Work has been suspended on one of two candidate fuels being advanced through an FAA program to develop a high-octane unleaded avgas that can work across the entirety of the U.S. piston-engine aircraft fleet.

Testing of 100M, a 100-octane unleaded avgas developed by energy company Phillips 66 and partner Afton Chemical under the FAA’s Piston Engine Aviation Fuels Initiative (PAFI), has been “paused,” both Phillips 66 and the FAA said Jan. 23.

Should 100M be withdrawn from the industry-government testing program, one other unleaded fuel candidate has progressed to this stage under PAFI. Last November, the FAA announced that UL100E, developed by a partnership of VP Racing Fuels and LyondellBasell, had passed a 150-hr. engine durability test phase, the first fuel to reach that milestone since PAFI was established in 2014.

“We can confirm that PAFI evaluation has been paused on the Phillips 66/Afton Chemical 100M unleaded fuel,” Phillips 66 said in response to an inquiry. “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.”

The FAA said PAFI evaluation of 100M “has been paused due to issues encountered during durability testing.”

Work toward producing a 100-octane unleaded fuel as a drop-in replacement for 100 Low Lead (100LL), the most common avgas, has accelerated in the two years since supervisors in Santa Clara County, California, made a controversial decision to stop supplying 100LL at county-owned airports. Supervisors took that step in contravention of FAA grant commitments after a study found elevated blood-lead levels in children living near Reid-Hillview Airport in San Jose.

Piston-engine airplanes and helicopters that run on leaded avgas are the largest remaining source of lead emissions into the air, according to the U.S. Environmental Protection Agency (EPA).

Nearly all of the roughly 170,000 active piston-engine aircraft in the U.S. burn 100LL containing the fuel additive tetra-ethyl-lead (TEL) as an antiknock agent, the Transportation Research Board says.

Aircraft with high-horsepower, high-compression piston engines consume 70% of the supply.

In October 2023, the EPA 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; the FAA will develop standards for the composition and properties of fuel or fuel additives to eliminate lead emissions.

Representatives of the Phillips 66-Afton Chemical partnership briefed pilots on the status of their candidate fuel last July during EAA AirVenture Oshkosh in Wisconsin. They said 100M uses a manganese-based octane enhancer instead of TEL and had been tested on a Lycoming piston engine.

Having successfully passed PAFI engine durability testing, the VP Racing-LyondellBasell fuel has advanced to full-scale engine and airframe testing on 10 engines and eight aircraft, which is expected to take 12-18 months. San Antonio-based VP Racing Fuels announced last month that it has formed a new company, VP Aviation, to commercialize high-octane unleaded avgas.

The advance of UL100E through the PAFI program lags FAA approval of another high-octane unleaded fuel through the agency’s supplemental type certification process. In September 2022, the FAA authorized most piston aircraft and engine models to use General Aviation Modification Inc.’s (GAMI) G100UL avgas.

GAMI has endeavored since then to gain acceptance of G100UL by aircraft and engine manufacturers and fuel distributors.

Another high-octane unleaded avgas developed by Swift Fuels, of West Lafayette, Indiana, is working its way through the FAA STC and ASTM product specification processes, with FAA fleetwide authorization anticipated in 2025. Swift since 2015 has produced a lower-octane unleaded avgas—UL94—that can be used in about 66% of the piston aircraft fleet, the company says. UL94 is currently available at 36 public-use airports.