Unleaded UL100E fuel advances as pressure mounts to replace 100LL avgas in US

A proposed 100-octane unleaded aviation fuel developed by LyondellBasell and VP Racing Fuels has moved closer to potential fleetwide approval, as pressure continues to build on the US general aviation industry to transition away from leaded 100LL avgas. 

According to the companies, ASTM International has published a new standard specification that clears a remaining hurdle for UL100E, an unleaded avgas candidate currently progressing through the FAA’s Piston Aviation Fuels Initiative (PAFI) approval path. With testing still underway, the companies claim that fleetwide FAA approval could follow later in 2026. 

UL100E is the only unleaded 100-octane fuel participating in the FAA’s fleet approval process, which aims to identify a single avgas replacement that can safely serve the broad piston aircraft fleet. The initiative was launched in 2014 to address mounting environmental and regulatory pressure to eliminate lead emissions from aviation gasoline. 

Published under ASTM designation D8631-25, the new specification establishes the testing and evaluation criteria that the FAA will use as it completes its review. LyondellBasell and VP Racing Fuels noted that recent milestones include material compatibility testing conducted by aircraft and engine manufacturers and testing at the FAA William J. Hughes Technical Center in New Jersey. 

Flight testing is ongoing on a Lancair Super Legacy equipped with a turbocharged Continental TSIO-550 engine and a Harvard Mk IV warbird powered by a Pratt & Whitney R-1340 radial engine. The FAA is also conducting endurance and detonation tests on large-block Lycoming and Continental engines, with PAFI testing expected to conclude in September 2026. 

The FAA will review the results to determine which aircraft and engine combinations are eligible for UL100E, as well as whether any operational limitations or modifications are required for a subset of the fleet, the companies said. 

If approved, UL100E would enter a market with few true alternatives. G100UL, from General Aviation Modifications Inc. (GAMI), received a broad supplemental type certificate in 2022 but has seen limited distribution and has not been submitted for ASTM evaluation. Meanwhile, Swift Fuels’ 100R has been approved via STCs for specific aircraft and engines and is being used at a small number of flight schools. 

The push for an unleaded replacement has taken on added urgency in California, where several local governments have moved to restrict or eliminate the sale of leaded avgas at publicly owned airports. Santa Monica Airport ended 100LL sales, and Santa Clara County stopped selling leaded avgas at Reid-Hillview Airport in 2022, citing public health concerns. Those actions have raised concern within the industry about a patchwork of local restrictions emerging ahead of a nationwide rollout of lead-free avgas. 

Aircraft Owners and Pilots Association President Darren Pleasance described the ASTM milestone as a sign of continued progress toward the industry’s goal of introducing a safe, fleetwide unleaded fuel in the US by 2030. 
 
“This is a meaningful milestone as the aviation industry continues to make progress toward an unleaded future,” Pleasance said. “We congratulate VP Racing and LyondellBasell for the work they’ve done and for contributing to the momentum needed to deliver safe and reliable solutions for general aviation.”