Nearly a half-century since automakers started building cars that run on unleaded gasoline and 16 years after Nascar switched to using unleaded fuel in race cars, general aviation in the U.S. is making steady progress, with some hiccups, toward phasing out leaded avgas.

The U.S. Environmental Protection Agency (EPA) started issuing lead emissions reduction standards in the 1970s and mandated the use of unleaded fuel in passenger cars as of model-year 1975. In its early enforcement of the landmark Clean Air Act, the agency said fuel containing lead could continue being sold for off-road uses in aircraft, race cars, farm equipment and marine engines. Pressured by environmental groups, Nascar switched to unleaded fuel in 2008.

Piston-engine airplanes and helicopters that run on leaded avgas are the largest remaining source of lead emissions into the air, the EPA now says. In October 2023, the agency announced a final determination that lead emissions from aircraft that operate on leaded fuel contribute to air pollution and endanger public health.

The long-anticipated “endangerment finding” triggered separate rulemaking processes: The EPA will develop regulations for lead emissions from aircraft engines, and the FAA will develop standards for the composition and properties of fuel or fuel additives to eliminate lead emissions.

EAGLE Takes Flight

It has been two years since industry associations and the FAA—eyeing efforts by local communities to shut down GA airports over various reasons including lead emissions—announced the Eliminate Aviation Gasoline Lead Emissions (EAGLE) initiative with the stated goal of moving the U.S. piston-engine aircraft fleet to unleaded avgas by 2030 or sooner.

Directed by an executive committee consisting of top leaders of GA industry associations and Lirio Liu, executive director of the FAA Aircraft Certification Service, EAGLE serves an over-arching role in the transition to unleaded avgas. It aims to facilitate not only the development of new unleaded fuels to replace 100 Low Lead (100LL), the most common avgas, but also the production, distribution and supply of those fuels to airports and fixed-base operators (FBO) nationwide.

Nearly all of the roughly 170,000 active piston-engine aircraft in the U.S. burn 100LL containing the fuel additive tetra-ethyl-lead to boost octane rating, according to the Transportation Research Board (TRB). Aircraft with higher-performance, high-compression piston engines consume about 70% of the supply, the industry says.

Airports and FBOs must maintain a supply of 100LL until a 100-octane unleaded fuel becomes commercially available, EAGLE’s principals say, to ensure that the aircraft engines that require it continue to operate safely and to protect the economic viability of the industry. The EPA’s lead emissions endangerment finding did not ban the sale of 100LL, they emphasize.

“We’ve all aligned that we have to do this,” says National Air Transportation Association (NATA) President and CEO Curt Castagna, who serves as the EAGLE industry co-chair. “We’re in this transitionary stage,” he adds. “We have to protect the 100LL, we have to protect the national airspace system and at the same time we have to show progress in the evolution to phase out 100LL. What we need is a continued rationale approach.”

In December 2023, Alaska’s U.S. Senators Lisa Murkowski and Dan Sullivan, both Republicans, introduced a resolution in Congress seeking to prevent EPA regulation of aircraft engine lead emissions. Such a regulation “ignores Alaska’s unique geographic reliance on aviation and will cause real harm to indigenous and rural communities across the state by potentially increasing fuel costs and impacting flight availability,” the senators said.

Rather than detracting from its mission, the case made by lawmakers from Alaska underscores EAGLE’s argument that the transition to unleaded avgas must be done carefully, Castagna says.

“Different places in the country have different needs relative to avgas,” he says. “In Alaska and Hawaii, it’s critical to their way of life and how aircraft are used in their economies. That’s not to minimize communities’ concerns over the impact of lead, but the movement of people and goods in Alaska is a primary responsibility. We have to look at this from a national perspective.”

Unveiled in public in February 2022, EAGLE helped reboot what had been a prolonged, FAA-led research and testing program, called the Piston Engine Aviation Fuels Initiative (PAFI), to qualify a high-octane unleaded avgas that could work across the wide variety of piston aircraft and engines. The FAA established PAFI in 2014; in November 2023, the agency announced that a first unleaded fuel candidate had successfully passed PAFI’s initial detonation and 150-hr. engine durability test phase.

The UL100E candidate fuel, developed by a consortium of VP Racing Fuels and chemical company LyondellBasell, has advanced to full-scale engine and airframe testing on 10 engines and eight aircraft, which is expected to take 12-18 months. VP Racing Fuels has said that it completed engine durability testing of the fuel on a turbocharged Continental piston engine. Plans called for using a mixed engine fleet to include participation by Lycoming Engines during full-scale testing.

Even as UL100E moved to full-scale testing, though, work on a second fuel being evaluated under PAFI was suspended. Testing of 100M, a high-octane unleaded avgas developed by Phillips 66 and Afton Chemical, “has been paused due to issues encountered during durability testing,” the FAA said in January 2024.

Phillips 66 issued a statement with similar language. “We can confirm that PAFI evaluation has been paused on the Phillips 66/Afton Chemical 100M unleaded fuel,” the energy company said. “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.”

Data from the PAFI testing supports development of an industry-consensus production specification by standards organization ASTM International—key for commercialization of a new product. Once a fuel completes the PAFI regimen and ASTM publishes a production specification, GA associations expect the FAA will issue a fleetwide authorization to allow its use across the range of piston-engine aircraft.

In December 2023, VP Racing Fuels announced that it has formed a new company, VP Aviation, to commercialize high-octane unleaded avgas. The motorsport fuel developer based in San Antonio projects annual demand to the tune of 300 million gal. for avgas worldwide. It did not respond to interview requests.

The advance of UL100E through the PAFI process lags FAA approval of another high-octane unleaded fuel through the agency’s supplemental type certification (STC) process. In September 2022, the FAA authorized most piston aircraft and engine models to use General Aviation Modification Inc.’s (GAMI) G100UL avgas. Pilots will be able to pump it by acquiring aircraft STCs costing in the hundreds of dollars based on the aircraft’s engine and horsepower.

A small engineering company known for developing precision fuel injectors and aftermarket turbochargers, GAMI started work on an unleaded avgas in 2009 and years later passed on joining the industry-government PAFI program. By pursuing FAA authorization through the STC process, its fuel recipe has remained proprietary and has not undergone peer review through ASTM. GAMI has made quantities of G100UL available for testing to aircraft and engine manufacturers that sign non-disclosure agreements.

Last October, Aircraft Owners and Pilots Association (AOPA) President Mark Baker, who preceded Castagna as EAGLE industry co-chair, flew a twin-engine Beechcraft Baron demonstration airplane with one engine running on G100UL to kick off an AOPA evaluation of new unleaded fuels.

GAMI has arranged with Vitol, a blending company in Houston, to produce G100UL by this spring, GAMI co-founder George Braly tells BCA. He understands that Vitol is in discussions with avgas distributors to establish a distribution network.

Dual Pathways Spark Debate

GAMI’s maverick status and the proprietary STC pathway to high-octane unleaded avgas have sparked a debate in the industry, with some trade groups expressing a preference for an industry-consensus fuel specification that has been vetted through the ASTM process.

“The paramount rule in this is that [fuels] are safe and we have to prove that to the FAA,” General Aviation Manufacturers Association President and CEO Pete Bunce told reporters in June 2023. “The way we have done this is we have had known fuels [that] we have been able to certify and test against this standard and that standard has been given to us by ASTM.”

The EAGLE position, Castagna says, is that it supports GAMI’s effort to organize a production and distribution network for G100UL, but that the market ultimately will decide which fuels make it to aircraft wings.

“The question of whether or not we’re going to have multiple fuels at the end of the day is not a decision that EAGLE makes,” Castagna says. “We know there’s 180 million gal. or so of avgas sold annually. Is there room for multiple fuels and different fuels? Really the industry, the consumers, are going to decide that ultimately. Our role with EAGLE is to facilitate the review of process and be a resource for the complete process, from science to the wing of the airplane.”

Among other unleaded avgas candidates, Swift Fuels, based in West Lafayette, Indiana, was pursuing FAA STC and ASTM specification of 100R, a 100-MON (motor octane number) fuel, which it expects to supply as a fleetwide replacement for 100LL in 2025.

Swift, which discontinued its participation in the PAFI program in 2018, has produced a lower-octane unleaded avgas—UL94—since 2015. UL94 satisfies the minimum octane requirements of about 66% of the U.S. piston aircraft fleet, the company says, and is available at 36 U.S. public-use airports. “The potentially significant obstacle to the greatly expanded use of UL94,” says the TRB, “is that thousands of small airports would need to invest more than $100,000 in a second avgas storage and dispensing system to accompany existing systems for supplying leaded avgas to aircraft that require fuel with enhanced octane.”

California Leads Avgas Transition

The FAA and GA associations unveiled the EAGLE initiative following a controversial decision by supervisors in Santa Clara County, California, to replace 100LL with UL94 at county-owned airports in 2022. The supervisors took that step in contravention of FAA grant commitments after a study revealed elevated blood-lead levels in children living near Reid-Hillview Airport (RHV) in San Jose. (FAA reauthorization legislation that was pending in Congress could mandate that airports continue to supply 100LL.)

Airport authorities in other states, including in Colorado and Florida, announced in 2022-23 that they had started offering UL94 alongside 100LL—or planned to supply it—by subsidizing its cost relative to the price of 100LL, helping pilots and flight schools buy the STCs needed to burn it or securing FAA grant funding to help FBOs install the fueling infrastructure.

NATA’s Castagna is president and CEO of aviation property and project management firm Aeroplex Group Partners, which has helped airports in southern California transition to unleaded avgas, beginning with Santa Monica Airport in March 2022. In August 2023, Castagna and Long Beach Mayor Rex Richardson were present as FBO Signature Flight Support pumped the first gallons of UL94 at city-owned Long Beach Airport (LGB).

Since it approved a plan to reduce lead emissions from piston aircraft, the Long Beach City Council has taken a series of steps to incentivize both the delivery and use of unleaded avgas, which is offered at LGB in addition to 100LL. The council voted in December 2022 to waive fuel flowage fees per gallon of avgas pumped at the airport for three years. In November 2023, it approved reimbursements of up to $300 for aircraft owners who obtain STCs to use unleaded fuel.

More recently, on Jan. 23 this year, the council voted to approve a subsidy program to offset the cost differential between unleaded fuel and 100LL, which can be $2-$4 more per gallon for UL94. Councilors appropriated $200,000 to implement the unleaded fuel subsidy program, which will be covered by airport revenue.

LGB joined the Oxnard, Santa Monica and Van Nuys airports in supplying UL94 in southern California, as well as Hayward Executive, RHV, San Carlos and San Martin among airports in the northern part of the state.

Last December, the Livermore City Council approved a resolution requiring unleaded fuel be made available at city-owned Livermore Municipal Airport near Oakland within two years. Authorities in Los Angeles County, which includes the city of Long Beach, plan to supply unleaded avgas at county-owned Brackett Field, Compton/Woodley, Gen. William J. Fox, San Gabriel Valley and Whiteman airports by June this year.

UND Encounters Engine Wear Issue

Among the early adopters of unleaded avgas are flight schools such as the School of Aviation Sciences at Utah Valley University in Provo, Utah, which received its first shipment of UL94 in April 2023.

The University of North Dakota (UND) John D. Odegard School of Aerospace Sciences, one of the nation’s largest public flight schools, announced in 2022 that it would switch its 100-aircraft training fleet to using UL94 in place of 100LL avgas. Following four months and 46,000 hr. of flying, the school resumed using 100LL in October 2023 after encountering an engine wear issue, a development first reported by AVweb.

Ongoing maintenance monitoring of UND aircraft running on UL94 revealed measurable exhaust valve seat recession, primarily in Piper Archers and Seminoles powered by Lycoming engines. The university sent cylinders to Lycoming for analysis and was also working with Swift Fuels to help them understand the issue.

Though independent of the EAGLE effort, UND’s experience points to some of the complexities the initiative faces in moving the U.S. piston-engine aircraft fleet wholly to unleaded fuel.

“EAGLE is aware of the fact that UND suspended the use of Swift’s fuel—actually mutually agreed [with Swift] to do that,” Castagna says. “EAGLE is aware of the fact that Lycoming and the FAA are working together to evaluate the recreation of that issue.”

The initiative has not been made aware of any similar concerns raised by other consumers of UL94, he adds. “I know of two different flight schools that are operating with it and they have not seen the same issues that UND experienced with their valve seating problems,” Castagna says.