US regulators have approved the use of a 100-octane unleaded fuel in all spark-ignition aviation engines, a move the general aviation industry says will help its transition away from leaded fuel.

The Federal Aviation Administration (FAA) has issued a supplemental type certificate (STC) for a fuel called G100UL, developed by Oklahoma company General Aviation Modifications (GAM), according to the Aircraft Owners and Pilots Association (AOPA) trade body.

Skyhawk 172

Source: Cessna

The STC means the unleaded fuel can be “used in every general spark-ignition engine and every airframe powered by those engines”, AOPA said on 1 September.

GAM did not immediately respond to a request for comment.

The FAA confirms the approval, saying the move follows a review of the company’s “test results and other documentation by an independent technical panel”.

Many high-compression, high-displacement aviation piston engines require high-octane fuel, which historically contains lead. “Octane is a measure of the performance of a fuel as it burns in an engine combustion chamber”, says the FAA. Octane levels also relate to a fuel’s ability to resist detonation, also known as “knock”.

The aviation industry has long relied on gasoline treated with an organic compound called tetraethyl lead (TEL), which is used to boost octane ratings, the FAA says. “All forms of lead are toxic if inhaled or ingested.”

While leaded fuel has been banned from automotive use, it remains as an aviation fuel due to performance requiremants.

“The use of leaded fuels is an operational safety issue, because without the additive TEL, the octane levels would be too low for some engines, and use of a lower-octane fuel than required could lead to engine failure,” the FAA says. “As a result, the additive TEL has not been banned from avgas.”

But the FAA has been supporting a transition to non-leaded, high-octane aviation fuels.

“We cleared the way for an unleaded aviation fuel to be used throughout the nation’s fleet of piston-powered aircraft,” deputy FAA administrator Bradley Mims says. “The FAA will require the fuel manufacturer to work with aircraft owners to track and report any unforeseen mechanical issues that might arise from introducing its fuel into existing fuel systems.”

AOPA describes the approval as a significant step in replacing leaded aviation fuel with its unleaded equivalent. It comes at a time the general aviation industry faces immense pressure to transition away from the leaded fuels used by many piston-powered aircraft due to environmental and safety concerns.

Pressure heightened in January when Santa Clara County in California banned the sale of 100-octane leaded fuel, commonly known as 100LL, over FAA objections. Also this year, the FAA and a range of industry groups agreed to eliminate the use of leaded aviation fuel by 2030.

In an interview with AOPA, GAM’s head of engineering George Braly says the FAA’s approval applies to “every single spark-ignition piston engine found anywhere in the FAA database”.

“This is a big day for the industry,” Braly adds. He notes many general aviation aircraft owners operate their aircraft in environmentally sensitive areas, like the US West Coast. “Relief is on the way,” says Braly.

The FAA already approved some unleaded fuels as substitutes for leaded gasoline in some piston-powered aircraft.

In 2021, the agency greenlit the use of GAM’s 100-octane fuel in some Cessna 172 engines. The FAA has also approved the use of a 94-octane unleaded fuel produced by an Indiana company called Swift Fuels. But that fuel is only used in lower-compression engines.