Bumpy flight

As multiple fuels progress through testing and certification, AOPA is focused on sharing our real-world experience with unleaded fuels so pilots can transition with confidence—and on ensuring 100LL remains available until we can safely move to unleaded alternatives.

AOPA is currently demonstrating the viability of unleaded fuel in a dual-fuel Beechcraft Baron, which has 170 hours and counting, with one engine burning100LL avgas and the other running on GAMI’s G100UL, the use of which has been authorized by the FAA. Throughout the process, AOPA is committed to sharing operating data from the airplane, including the similarities and differences between 100LL and the new fuel, and will do the same with other fuels as they are authorized for use.

In the meantime, the president has signed legislation that would safeguard the availability of 100LL avgas—anywhere it was available in 2022—until the FAA determines that an unleaded alternative is available as local governments, seizing on the long-expected endangerment finding by the Environmental Protection Agency in 2023, have already stopped or are threatening to stop selling 100LL. Multiple fuels currently going through the certification process have faced challenges.

G100UL 

After receiving approval of a fleetwide supplemental type certificate for airplanes in September 2022 (helicopter engines are still in the works), GAMI has been working to find production and distribution avenues to get its G100UL into the field. GAMI’s George Braly said that fuel refiner Vitol has 1.3 million gallons ready to go and capacity to produce 12 million gallons a year in only one of its tanks. Braly said that’s enough to supply California’s needs, and there’s a 5-million-gallon tank in Houston that can be put into service when the demand is there, which could supply the entire western part of the country.

The current challenge is distribution. Braly said avgas distribution is controlled by a small number of providers, and his company has thus far been challenged to find a distributor that is willing to get the fuel from the refinery to FBOs.

That logjam isn’t likely to be broken anytime soon, as NATA, which represents airport businesses, and whose CEO, Curt Castagna, currently serves as the industry head of the Eliminate Aviation Gasoline Lead Emissions (EAGLE) program, recently released a paper advising members not to sell G100UL because it doesn’t meet an ASTM standard.

“Unleaded aviation gasolines should not be offered as a standalone fuel without an industry consensus standard such as ASTM International, confirmation of materials compatibility throughout the supply chain, and FAA testing for compatibility with other FAA-approved unleaded avgas formulations,” the paper said.

In a separate statement, Castagna said, “For NATA’s FBO and fuel distributor members, an industry consensus standard such as ASTM International is an especially relevant component in bringing new fuels to market, as it evaluates compatibility with materials throughout the supply chain upstream of the aircraft.” He also cited liability and the need for engine manufacturers and others to test and weigh in on the fuel’s suitability as key factors in GAMI’s current challenges. While the ASTM standard does give some assurances to fuel providers, proper liability protection is still an issue.

Braly dismissed the paper’s concerns and said the distributors he’s spoken with aren’t concerned about G100UL’s safety.

GAMI initially pursued ASTM certification, a lengthy and expensive path separate from the STC approval received from the FAA. While the STC process involves only the company and the FAA, ASTM is an industry consensus standard where competitors come together to create protocols and approve applicants. There are controls in place that are designed to protect the applicant’s intellectual property. However, after some initial meetings and an application with ASTM, Braly alleges that an oil company attempted to file patents on GAMI’s candidate fuel, at which point he abandoned the process.

Swift UL94

Some pilots are understandably skeptical of switching to an unleaded fuel for fear of what it will mean for their engines and safety. Some of those fears were confirmed recently, when after operating Swift Fuel’s ASTM-approved UL94 for 46,000 hours, the University of North Dakota late last year was forced to revert back to 100LL because of numerous valve issues. The university had made the switch to UL94 in its Piper Archers and Seminoles last summer to bolster support for unleaded fuels. The fuel has an ASTM approval, an STC from the FAA, and is approved by Lycoming for use in the Archer’s IO-360 engine.

School maintenance technicians first learned of a problem when an Archer returned to the school’s Grand Forks base with a rough-running engine. Dan Kasowski, the head of maintenance, said they removed the cylinder and found valve seat recession. “At that point it was a one-off and we didn’t think much of it,” he said. The discovery of a second recessed valve seat soon after spurred them to report the issue to Lycoming and Swift Fuels representatives.

A full inspection program revealed issues with 135 cylinders, many of them with recessed valve seats. Lead has long been known to prevent valve seat recession, but the transition to unleaded auto fuel taught engine makers that hardened valve seats can prevent the problem. UND’s Lycoming engines have hardened valve seats.

In a statement, Lycoming said, “Lycoming Engines conducted a root cause analysis of this issue, including further testing of UL94. This analysis has determined that the engine components and airframe components conform to specification and are not believed to contribute to valve seat recession”. It goes on, “Analysis indicates that aromatic concentration tolerances for UL94 may contribute to valve seat recession in certain flight profiles. Aromatic limits are not directly specified by ASTM D7547, and elevated aromatic concentration may result in slower flame speed, radiant heat from particulates, and particulate abrasiveness to valve seats that may contribute to valve seat recession.”

In other words, Lycoming blames Swift, and in a rebuttal, Swift CEO Chris D’Acosta blames Lycoming. “Lycoming’s press release on the root-cause analysis of UND’s issue with exhaust valve recession is at best inconclusive. High aromatic hydrocarbon concentrations in UL94 are not the culprit in this analysis. Moderate use of toluene (typically 10 to 20 percent) in the fuel is entirely reasonable and not likely to contribute to valve seat recession.”

One of the challenges of transitioning to an unleaded fuel is getting the octane high enough to prevent detonation. At 94 octane, the Swift fuel is lower than avgas, which has been suggested is a problem on the engine’s 8.5:1 cylinders. Lycoming and Swift alluded to the detonation question in their statements, saying that mixture settings could have played a role. Kasowski said none of the cylinders that came off had any signs of detonation.

UND is still looking forward to using unleaded fuels, as soon as they can ensure the fuel’s safety. Bob Kraus, the dean of UND’s aviation program, said the ASTM standard requires 150 hours of testing. “At 150 hours, we didn’t see anything on most of the aircraft,” he said. He doesn’t think switching was premature because of the robust testing that went into the fuel. But their experience raises questions about the design of the testing and whether it’s rigorous enough.

Swift is also working on a 100-octane unleaded formulation through the STC process and says it expects FAA approval later this year.

EAGLE/PAFI

Meanwhile, the joint industry and FAA EAGLE initiative, which seeks to remove leaded fuel from the market by 2030, continues. The Piston Aviation Fuels Initiative is currently testing one fuel by LyondellBasell Industries/VP Racing. In fall 2023, the fuel passed out of the initial 150-hour durability stage and is now in the process of full-scale testing on aircraft. This second 150-hour phase includes 10 engines on eight aircraft.

At an EAGLE update in May, an FAA representative said the LyondellBasell/VP Racing fuel is progressing well and the agency has a high confidence level that it will come out of the testing positively, but that it probably won’t be a drop-in replacement for the entire fleet. A subsection of engines will likely require modifications in order to work, the representative added. If approved through the PAFI process, the LyondellBasell/VP Racing fuel will also receive an ASTM standard approval.

What’s clear to EAGLE members and others is that the will to move away from 100LL is, after decades of effort, finally a clear and shared goal. What happens between now and full commercial availability appears to be not as straightforward as everyone would hope, however. “We want this to succeed,” Kraus said. “There are so many reasons to move away from avgas.”
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