for aircraft engines, including better protection against detonation. That’s something I would love to see major manufacturers pursue.

    • None of the things you suggest here would be practical in the modification of the current fleet of engines that currently need 100 octane fuel, however, there are some solutions that would allow the use of 94UL fuel. Water injection, variable electronic ignition, and larger more efficient intercoolers for turbo charged aircraft are solutions that would work without causing power loss.

      • Wow…. and you’re inventing some new science and thermodynamic principles along with revised combustion dynamics, that’s so cool! Please share all your calculations with us!

  3. Methylcyclopentadienyl manganese tricarbonyl (MMT) is a gasoline octane enhancer produced by the Afton Chemical Corporation (Afton), formerly known as the Ethyl Corporation. MMT is allowed in U.S. gasoline at a level equivalent to 1/32 grams per gallon manganese (gpg Mn).Dec

  4. Phillips 66 could do all of Aviation a big favor and help itself by supplying airports with inexpensive, lead-free, ethanol-free, 93 AKI Mogas. Just as their peers in Europe have done for decades.

  5. “1/32 grams per gallon manganese” That just hurts my brain. What is that in stones per bushel? Tons per cubic furlong? And what’s with that fraction in front of a metric unit?

    • I laughed at that “1/32 grams” also. Grams is a metric measurement, NEVER preceded by a fraction. Should always a digital numerical amount such as 300 or 5 or .01, etc.

    • It is like adding salt to soup. One finger pinch to a pot… And 1/32 of a gramme (with the double m and an e, please, we are not savages) is so small you can call it nothing.

      • In the oil industry, it is standard practice to use the term, “grams per gallon,” to refer to content of metallic anti-knock additives in gasoline. The weight only refers to the metal, not the whole molecule. Back in the lead additive days, a mid grade auto gasoline would typically have about 5 grams per gallon lead. As for “1/32?” I never saw fractions used. But it somehow sounds better than 0.03125.

  6. This is how every FAA-led fuel initiative goes. The purpose isn’t to come up with an unleaded fuel. It’s to throw money at political donors, then have them slowly “withdraw” until the program “fails.” Then they come up with a new acronym and more funding and do it all over again.

    • You missed a good pun there: FAA-led lead-free fuel initiative” 😀

      But yes, when PAFI died and EAGLE/PAFI were reborn, my eyes nearly rolled out of my head from the eye-roll they did.

  7. Forget lead free avgas. Let ancient technology Lycoming/Continental gas recips die a slow death. Put research money into Jet-A burning “Diesels” like Delta Hawk. Their V-4 seems like a viable engine. So let’s develop that to become a V-6 300+ hp engine for GA aircraft. Also check the Steven Higgs design….even better.
    I saw the prototype Higgs engine displayed at 2022 Sun & Fun. However now, they seem to be based in Japan. So I can’t get an update on what’s going on with this engine.

    • Too bad “they” didn’t start that process 40 years ago. If they’d have stayed working on replacement diesels then, by now most of the gas ones would have been replaced, tbo’d, whatever. No “forcing” anyone to replace anything.

    • Some of these “ancient tech Lyco/Conti” engines already run just fine on unleaded avgas. 91UL is approved in most Lycoming engines. Quite a few airfields in Europe have 91UL for sale.

  8. Now, can we start getting 94 octane unleaded at airports? 100 octane unleaded isn’t coming, and if it does it will come under stack for whatever new additives are in it. Yes, lots of airplanes with engines that should never have been installed will need a “fix” so they can burn 94 octane. That’s going to be easier, more practical, and more permanent than all this illusory chasing of 100 octane unleaded.

    • As soon as you guys start traveling around the country in your planes buying enough of it to make it profitable. Of course, if you all start using your plane for travel, many of you will want high performance planes.

      • I had to laugh at that. When I make my long distance cross country flights, I purposely go over GA airports. In the event that I have to put down for any reason.

        I see no aircraft in the pattern, none coming or going, none on the tie downs, and the hangar doors closed. Vfr weekend.

        Matter of fact, rarely have I seen any vehicles. If this keeps up, forget about the fuel issues. Me thinks that the For Sale signs are going up for the airports them

        • Things were hopping from mid 90’s to early aughts, but even during the comeback we kept losing airports, tax law changes (justified by class warfare attacks on Bizjets) swung against us, and the kerosene club started getting nasty.
          People who were using pistons for business travel either moved up to turbines or went back to airlines and cars. 2008 put the nail in.
          Using a plane for travel just got to be more trouble than it was worth for too many situations.

      • Anyone who wants a plane for serious travel is going to be burning JetA. Most of them already are – as The old Eric W notes. I was surprised that Cirrus put another “100LL only” motor in its latest $1 million four-seater.

        The rest of us, burning Avgas, are mostly in it because we love it.
        And yes, I’ve flown mine across the country, sure.

  9. “The GAMI fuel has been in limited use for more than a year.”

    G100UL continues to be in limited testing, but it is not yet commercially available nor is it in use outside of testing. Can you provide an update on GAMI’s plans to turn its project into a commercially available solution?

  10. If I owned an airplane that burned Avgas I’d sell it ASAP for as much as I could. Diesel and Jet-A burning engines for GA is where research dollars need to be spent.

    • As there are many planes with compression ignition engines, not much research needs to be done. But remember: the 100 UL is for the high-performance “working” planes that burn most of the fuel. By swapping to diesel, if power is the same, you are adding 200+ lbs (because most will be twins) and adding more than 10% in fuel weight per gallon. Ain’t gonna be a blanket engine replacement rule from the FAA.
      Not to mention that today’s hi-perf petrol burners run close to six figures per engine; the kerosene versions will be more.

  11. Glad to see VP racing fuel is working on making a 100ul fuel. They sell leaded race fuel for about $20 gal so I cant wait to fill the tank on my plane for the ONE flight around the pattern that I would be able to do each year. What a joke the entire Lead in the fuel thing is. Leave the lead in the 100 LL, Put in a tank and sell Mogas next to it ant the 100LL sales will drop making the hole thing a Moot point.

  12. I enjoy reading this section of AV Web but many times it makes me realize how pilots may be intelligent, but technical knowledge about engines is quite another matter. (turn the big switch over there and pull the red knob). I guess if you spent your career in engineering or a any
    time in motor sports you scratched your head looking at 1937 designed engines being installed in 1957 designed aircraft being sold in 2023 for $500,000 and wondered why GA was still living in the past and charging a fortune for antique technology. No wonder people buy 60 year old aircraft. Then comes those who would like to fill up their tanks with $4/gal pump gas instead of $7/gal 100LL or believe that leaded gas in antiquated and unhealthy; all reasonable ideas. This all goes back to the strict FAA requirements that make it almost impossible to improve or change anything without “certification” and that means hundreds of thousands, perhaps millions of dollars. Vans made it possible for those of us who could not afford a new GA aircraft to build an airplane and Rotax and UL engines (Austria and Belgium) plus a score of other US based companies modified automobile engines which could run on either mogas or 100LL. But getting mogas on airports became almost impossible because selling 100LL is a profitable market and we can’t let those antique engines use unleaded fuel. The engine I installed in my RV- burns either fuel (although 100LL is like drinking a cup of coffee grounds) The engine is FADEC. starts and runs smoothly. I have had zero problems with running either fuel but prefer 91 pump gas which I Jerry Jug in. I am forced to run 100LL when I refuel at an airport but would prefer 91-94 unleaded.I add a lead scavenger in an attempt to protect the oil from absorbing lead. So much complication when you have a modern engine in your airplane. A few rotations of the starter and my FADEC engine starts; I listen to hangar buddies start, and start and adjust mixture and then the engine coughs and maybe starts and I wait to hear the roar of an R2800 (after that much effort) but it’s only a 4 cylinder Lycoming or Continental sputtering. When I shut off my engine I throw a switch and the ECU turns it off just like the one in my car, I don’t pull the mixture out just as I don’t in my car. I bought a new pickup truck this month I wanted to ask them if they had a carbureted engine with a big 4 barrel on it, maybe Tri-Power but that hasn’t been available since 1970, Nuff said.

    • You leave a few key things about Mogas out. No.1 it is not stable, No.2 it most likely will be tainted with ethanol which will attack most planes fuel systems, No.3 it is blended for seasons and is not consistent like Avfuel is and No.4 it’s vaporization point is lower so it is not suited for higher altitudes. That said if you can find untainted mogas and you fly enough that it won’t sit in your tanks more then a couple of months it is an alternative.

    • The vast majority of fuel is burned by high performance aircraft that cannot safely run on Mogas (I’m not knowledgeable enough to explain why, yet that fact is on my side). It’s not about getting rich on 100LL, it’s about there being no profit at all on most fields to sell anything else.
      You are quite overqualified to sell fuel on your field. If you want to sell Mogas, go sell Mogas.

        • Touché.

          Interesting that’s true though since diesel is the new leaded gas in the car world. Shopping for a new SUV revealed that if I could get the diesel versions offered overseas I could get half again or more mpg.
          Is it really that much dirtier than gasoline?

  13. So we already have a viable 100UL avgas formulation, but we are waiting to move forward on that while holding the door open for others to develop alternatives that apparently doesn’t really work, or are put on the back burner of development. Sheesh.

  14. Unstable and tainted. Vapor-lock. High altitude. Seasonal blends. While flying over the Mojave last June 105F on the ground and 60F at 9,500′ engine smooth running. Plane climbs out at 900 fpm at Gross take off. have flown it to 10,500′ many times with zero problems on 91UL. The FADEC engine has an anti-vapor design in the FI system, works flawlessly. That nasty ethanol is not a problem if you don’t let it sit in the tank for a year, actually fly the airplane. If you do, then you should drain it. Always check the fuel, pre-flight for water. Ethanol can absorb water. Zero Ethanol is also available at Circle K. My fuel lines checked every year for 6 years and they have not been “attacked” cracked or leaked. But then I built her and do my continuation of inspection every year. Many of the Lycoming/Continental folks mix 100LL with Pump gas here and tell me it works well. I use both with zero problems. “Back to the future.” In the end economics and pilots who have had enough will force this issue or perhaps as someone else posted diesel engines burning JetA will take over $$$$.

  15. One thing that doesn’t reduce knock: paperwork. George said it took GAMI 90 days to come up with a formula that worked. Now we have the technical solution but a paperwork issue. The FAA should buy GAMI’s IP, release it into the public domain, and cancel the paperwork requirement.

  16. I’m frustrated every time I read about the STC requirement for GAMI (and Swift) without any mention of the significant percentage of aircraft (experimentals) that not only aren’t subject to any STC requirement but couldn’t get one if we wanted to (since there’s no Type Certificate to Supplement with an experimental A/C). Russ, I hope you and AVWeb will consider including that additional context in any articles on the topic, especially if manufacturers come up with some alternate requirement that we’d have to fulfill to use their fuels.

  17. So the EPA is pushing for this change, the FAA is encouraging it and it eventually will become a mandate and we have to buy the STC? That’s sounds about right for this government.