The 100R Avgas Transition to Unleaded

Every summer, the ramps and campgrounds of Wittman Regional Airport (KOSH) fill with the same question in a hundred different forms: When will unleaded fuel actually work in my airplane? This year at EAA AirVenture Oshkosh, Swift Fuels arrived with an answer that was bigger than almost anyone expected.

Just days before the show opened, the FAA published an expanded approved model list under Swift’s existing supplemental type certificate for 100R, its 100-octane unleaded avgas. The update added more than 1,200 engine models and 1,600 airframe models. Jonathan Welsh of The Aviation Consumer visited with Swift Fuels CEO Chris D’Acosta at Oshkosh to walk through how the list got so long, so fast, and why he believes the industry is finally within sight of a lead-free ramp.

D’Acosta said that this was a long-awaited update.

“We started commercially selling unleaded fuel back in 2015 with a proprietary version of avgas, which was called UL94,” D’Acosta said. “Our UL94 STC had a rather large expanse of lower octane TCDS approved engines and aircraft, and we utilized that tranche as the starting point for our Phase I deployment of our 100-octane fuel, which is called 100R.”

That Phase I approach has paid off. Swift now counts 1,200 FAA-approved engines and 1,600 airframes under its 100R STC, using what’s known as an approved model list (AML), a mechanism that allows an FAA approval to cover broad categories of aircraft. This first tranche of various FAA approved engines and aircraft types are monitored by Swift Fuels and records show these aircraft collectively consume an estimated 56 percent of the 100LL typically consumed each year by certificated aircraft in the U.S. piston fleet.

Welsh, who flies a Rockwell Commander with a Lycoming IO-540, admitted he had already found his own airplane on the list. But he also flagged something plenty of owners are running into—the search isn’t always intuitive. 

D’Acosta said that the system relies on a wide variety of documentation and data formats, but that Swift Fuels is working to help the FAA.

“Now we’re being asked to put things into a standard form in the DRS system of the FAA, which results in a transposition of some of the data,” he said.

Compounding the confusion, decades of corporate mergers mean an owner’s airplane may be filed in the Swift Fuels AML under a name that no longer appears on the type certificate.

“The classic example is something like, ‘I’m looking for my Cessna, but it’s in the DRS under Textron,’” D’Acosta said.

Swift’s fix is a purpose-built search tool that lets pilots type in their engine model, airframe, or even an old or informal manufacturer name. An AI-assisted search sorts out the rest. 

Behind the announcement sits a certification process D’Acosta describes as a three-legged stool inside the FAA’s traditional certification branch—engineering, production, and flight safety, all of which have to align before a certificate is issued. Because Swift isn’t an airframe or engine manufacturer, its path runs through a supplemental type certificate (STC) rather than an original type certificate, layered on top of the underlying products it doesn’t build itself. 

“The time-consuming part is getting all those parties involved, participating in an agreement and alignment before it’s issued,” D’Acosta said.

The paperwork doesn’t stop at the AML, however. Swift is also a PMA-authorized parts manufacturer, which means the fuel placards used on customer aircraft need their own sign-off. Every customer who has ever purchased a Swift STC, going back to the earliest UL94 buyers years ago, will now receive an updated packet at no additional charge. D’Acosta calls this a “Forever Avgas STC.”

The new packet includes both a fuel placard and a small metal engine tag that an A&P installs under the cowling. Pilots who haven’t yet bought an STC will still need to purchase one before they can legally burn 100R.

The certificate requirement applies specifically to certificated aircraft, not the roughly 35,000 to 40,000 experimental, light sport, or special category airplanes that fall outside that framework. Of the 225,000 piston aircraft on the U.S. registry, D’Acosta puts about 180,000 in the certificated category that will need an STC. 

Many pilots grew up on 100LL but might not understand the significance of this transition. According to D’Acosta, who remembers when lead came out of automotive gasoline in the 1990s, it’s more of a chemistry problem than a mechanical one. 

“Lead is a tool used in a piston engine to suppress the tendency of an engine to detonate,” said D’Acosta, describing how compressed fuel-air mixtures in the cylinder want to explode prematurely under heat and pressure. “Lead was found to be a high-octane contributor to gasoline to suppress detonation, so the combustion event will wait for the spark to fire. The trouble is [that] lead is also toxic, and it has byproducts like fouling.” 

Those deposits from lead, according to D’Acosta, foul spark plugs, blow by into the oil, and contribute to the acidity that corrodes engines over time. 

Swift’s answer is a proprietary additive package dosed at 250 parts per million that prevents valve seat recession, resists intake coking, and includes antioxidants to keep the fuel and engine internals clean. That formulation, along with the base fuel itself, is patented, and it underpins the production specification Swift earned through ASTM International. That consensus standards process is the result of a collaborative effort among oil companies, engine makers, and airframe manufacturers. 

“We’ve earned an ASTM International production specification,” D’Acosta said. “That’s a big deal. It took a long time and a lot of extraordinary work with our fuel and aerospace industry collaborators across the globe.”

With the current tranche of engines and airframes approved, Swift’s attention is already shifting to the higher-compression “550 series” engines, including the 540s, 520s, and 390s that power much of the high-performance piston fleet. D’Acosta recently bought a Cirrus SR22T, powered by a turbocharged TSIO-550-K, specifically to generate flight data supporting the certification push, alongside dedicated endurance testing at an underground test cell at Purdue University and upcoming work with Continental Motors in Mobile, Alabama.

The AirVenture flight line itself became something of a proving ground. A visiting 9-cylinder radial warbird, powered by a Wright R-1820, had flown from its home base in Iowa to Swift’s Indiana headquarters, refueled there, and continued on to Oshkosh. After the show it was headed back to Indiana, where D’Acosta said it would run on Swift fuel exclusively for the following three to four weeks.

Elsewhere on the Warbirds grounds, Cory Miller from Strix Aero set up an Allison V-12 engine test cell operating exclusively on Swift 100R fuel. A California owner  Jose Flores has been testing a Rolls-Royce Merlin V-12 engine on 100R.

In parallel, Swift has joined testing programs at the FAA’s William J. Hughes Technical Center in Atlantic City, New Jersey, which runs its own engine evaluations under a separate approval process that, unlike an STC, doesn’t result in a certificate but still requires a significant degree of FAA oversight.

D’Acosta credits a long list of FAA personnel—from Administrator Bryan Bedford and Deputy Administrator Chris Rocheleau down through the Central Certification Branch, the FAA Tech Center and FAA’s fuels research branch—for staying engaged through what has been a multiyear, multioffice process. 

“We’ve had bumps in the road, from government shutdowns, staff turnover, unexpected delays, etcetera,” D’Acosta said. “That’s just normal. Happens all the time. But they’ve committed themselves to talk to us, listen to us, and adjust things where they can.”

For Welsh and thousands of owners now checking Swift’s list for their own tail numbers, the moment carried a personal note.

“I’m looking forward to being able to pull up to the fuel farm and not have my kids asking me, ‘Does that stuff still have lead in it?’” Welsh said.

Based on the pace of approvals rolling out of Indiana this summer, that day looks considerably closer than it did a year ago.

Click here to learn more about Swift Fuels.