FAA To Approve Use Of 91UL Fuel In Two-Thirds Of Piston Fleet


The FAA will issue a fleet authorization this year that will allow more than two-thirds of the gasoline piston fleet to operate on 91UL avgas. In a news conference held to update progress on the development of an unleaded high-octane replacement for 100UL, Lirio Liu, the head of certification for the FAA, said the agency plans to issue the blanket approval as part of the initiative to get rid of lead in avgas. “We expect approximately about 68 percent of the general aviation fleet will be able to use the UL91.”

In addition to reducing lead emissions from piston aircraft, the fleet authorization for UL91 will give a glimpse of a lead-free future for GA aircraft. “That will facilitate broader use and experience with the transition,” Liu said.

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    • Because most of the piston fleet is already designed to run on 91 octane AvGas (leaded or not).

      100UL and 91Mogas are different blends entirely. Also, you have to convince companies to specially make 100UL and 91 no-alcohol Mogas (whereas refiners would shout “alleluia” if told that they can just leave out lead out of their regular AvGas blend!).

        • The refinery is not putting out 91 octane…. which is WHY they need to add ethanol later. The refinery will need to have a different blend to get 91 minimum which is silly because unleaded AvGas is already available by not adding lead. No change at the refinery is needed.

    • In a word, no.

      Old cars weren’t ‘designed’ to use lead. And lead never ‘lubricated’ or ‘cushioned’ anything.

      The main problem with cars using unleaded is the fuel had lower octane than before. Oh, the sticker on the pump still said “89 Octane”, but now the unleaded fuel had *exactly* 89 octane (or even 88.9), whereas before with leaded gas it was often a couple of points higher. That’s because it was cheap and easy to add a bit more lead to guarantee the octane rating. But with unleaded it was much more difficult and expensive just to reach the rating. So cars that ran fine on the old “89” and pinged on unleaded “89” were really cars that needed 91+ octane all along.

      The problem wasn’t the lack of lead. It was the lack of octane.

      Add if those cars had unhardened valve seats, where seats were cut directly into cast iron cylinder heads, the continuous pinging would result in valve-seat recession. But airplane engines use hardened valve seats. They have to because aluminum is too soft to machine a seat directly.

      Getting the lead out will make airplane engines run better, not worse.

    • Even for old cars, the statement that unleaded gas burned the valve seats was a myth.
      There is actually only one car model (Pontiacs) that that happened to amd that was due to a design flaw in the vakve seats pertaining to that particular model.

  1. I don’t think it’ll affect the valves. Based on an experience of mine 30+ years ago, where I ran a few tanks of 100LL through my ’79 subaru (which had unleaded valves but no cat). I thought I’d be ok, nope. I later spoke with an A&P asking why 100LL still burned the hard valves but doesn’t hurt aero engines. I was told aero’s use titanium valves.

    • Well, yes and no. Most aircraft engine exhaust valves are a nickel-based super alloy known as Nimonic 80A. The metallurgical composition is about 76% nickel, 19.5% chromium and a small amount of aluminum, titanium and other ingredients. The titanium amounts to less than 2.5% of the total, which is there for some minor performance enhancements, but the main nickel and chromium give it the heat resistance necessary for operating at the extreme temperatures when the valve first opens. With aircraft engines, the main concern is not the valves, but the valve seats. Unlike car engines that are historically cast iron, the aluminum cylinders on airplane engines have always had some level of hardened valve seats to allow the cylinders to stand up to the stress of the valves opening and closing. Despite the myth about the lead in TEL acting as a valve lubricant, it actually causes problems in the form of deposits on the valve stems that eventually result in stuck valves. For a good description of the unleaded fuel issue and its effect on valves, I suggest you view Paul Bertorelli’s good video on the subject (Whadayamean Unleaded Fuel Will Trash My Valves?).

  2. Why are we still talking about unleaded AvGas like it’s not here yet? G100UL is already certified in 100% of the fleet, which sounds much better than “68 percent of the fleet.” The lead-free future is here, and it’s G100UL; we just need to get it pumping!

    • Because most of the fleet NEVER needed (or wanted) 100 octane in the first place! Your kind of thinking is why we all are stuck with 100LL to this day. Pushing a more expensive fuel is a terrible “solution” for most of the fleet when we already have a drop in perfect solution that may even be less expensive.

      • You keep glossing over the fact that most of the fleet that doesn’t need 100LL also consumes the least amount of it.

        Small, low-compression engines are found in many privately owned and flown aircraft. Their numbers are large, but the amount of hours flown are small, as well as the amount of fuel they consume per hour. In all, they amount to about 30% of fuel sales.

        High-compression engines are found in larger/faster aircraft, typically revenue-producing or business use. While there are not as many, they have more engines (think twins), fly more hours and consume a LOT more fuel, both per hour and in total. They consume about 70% of 100LL fuel sales.

        A low-octane unleaded “solution” only solves 30% of the problem.

          • Right there with you. Can we PLEASE just have access to avgas without any of the additives – lead or otherwise – needed to get it to 100 octane?

          • I remember fueling up my Cessna 150 with red 80/87 octane. It’s no longer available.


            Because the avgas market is so small. Splitting it into two even smaller pieces didn’t make financial sense.

            With 80/87 (red) and 100LL (blue), an FBO has twice the number of pumps to buy and maintain. But one pump (blue) outsells the other by over 2-to-1. Now they have a pump (red) that costs just as much as the other, but sells less than half the volume. Which means the cost per gallon will go up a bit due to the lower volume. And you can’t sell more of it because the red only works in some planes, but the blue stuff works in everything.

            It didn’t take long for the industry to settle on one pump selling 100LL.

            Now, 91UL is a bit different, being essentially a by-product of mogas production so it’s already widely available. But as others have pointed out, it likely only makes sense for an FBO to put in a second pump for it if there’s a large population of low-compression singles based at their airport.

            Why don’t you talk to your local FBO and ask them what they would do?

        • It’s not a screw the small planes solution, it’s a keep one fuel that works solution.

          Notice the choice of words to “have access”. What you guys really want is for someone else to make an investment to sell you something you don’t buy enough of for it to be profitable.

          And, I suspect what a lot of people want is for the high performance fuel to actually become unobtainable so those planes will be scrapped which you guys think will make your planes more valuable. I don’t think it will really work out that way, but the mindset in piston GA is unexplainable if you assume rational actors, so I stopped doing that.

          • For the last 30 years of politics and funding cycles, we could have fully funded 91UL as an airport improvement. Lord knows it would have been the least expensive airport improvement program ever.

      • That assumes that the economics of having 2 or 3 avgas grades of fuel is the same as the economics of having mutiple grades of mogas, but it’s not. It may not be economically worth it for the refiners to make two grades of specialized fuel to be available at every airport.

        • Yeah not just the refiners but the airports and FBOs who would have to absorb the cost of installing and maintaining new tanks and pumps – not likely to be feasible for most of them. That’s why we don’t even have 94UL or mogas at most airports in spite of lots of planes (including more and more experimentals) being able to use it (and wishing they could).

  3. Does this mean we will no longer need an STC to us 91UL? If yes, then does that mean the “inventors” of 91UL lose all that money they would have collected? Or is the FAA or someone going to “buy” the rights to 91UL and make it available for everyone? If no, then what does this announcement actually do? All the planes that qualify already can buy an STC and use 91UL.

    IF the FAA really wanted to support UL fuel they would”buy” the rights to 100UL and 91UL and allow everyone to make, sell, and use it.

    • I’m not sure I agree with your solution to subsidize the inventors with money confiscated from all of the taxpayers instead of just the ones that actually benefit from the fuel. Why not just let the inventors license their formulas to the big producers (assuming that they still have any enforceable patents to lean on), like other patent holders do with basically everything else?

    • If the FAA does what is proposed, and you have an engine listed on Lycoming SI 1070AB as able to run on UL91, you will not need an STC. UL91 and UL94 are ASTM D7547 spec fuels. They are not “mogas” or automobile ASTM D4814 fuels which require an STC. Swiftfuel UL94 does not require an STC for grade 80 aircraft and engines. CURRENTLY, Aircraft whose minimum approved fuel grade is listed as Grade 100LL, Grade 100/130, or Grade 91/96 CANNOT USE Swift Fuels’ Unleaded UL94 Avgas without purchasing an appropriate Supplemental Type Certificate. This is crazy as the EU has allowed UL91 for more than 10 years in aircraft with engines on the Lycoming SI 1070 list. In the USA the AIRFRAME needed to be approved as well. Not so in Europe.

  4. Will they require a new color dye for 91UL fuel so we know the refinery didn’t ship the same fuel they sell as MOGAS with ethanol in it? Or will it be up to the pilots/FBOs to test the fuel themselves?

  5. Just a reminder about Avgas and Mogas… Swift’s UL94 is pretty much 100LL minus the addition of a few drops of liquid tetraethyl lead. That is why it is ASTM certified, meets requirements for stability, vapor pressure, density, etc.. And UL91 is pretty much Mogas, excluding any ethanol, which is not available in some places (California…) and doesn’t meet aviation stability specs, but works in engines designed for old mid-grade 91 Octane Avgas.. The new compounds from folks like VP, Shell, GAMI and Swift have been tested in some aircraft, but have varying weights, unknown compounds and may or may not dissolve paint, rubber or plastic parts.. If you can use UL91 or UL94, it works better in engines than 100LL (just ask old Cessna 152 owners about lead fouling with 100LL!) but if you need all of the 100 octane rating (measured – not calculated – to prevent detonation..) then hang on until plane makers have fully tested their products and refiners have worked out supply chain that is greater than tank truck lots..

  6. UL 94 is the future. The drop in UL 100 will be way to expensive because it is synthetic blend. The aircraft that now require 100 octane fuel can be modified as they should have done years ago, instead of holding all the other aircraft hostage.

    • You are still ignoring the fact that the lower number of high performance engines still use the majority of aircraft fuels and provide most of the revenue to commercial operators. Asking them to derate their engines so that you can run a lower octane fuel (in spite of the fact that your engines will run on their high octane fuel) is like asking you to stop driving your gasoline powered car to give more room for diesel trucks on the road. Derating high performance engines would be incredibly expensive and could actually render the aircraft unfit for its original purpose. The FAA’s pronouncement of approving engines to use 91UL is creating another can of worms – mainly because it gets back into the argument of whether an FBO should have two separate pumps for Avgas, in addition to many that also dispense Jet A. We have enough problems with misfueling incidents between 100LL and Jet A. Dispensing two grades of Avgas creates another misfueling potential and exposes the FBO to the expense of operating the two grades along with the liability for fueling with the wrong grade.

      • No, I said just dump, 100 octane. No two grades. All these aircraft engines could be modified without degrading their power outputs. Most turbo charge airplanes would just need to add inner coolers or larger inner coolers to handle the new 94 UL as the only fuel available.
        Most turbo charged piston engines only have compression ratios of 7.5 to 1. The big myth, is that these are high compression engines and have to use 100 octane fuel.

        • “Most turbo charged piston engines only have compression ratios of 7.5 to 1. The big myth, is that these are high compression engines and have to use 100 octane fuel.”

          That’s because the turbocharger is compressing the air before it even gets in the cylinder. The result is even greater peak pressure and power (and chance for detonation).

          The piston/cylinder compression ratio may be low, but the overall compression ratio (turbo+piston/cylinder) is even higher.

          Which is the whole point of turbocharging an engine.

        • There is already a Cirrus approved for UL 94 that uses a water injection system. But this is one of the wonderful ideas that could be used to just convert all airplanes over to UL 94 and stop the nonsense that isn’t going to happen of having a UL 100 fuel and no longer hold the whole industry hostage to this nonsense.

  7. Clarity will come later. But a big part of the problem is that it’s not 1970 when one could stick in any old tank, put on a pump and call it a fuel farm. Not only does it now have to meet many more safety requirements than before (good), it usually has to have a credit card payment system attached. Hence a $3k installation becomes a $30k (or more) installation. Hence, most small airports can’t easily afford two fuel choices. From a return on investment standpoint, without subsidies, the at the pump price required for a single 100UL installation might well be less than what one would have to charge for a new 9xUL choice from a second added station, even if the $/gallon is inherently lower. And adding a second truck run from the distributor isn’t free either.

  8. It should be an all or nothing approach on fuel IMO, for the small engines that are already approved for mogas this new fuel is moot and only going to drive up the price of 100LL, lets base the rules on helping the consumer and not a destructive political socialist agenda that benefits no one.

  9. My concern is that we have airports that struggle to keep up with emails, much less making sure they note somewhere available in EFB’s and in online planning tools what type of fuel is available. My Turbo Saratoga will be part of that one-third NOT allowed to use 91UL. I have an intercooler (it is NOT an inner cooler – sorry, pet peeve) and I don’t plan on “trying” lower octane fuel and going up to 15,000 feet and just “screw around and see” if detonation occurs. For those guys content at flying at 3,000 feet all day long and never flying more than 100 miles from home, this is probably not that big of a deal. For those of us whose typical missions has us flying 10,000 feet or higher, in the mountains, and loaded for real traveling, the risk of not being able to find 100LL or 100 octane equivalent is a real concern.

  10. So how is this fuel issue affecting the so-called (real or otherwise) pilot shortage? Does the algorithm used to come up with the 70%-of-the-fuel-is-burned-by-30%-of-the-fleet consider that pilot training is done in the 30% category. Imagine the cost of you PPL if it was done in a hot twin?

    Wouldn’t help said pilot shortage

    • I’m not sure what your logic is here. If an FBO and flight school share the same owner, or are even the same business entity, you’d think they would put in the second fuel for their old 172’s if it saved money.

      They don’t, so I suggest it’s likely not worth it. I could be wrong, but if you ever do the calculations on fuel cost as a portion of TOTAL operating cost including depreciation, and actual repairs, you’ll see why so many people who didn’t need body on frame trucks kept buying them despite fuel price spikes. You gotta be on the extreme far end of the mileage chart before fuel costs really matter. One reason I’ve been suggesting forever to dump all the stupid mandates and greatly raise fuel taxes so the market will favor efficiency.

  11. The FAA should install one self serve UL94 tank and pump at every federally funded airport. The cost should be shared between the FAA and the EPA for being complete failures. This pump should have short term transient tie downs, and through the fence access. This will help with the pilot shortage and improve safety. I can fly for an hour for less money than it costs to take a piss at Signature.

  12. Oh, goody. Now we’re back to two fuels, 91UL for “the 68%” and some version of 100 for the rest of us? After 50 years of airports having one supply tank, airports are gonna need two tanks?

    (For folks thinking installing a second tank is just an “FAA thing,” they have no idea what kind of effort it is to install a second tank – funding calisthenics, engineering work, dedicating ramp the space, installing the tank and bump protection, adding second pump, getting the spotted owl EPA approval, obtaining fire safety certification. Even installing a small second system is upwards of $350,000 that your tax dollars or inflation get to pay for. And last but not least, the airport gets to bankroll an additional $30,000 of fuel inventory. Fun.)