G100UL, the Unleaded Avgas the FAA Approved for Every Piston Aircraft, and the Lead Phase-Out That Changes What Every Piston Pilot Puts in the Tanks

In September 2022, the FAA issued a Supplemental Type Certificate for G100UL, a fully unleaded avgas developed by General Aviation Modifications Incorporated (GAMI) that achieves 100 octane without tetraethyllead. The certificate covers every aircraft in the U.S. certified piston fleet currently approved to use 100LL - no individual aircraft modifications required. After decades of partial progress and one government-led initiative that fell short, a certified, universal drop-in replacement for leaded avgas now exists.

Why Aviation Gasoline Still Contains Lead

Aviation gasoline has contained tetraethyllead, an organolead compound, since the 1920s. The reason is straightforward: lead raises octane rating, allowing engines to run at higher compression ratios and manifold pressures without detonation - the destructive premature ignition that hammers pistons, exhaust valves, and cylinder walls at high power settings.

The fuel designated 100LL - the LL standing for “low lead,” a standard that emerged in the 1970s as a step down from earlier high-octane grades - contains approximately 0.56 grams of tetraethyllead per liter. Less than earlier formulas. Not zero.

Lead does two distinct things inside an aircraft engine, and both proved difficult to replace. First, it provides the octane margin that 25 to 30 percent of the piston fleet requires to operate at full rated power: high-compression engines, turbocharged powerplants, large-displacement designs like the Continental IO-550 and Lycoming TIO-541. Remove lead without replacing that octane contribution and those engines must be derated - less power, lower manifold pressure limits, degraded climb performance. Second, lead lubricates exhaust valve seats in older engines that lack hardened valve seat inserts, preventing a wear failure called valve seat recession. Without the protective layer lead deposits form, the valve seat material erodes, compression falls, and the engine eventually fails.

The Government’s Attempt: What PAFI Found

In 2014, the FAA launched the Piston Aviation Fuels Initiative (PAFI) - a structured, government-led program to evaluate candidate unleaded fuels against the full range of certified piston aircraft and engines. The stated goal: a single, universally approved drop-in replacement. Approved once. Works everywhere.

PAFI was thorough. Independent test facilities, engine manufacturers, and the FAA’s own technical centers all participated over several years. The program found that the problem was solvable - but not through any single formula. High-octane candidates raised material compatibility concerns with rubber seals and fuel bladders in older airframes. Fuel-system-compatible formulas couldn’t achieve full 100 octane without compression tradeoffs. Handling and distribution characteristics complicated the supply chain further.

PAFI concluded without a certified fuel. That outcome was widely reported as a failure - but work was continuing outside the government program.

What GAMI Built and What the FAA Approved

General Aviation Modifications Incorporated, based in Ada, Oklahoma, is best known for the GAMIjector fuel injection nozzles that balance fuel distribution across cylinders, enabling lean-of-peak operations without rough running. George Braly and Tim Roehl, GAMI’s founders, have spent decades working on aircraft engine thermodynamics at a level few in the industry match.

Their unleaded avgas formulation, G100UL, uses a combination of aromatic compounds and proprietary additives to achieve 100 motor octane without lead. The full chemistry is proprietary and has not been publicly released. The formulation also addresses valve seat recession: a protective film component performs the lubrication function that lead deposits previously provided, and the FAA’s testing validated acceptable wear rates in engines otherwise vulnerable to this failure mode.

After years of testing, legal disputes over the approval process, and an FAA review that generated controversy around testing methodology, GAMI received its STC in September 2022. The scope was unlike anything that had come before: every U.S. certified piston aircraft currently approved to use 100LL fell under a single certificate. No individual airframe or engine approvals. No aircraft modifications. One certificate for the entire fleet.

Where G100UL Is Available Today

The certified pathway exists. The supply chain is still catching up.

G100UL is currently available at a limited number of locations. California and the West Coast are ahead of the rest of the country, driven in part by state-level pressure on lead emissions near airports. Epidemiological studies have documented elevated blood lead levels in children living near general aviation airports compared to populations farther away. Lead binds to soil and enters water; the exposure pathway from piston aircraft exhaust is well established. That data has generated regulatory pressure in California that other states have not yet matched - but that pressure is spreading.

GAMI has been licensing the G100UL formulation to fuel producers. Getting the product to airports requires refineries willing to manufacture it, distributors to move it, and FBOs to stock it - sometimes requiring new storage infrastructure investment. That supply chain moves slowly.

Where G100UL is available today, it carries a price premium of approximately $1.00 to $1.50 per gallon over 100LL. That gap is expected to compress as production scales, but pilots switching now will pay more at the pump in the near term.

The Other Unleaded Option: Swift UL94

Swift Fuels, based in West Lafayette, Indiana, has been selling an unleaded avgas called UL94 for several years, and it is currently available at more locations than G100UL.

As the name indicates, UL94 delivers 94 motor octane. For the large portion of the piston fleet with naturally aspirated, lower-compression engines - a Cessna 172 with its Lycoming O-320, a Piper Cherokee with a comparable powerplant - UL94 is a legal, viable option available today.

The limitation is real: UL94 does not cover the high-performance end of the fleet. Engines that require full 100 octane cannot operate within their certified envelopes on UL94 without derated power and modified procedures. For pilots of turbocharged or high-compression aircraft, it is not an acceptable replacement.

The Two-Product Future of Avgas

The picture taking shape is a two-fuel world. UL94 handles the lower-octane portion of the fleet: trainers, basic singles, vintage aircraft with modest compression ratios. G100UL covers everything that requires full 100 octane, including turbocharged and high-performance engines. Both unleaded. 100LL eventually phased out entirely.

The FAA has stated publicly that leaded avgas will end in the civil aviation fleet. The timeline and transition mechanics are still being formalized. Rulemaking to establish the phase-out schedule is in progress, and public comment periods will follow - pilot and aircraft owner input on realistic transition timelines and economic impact will carry weight in that process.

What This Means for Pilots Right Now

Check your POH. Your pilot’s operating handbook specifies the fuel grade your aircraft requires, and that specification determines which unleaded option applies to you.

If your aircraft is approved for less than 100 octane, UL94 is available today at airports that stock it. The Aircraft Owners and Pilots Association (AOPA) maintains a fuel availability database showing which airports carry which unleaded alternatives - worth bookmarking before planning a cross-country.

If your engine requires full 100 octane, G100UL is your certified product. It is approved for your airplane. It may not be available at your home field yet, but the distribution footprint is growing. GAMI’s website documents current stocking locations.

If you own an older aircraft with an engine that has never been overhauled, or you are uncertain whether the valve seats have been upgraded to hardened inserts, consult your engine shop before switching fuels. Most engines overhauled in recent decades have hardened valve seat inserts installed as part of the overhaul. A genuinely vintage engine that has run continuously on 100LL since it left the factory warrants a specific conversation before any fuel change.

The technical barrier and the regulatory barrier are both down. What remains is infrastructure - and infrastructure changes when demand forces it to.

Key Takeaways

• GAMI’s G100UL received FAA STC approval in September 2022, covering the entire U.S. certified piston fleet under a single certificate with no aircraft modifications required.
• G100UL achieves 100 motor octane without tetraethyllead, replacing both the octane contribution and the valve seat lubrication function that previously required lead.
• Distribution remains limited, with California and the West Coast ahead of other regions; where available, G100UL carries a $1.00–$1.50 per gallon premium over 100LL.
• Swift UL94 is a viable option today for lower-compression, naturally aspirated engines and is available at more locations - but it does not cover aircraft requiring full 100 octane.
• The FAA has committed to phasing out leaded avgas; rulemaking on the timeline is underway, and public comment periods will give pilots a direct voice in shaping the transition.