G100UL, the End of Leaded Avgas, and the Unleaded Fuel Fight Playing Out on the Oshkosh Ramp

The aviation industry is working to eliminate leaded avgas - the last leaded transportation fuel in America - and replace it across the entire piston fleet by 2030. The leading unleaded contender is G100UL from GAMI, approved by the FAA through a supplemental type certificate, but it faces competition from Swift Fuels and a slower industry-consensus effort, plus open questions about material compatibility. That fight is playing out right now on the ramp at AirVenture Oshkosh, where thousands of piston aircraft get fueled every day.

Why is aviation still using leaded fuel?

For roughly a century, piston aircraft have run on gasoline containing tetraethyl lead. That is what the “LL” in 100LL stands for - low lead - which is a strange name given it is the only transportation fuel in the United States that still contains any lead at all. Cars lost lead in the 1990s. Boats, motorcycles, and lawnmowers are long gone too. Aviation kept it.

The lead is not there by accident. It does one specific job extremely well: it raises the fuel’s octane rating. Octane is not a measure of energy - it is a measure of resistance to detonation, the condition where the fuel-air mixture in a cylinder explodes all at once instead of burning in a smooth, controlled wave. That hammer blow can crack a piston, burn a valve, or destroy an engine in minutes.

High-compression, high-horsepower engines are the most at risk - the big-bore Continentals and Lycomings, and the turbocharged singles hauling six people over mountains. For a hundred years, a tiny amount of lead has been the cheapest, most reliable way to give those engines the octane they need. The catch is stark: the thing poisoning the fuel is also the thing protecting the engine.

Why does leaded avgas have to go?

General aviation burns a small volume of fuel compared to the airlines. But because it is the last leaded fuel in wide use, piston aircraft are now the largest source of airborne lead emissions in the country. Lead has no known safe level of exposure, particularly for children, and communities around busy airports have pushed on this issue for years.

The Environmental Protection Agency (EPA) issued an endangerment finding - the legal trigger requiring the problem to be addressed. This is not optional, and the clock is running.

What is the EAGLE initiative and its 2030 deadline?

The industry’s coordinated plan is called EAGLE - Eliminate Aviation Gasoline Lead Emissions. It is a joint effort involving the FAA, engine makers, fuel producers, and the major aviation associations. The stated target is to move unleaded fuel across the entire piston fleet, safely, by the end of the decade - 2030 - without stranding any operator.

The word “everybody” is the whole challenge. It is not enough to make an unleaded fuel that works in a Piper Cub - a low-compression engine can run on almost anything. The hard part is the other half of the fleet. Roughly two-thirds of the avgas burned in the U.S. goes into the minority of high-compression, high-horsepower engines. A fuel that cannot feed those engines is not a solution; it is a boutique fuel for trainers.

The real engineering bar is a drop-in replacement: same tank, same lines, same seals, same engine, no modifications - delivering the same octane as 100LL, without the lead. That is a brutally high standard.

What is G100UL and how was it approved?

The name heard most often is G100UL, developed by General Aviation Modifications, Inc. (GAMI) of Ada, Oklahoma - the same company known for balanced fuel injector sets that allow lean-of-peak operation. GAMI spent roughly two decades developing its unleaded 100-octane fuel.

GAMI’s approval path was unconventional. Instead of going through the traditional consensus specification process run by the industry standards body, GAMI used the Supplemental Type Certificate (STC) route - the same mechanism used to add a new autopilot or engine to an aircraft. The FAA approved the fuel engine by engine and airframe by airframe until the STC covered essentially the entire piston fleet, every spark-ignition engine. An owner buys the STC, places a placard near the filler, and is legally cleared to burn the unleaded fuel.

It was a genuinely novel path, and on paper it worked. The fuel is approved.

Is G100UL safe and available yet?

This is where honesty matters: approved is not the same as available, and it is not the same as trusted.

There are real questions about material compatibility - whether G100UL plays nicely with every paint, sealant, fuel bladder, and O-ring in a fleet of aircraft that in some cases are older than the pilots flying them. GAMI and its partners say the data supports the fuel, that the concerns are being tested and addressed, and that some of the noise is competitive. Skeptics want to see many more aircraft fly many more hours before calling it settled. Both positions are reasonable - that is what a real engineering transition looks like, and it is not clean.

What other unleaded fuels are competing?

G100UL is not the only contender.

Swift Fuels already sells a lower-octane unleaded fuel, UL94, that many low-compression aircraft can burn today, and the company is developing its own 100-octane unleaded product.

Separately, a joint effort involving Phillips and other majors is working through the traditional consensus specification process - the slower, more conservative path aimed at an industry-wide spec rather than an STC. That approach takes longer, but if it succeeds it gives fuel producers and airport fuel farms a single standardized product they can all manufacture and distribute the same way.

So two philosophies are fighting it out: get an approved fuel into the trucks now, aircraft by aircraft, and iterate - or do it once, do it slowly, with a full industry standard so no one is surprised in five years. Aviation has a deep bias toward the second approach, because the failure mode is a smoking hole.

Why does Oshkosh matter for the fuel fight?

AirVenture Oshkosh at Wittman Regional Airport is aviation’s launchpad, and fuel is no exception. This week the fuel companies are there with booths, data sheets, and engineers ready to argue octane numbers with anyone willing to stand and listen. It is where the industry has its family argument in public.

More importantly, for one week Oshkosh becomes one of the busiest airports on the planet, fueling thousands of piston aircraft. That fuel operation is a genuine logistics feat, with tankers running nonstop. It is exactly the kind of environment the industry looks to when it wants to prove a new fuel can move at volume - into real aircraft flown by real people who then fly home safely. A tired pilot topping off at the end of the week and flying home over three states is a more meaningful test than any lab bench.

What should pilots do right now?

100LL is not disappearing next month. There is a transition window, and the entire stated goal of EAGLE is that no operator is left without fuel. There is no reason to panic-sell a high-performance aircraft.

If you fly a low-compression engine, an unleaded option such as UL94 may already be available at fields near you, with more coming. If you fly a high-performance engine, watch the material compatibility data closely, talk to your engine manufacturer, and make your own decision about when to switch. You do not have to be first.

Finally, pay attention to how this gets solved, not just whether it does. The method used to retire leaded avgas - STC versus consensus spec, fast versus slow - is likely to become the template for every hard transition coming behind it, from sustainable fuels to new engine technology. The lead fight is aviation learning how to change something fundamental in a hundred-year-old fleet without breaking it.

Key Takeaways

• Leaded 100LL is the last leaded transportation fuel in America and is now the largest source of airborne lead emissions in the country; an EPA endangerment finding legally requires it to be phased out.
• The industry’s EAGLE initiative aims to move the entire piston fleet to unleaded fuel safely by 2030.
• G100UL from GAMI is FAA-approved via a fleet-wide STC, but faces open questions about material compatibility and real-world availability.
• Swift Fuels (UL94 today, 100-octane in development) and a slower Phillips-led consensus-spec effort are competing alternatives representing a “ship and iterate” versus “specify and be sure” philosophy.
• Roughly two-thirds of avgas goes to high-compression engines, so any real solution must serve them - not just low-compression trainers.