Unleaded Avgas Q&A: Closer To Pumps?

New U.S. leadership could do more to nudge a transition to high-octane unleaded avgas. Four major players continue to work the science involved.

few years have passed since the unleaded avgas discussion came full circle, but we’re still pumping 100LL. Make no mistake, it’s still an EPA focus.

We could find ourselves modifying engines to make unleaded gas work in our airplanes. It’s worth a Q&A discussion, which I share here.


Full disclaimer: I was an engineer at Chevron for 38 years. The last decade plus, I was involved in unleaded avgas development. That’s until Chevron became so frustrated at the FAA’s inability to understand science that it walked away (as did many of the majors). I’ve provided some small consulting to Swift and GAMI since then, but am not privy to their confidential stuff, and am not representing either of them in expressing my observations on the passing scene. 

While the FAA’s PAFI (Piston Aviation Fuels Initiative) is stuck— because ignoring science is a bad approach—there are five companies working the issue. In diminishing order of successful probability, in my view, there’s GAMI, Phillips, Swift and Shell. Let’s look at each approach, and some crystal ball gazing about what the future may hold.

GAMI has been working on this issue since 2010, and it does observe the science, while its formulation and additives have evolved. The FAA has streamlined the approval process, and GAMI is about ready to begin the final testing for approval this year. That will likely result in fleet trials (maybe at flight schools), with a broader rollout following.

Phillips/Afton came to this game late, announcing its plans at AirVenture 2019. Phillips plans to replace the lead with Methylcyclopentadienyl Manganese Tricarbonyl (MMT). Manganese is much less toxic than lead, although when MMT was used in mogas, there were issues with spark plug fouling. Phillips thinks they have a scavenger for the aviation application that will spare our expensive plugs. Phillips expects commercialization in 2025/2026.

Lyondell/VP-Racing has been developing their fuel without publicity, save presentations to the ASTM unleaded avgas group. They’re adding ETBE, manufactured by them, to conventional avgas, as well as MMT (see above). They are cooperating with the FAA’s PAFI process, and according to industry insiders, are pleased with the FAA’s renewed emphasis on making PAFI succeed. Certification is two to four years away, they estimate.

The market still waits for Indiana-based Swift Fuel’s in-development 100-octane unleaded avgas. On its website (www.swiftfuelsavgas.com), it says its goal is to fully replace 100LL with a 100-octane unleaded fuel within three years, and I think the underlying science should be workable, if not slowed by lack of resources. 

Swift is selling an interim 94UL fuel that it now says it may or may not discontinue when 100UL is available. Since the supply logistics on the 94UL fuel seem sketchy, what does that imply about Swift’s viability? Swift says new investment is coming online next month to resolve supply issues.

Shell was the primary contender in the FAA’s PAFI, and that effort was unsuccessful, even after the FAA changed the rules to accommodate it, versus being left with no players after Swift Fuels withdrew from PAFI.

It’s important to note that it’s NOT the Shell refining company that’s pushing this effort. Instead, it’s kind of a speculative effort by its Global Solutions company, which was spun off a few years back. The company is hoping to develop a salable solution, rather than developing a solution for the Shell corporation. Will they succeed with their heavy-alcohol approach? Shell promises an update soon. 


As the technical advisor within the Cardinal Flyers Online group, I’ve fielded a variety of questions, comments, theories and technical concerns. Some are worth talking about here. Obviously front and center is equipment compatibility.

Unleaded avgas can be made to equal the current 100LL. I do not know what the holdup is.

There are subtleties when one starts looking at the entire fleet, and then there’s the certification process itself. The FAA has very limited experience in certifying fuels, and has made some missteps.

It’s not the politics that has held up the rollout of high-octane unleaded avgas, it’s that pesky physics thing. It’s got to support the high-compression engines that burn most of the 100LL.

It’s a combination of factors, actually. The FAA has to follow the scientific method, but as a federal agency it is an inherently political animal. So there tends to be a combination of confounding factors. 

Maybe they need to reduce the compression in those engines—it’s not a big deal to change pistons. Otherwise, unleaded mogas can offer a real solution.

Some simply underestimate the enormous certification burden for the fleet. Each engine/airframe combination would have to be recertified, with new performance tables compiled from flight testing. This is a stupidly expensive effort, and not one that’s popular with aircraft owners, partly because it will result in reduced gross weight and performance. No one is excited about spending the money to make that happen, either.

It’s been 25 years since the EPA’s ban on lead in fuel, just how long do you think John Kerry and the new administration are going to wait?

You overlook that the EPA and FAA duked this one out in federal court during the George W. Bush administration (the White House didn’t want to take the heat, I guess). The SCOTUS decided that the FAA is in charge of avgas regulation, so the EPA enters a holding pattern waiting for the FAA to do its thing. The EPA could make a finding of endangerment, which might adversely motivate the current producers of leaded avgas. But, that didn’t happen under the Obama leadership. Perhaps a Biden leadership might be more interested in observing Supreme Court rulings in letter and spirit than running roughshod over them.

Will this lower the price for avgas?

It’s hard to say. The unleaded fuel will probably cost slightly more to make, perhaps another 25 cents per gallon. On the optimistic side, since high-liability lead facilities aren’t required, any number of additional blenders might choose to enter the marketplace, introducing more competition than currently exists. My guess is avgas prices will be roughly the same: At wholesale, avgas sells at 80 cents to $1 per gallon more than premium unleaded.

Mogas isn’t really a replacement. The vapor pressure is too high. A good chunk of the fleet would have to be retrofitted with in-tank boost pumps to keep airframe fuel lines from forming vapors.

There are a number of issues with mogas, although in the summertime, mogas vapor pressure and avgas vapor pressure are generally about the same. 

Ethanol content and octane are the big showstoppers. Also, the mogas you buy at the corner gas station doesn’t have the quality control that aviation fuels do.

Dilemma: You certainly won’t be able to pump unleaded into a typical GA airplane without sizable engine modification.

Wouldn’t we have to retrofit our engines with electronic ignition, knock sensors and variable timing? That’s a big investment.

There are technical problems with knock sensors in aviation engines; lots of the aerodynamic noise looks like knock to conventional sensors, which is why GAMI tried developing fiber optic-based sensors. But none of that is certified and ready for prime time. And you’re right about the big investment.

Many argue that there will be an environmental benefit to unleaded avgas, but the amount we burn is microscopic, no?

Perhaps, but there’s no safe lead exposure—from molecule one the best we understand it—lead exposure to children reduces IQ. Today, one-third of the lead entering the ecosystem is from our microscopic avgas use, so many argue that it needs to go away as a matter of social policy. 

There are studies implicating the one remaining tetraethyl lead plant in Liverpool, England, in adverse impact on children living downwind. How does one stand tall about that?

Leaded fuel supposedly helps lubricate the valves, but I think that can be addressed in other ways, couldn’t it?

Leaded gas is actually bad for the valves. There’s scholarship and industry papers explaining that the valve problems we saw when lead was phased out of mogas came from the drop in octane, not the lack of lead, and the FAA has done at least one validating study on aircraft.

Ethanol contamination is a ridiculously easy check. Just add 10 percent water, shake and see if the apparent water volume increases.

So, what do you do if your airport tank is full of mogas-derived avgas that flunks that test? It’s not pretty. It needs to be controlled for upstream, not remedied after the fact.

High temperature and high humidity reduce horsepower now, yet we accept it. Maybe just reduce the max gross weight?

But the impact of temperature and humidity can be calculated in an FAA-approved manner, per the POH and the AIM. To reduce horsepower will require a certification effort with flight testing—and that’s expensive.

I’m not interested in paying for expensive engine modifications to burn lower-octane fuel and thereby get less horsepower, less gross weight, less climb performance and also a hit to speed.

Don’t forget, less range too, as fuel efficiency declines when you reduce compression ratio. The lower compression turbo Cirrus burns about one gallon per hour more than the more efficient turbonormalized Cirrus with higher compression ratio.

Fueling stations like Kwik Trip sell 91 octane with no ethanol, and many of them also pump pure 87.

Don’t forget the plethora of different octane scales. That Kwik Trip 91 is actually about 86 octane on the aviation octane scale. And, the 87 pump fuel is 82 octane on the aviation octane scale. 

You’re overestimating the impact of a reduction in compression ratio. It would likely result in less than 5 percent decrease in peak HP.

I don’t think that’s the issue as much as the certification and modification expense.

There are more turbocharged engines in autos today than ever before, and most are running 87 to 91 octane.

They’re accomplishing that today with scheduled direct injection systems, which doesn’t seem to work well with very large diameter pistons like the ones in aviation engines. And, don’t forget that the certification burden would be significant.

I have a Lycoming IO-360 with the SureFly variable timing and 8.7 compression ratio. I am guessing that I could run on the 94UL, without a problem.

Your SureFly ignition is only advancing the timing from stock, not retarding it. There’s no detonation detection. The certification standards require you demonstrate a 10 percent fuel flow detonation margin at redline CHT and oil temperature. Flying behind an angle-valve IO-360 myself, I’d be very surprised if you could make that work. So, you’ll need to lower the CHT and oil redline temperatures, then do climb cooling tests to prove to the FAA that you still have adequate margin for the 100-degree F day. None of that is cheap, even if it “might” work.

All they know is lead is bad, so we must rid it from our fuel. The unintended consequences of that are rarely a thought. Their ignorance is matched by their zeal for quick solutions.

This is a quick solution? Lead was regulated out of mogas in the early 1990s, and the industry unleaded avgas task force began work in 1991 and declared failure in 2011. This has been anything but a quick solution. Fortunately, it turns out the task force was trying to solve the wrong problem; the real problem was much more amenable to a solution.

It’s been decades since I first heard the trope that 20 percent of the sales base burn 80 percent of the 100LL. I just wonder if there’s been a more recent snapshot. That study is 20 to 30 years old.

The FAA does an every-three-year survey of aircraft utilization. My recollection is that its more recent analysis is that 30 percent of the aircraft are burning 70 percent of the fuel—a distinction that doesn’t make much of a difference. See the PAFI papers on www.faa.gov.

It seems like a competent FADEC system could take care of the knock issue, no?

It could, but not without reducing performance, which introduces the large cost certification issue and the very large cost engine modification issue. There’s also the time to comply problem.

Having computer control over our ignition and fuel would let us run a lower octane fuel without losing horsepower. It shouldn’t be difficult to develop a system for our engines.

I don’t think the science supports your assertion because it’s been attempted in the engine test cell. Still, it depends on your definition of difficult as it relates to time, cost, installation details and regulatory recertification.

I could understand the lack of STC approvals for older planes. But new aircraft manufacturers have done nothing to try and adapt to new unleaded fuel.

Cessna attempted it with the IO-580 in the 206 Stationair, and the IO-360 in the 172 Skyhawk. But the IO-580 blew the heads on the cylinders, and the marketplace didn’t want the lower compression 172. Folks converted them to higher compression to improve power and fuel efficiency, so Cessna simply responded to the marketplace.

The Jet-A-burning aviation diesels do have slightly less horsepower; however, all of the ones I know of are turbocharged, so at cruise altitude the diesels end up being far more efficient. The Diamond DA62 will carry seven adults on 12 to 14 GPH.

Careful how you measure efficiency. From an engineering and aircraft performance perspective, it’s horsepower per pound of fuel. Diesels may appear to be more efficient because the fuel is heavier, so in horsepower per gallon it looks better. But, airplanes aren’t inherently limited by volume, but instead by weight—and in horsepower per pound, diesels don’t have much or any advantage.

The latest FAA update (Aug. 20, 2020) just says, “The FAA, fuel suppliers, and aerospace manufacturers continue to develop high octane, unleaded fuel formulations. The goal of these efforts is to identify fuel formulations that provide operationally safe alternatives to 100LL. The PAFI program continues to support the efforts of fuel producers as they bring forth alternative, unleaded fuels for testing and evaluation.” Kind of a kick-the-can-down-the-road update from the FAA: https://www.faa.gov/about/initiatives/avgas/.

The excitement in 100UL isn’t from FAA leadership, but from the innovations the five contenders bring. If the FAA can continue to support the certification efforts, life will be good.

Instead of a two-decade sunset period that the EAA was prepping us for, it will be a two- to five-year sunset period.

I see no evidence that the EPA had any sunset time period in mind. In any case, the Supreme Court handed this responsibility to the FAA. That said, once a viable unleaded fuel is identified, the FAA will move quickly to require its use,  but perhaps not as quickly as local jurisdictions may.

Dilemma – Supply – Will fuel farms be stocked with both unleaded and 100LL?

Will there be two grades of aviation fuel offered at most airports?

As avgas demand continues to decline, no one has an appetite to invest in a second set of facilities. The diminished demand for both types of avgas would doom them to early extinction. The industry considered this in the 1990s and moved on, and factors today even more strongly support that decision.

How about shipping one grade of unleaded fuel, and very small amounts of octane booster (lead or other) and mix it at the pump?

Lead is too toxic to be distributed that way, and no one wants to spend money on lead-containing “solutions” to the no-lead problem.

You could buy real lead additive at auto parts stores.

That was a brand name, not a description. Lead in a can was never available. Some marketed lead substitutes; however, they didn’t boost octane.

The EPA requires mogas to be oxygenated.

No, Congress required that to appease corn farmers. There’s no technical justification for oxygenates for pollution control any longer. In fact, some studies show that pollution is greater with the ethanol mandate.

You still need lead for valve lubrication, if the valve’s guides and seats were constructed in accordance with the latest specifications, no?

Lead erodes valves, it doesn’t lubricate them. The last change in the Lycoming valve hardness spec was 1974, so there aren’t many engines running valve seats older than that.

Continental believes its engines can make rated power on any of the proposed 100LL replacements even at slightly lower octane.

Continental made that claim, and was flying a Bonanza around the country with its supposed lower octane engine. Unfortunately, the cylinders cracked.

The horsepower difference from a lower compression ratio is comparatively minor, isn’t it?

It still requires unworkable certification costs.

Fuel injection and angle valves raised the Lycoming O-360 engine to 200 HP on 100LL just by these two items. I don’t think compression was raised between the 180-HP O-360 and the 200-HP fuel injected IO-360.

The compression ratio was raised from 8.5:1 to 8.7:1, and the angle valves are larger, allowing more fuel/air mixture into the cylinder, hence more power out.

Where is the lead mixed into the avgas in the distribution chain? I imagine it isn’t at the refinery, it’s at the distributor. And I doubt they have a truck or two that are specialized to only deliver avgas.

The lead is blended at the refinery by circulating the avgas tank, with its four components already batched in, through an eductor that sucks the lead out of the ISO container it arrived in from Liverpool. This circulation also serves to mix the tank. The dye and antioxidant additive are added through this same technique.

I don’t believe any pipelines will carry leaded fuel anymore either, it’s all being trucked.

Avgas stopped moving by pipeline in the 1960s as the volumes became too small to tolerate the transportation mixing (transmix) that occurs in pipelines.

Lots of fuel crosses the border both ways (U.S. to Canada) as these two Canadian refineries do not produce avgas continuously.

Avgas is a batch product, unlike mogas that is continuously blended. There’s no reason a refiner couldn’t make batches consecutively, though, so that avgas would remain in inventory, although it may not be economical to do so.

How many engines are running in test cells to establish that difference between mogas and 100LL?

GAMI has done extensive test cell work with a variety of fuels; their work does not support a feasible mogas path forward.

Refiners sample and test every single run of avgas to ensure it meets the specifications before anything gets loaded in a truck.

Just like they do with mogas.

Don’t know if any of the West Coast refineries in Washington State produce aviation gasoline.

Chevron in Richmond, California, is the only West Coast producer.

I’ve always wondered how the quality of avgas (100LL) changes over time. Some of these storage tanks at airports are huge, and I can’t imagine most of them are filled more than once a year.

Avgas has a one-year stability spec, unlike mogas that has a few months stability expectation. There aren’t many huge tanks at airports with avgas in them—most are holding monthly quantities.

I think the cross-contamination argument is a bit overblown. If the tanker is actually empty and you put 500, 1000 or 5000 gallons of avgas in it, it won’t matter which petroleum was in it before, in practice operationally. Regulatory wise, it’s another matter.

And liability wise. Defend that decision in front of a jury.

There are a lot of neighborhoods around airports that have higher lead contamination levels. IIRC this is one of the arguments that the people trying to close Reid-Hillview are making.

That’s true, and the EPA has been supporting that by conducting new “ambient” lead testing around airports, with sensors located just a few feet from runup areas, for example. Since there’s no safe amount of lead ingestion, the EPA has also reduced the action level by two orders of magnitude.

Seems to me those folks need to be more concerned with drunk drivers around Reid-Hillview.

What aboutism is seldom an effective argument in a regulatory discussion.

It seems reasonable (to me anyway) to have two fuel grades now: unleaded and leaded.

It doesn’t seem reasonable to anyone who has to pay for leak-detection-equipped tanks and segregated production facilities. Nobody in the industry is interested in that expense, including pilots—especially once they find out how it might affect avgas pricing.

In clean-and-green California, unleaded is only available at San Carlos, and it is the new Swift stuff.

The Swift 94UL is the only unleaded avgas being produced, but there’s not enough of it; San Carlos runs out regularly. But Swift doesn’t believe it’s economic to make two grades. The company says when its 100UL is approved, the 94UL will go away; it can only afford one supply chain.

Is there really not enough interest or is it willingness in adding unleaded as a fuel choice in the areas that have the volume of traffic to support it?

The economics simply don’t work.

Big-bore Lycomings and Continentals require 100 octane because they run higher compression ratios. But does that mean they won’t run just fine on 94UL? At what load/RPM do they start to suffer from detonation? If they start to detonate at max RPM, would planning a limitation of 100 to 150 RPM less make the aircraft’s performance useless or undesirable?

Combustion chamber size and compression ratio are the biggest drivers. Higher RPM actually helps avoid detonation. That’s why you see motorcycles with tiny cylinder volumes and 12:1 compression do fine at 10,000-plus RPM on pump gas. To make the high-compression (higher than roughly 8.5:1) big-bore engines happy on 94 octane would mean reducing compression ratio, which hurts efficiency and power.

The NA IO-550 is an 8.5 compression ratio though. No different than the O-320-D3G was in my Warrior II. No need for 100LL.

That’s an overbroad statement. Most IO-550’s breathe much better than your O-320, putting more fuel/air mixture into the cylinder per displacement cubic inch. That means more power, more heat and more tendency to detonate.

On FlightAware, I’m looking at the “airborne aircraft by type” page. First on the list (the most common type showing up there) is the Cessna 172, with 565 airborne. The next most common GA type is the Piper Cherokee of the PA-28 persuasion, with 244 flying. Number three is the Cessna 182 Skylane, with 146. Fourth is the Cirrus SR22, with 73. After that, it’s the Cessna 150 with 62, Cessna 152 with 46, Mooney M20 with 45, Beech Bonanza 36 with 32 and the Diamond DA40 with 29. 

I believe there is an 80-20 or so split, but the 80 and 20 are reversed from what “everybody knows.”

Have you corrected your calculation for fuel flow? Is your sample statistically valid? The FAA’s relook a few years ago was not far from the 80/20, maybe 70/30.

The quickest, easiest solution is to make flying our “religion.” The AIM is our “Bible.” A flight is a religious “service” during which we commune with Sky (God).

You’ve heard of the Church of the Lean of Peak, and their patron saint, Saint George of Ada?

Got more questions? Hit us up and let’s keep the discussion going.