The End of One Hundred LL, GAMI's G One Hundred UL STC, and the Supply Chain Risk That Every Piston Pilot Needs to Understand

Every gallon of 100LL avgas on the planet depends on a single manufacturer. That supply chain vulnerability, combined with a formal EPA endangerment finding in 2023, means the transition away from leaded aviation gasoline is no longer a hypothetical. It is a regulatory and logistical process already underway - and every piston pilot needs to understand where it stands.

Why 100LL Has Always Contained Lead

The pale blue color in your fuel tester is not aesthetic. It identifies the presence of tetraethyl lead, the additive that allows 100LL to reach its octane rating. Without it, the fuel cannot deliver the performance high-compression and turbocharged piston engines require.

Leaded avgas dates to the 1930s, when the discovery that tetraethyl lead could dramatically boost octane ratings became central to aircraft engine development. During World War II, Rolls-Royce Merlin engines in Spitfires and Hurricanes were optimized for high-octane fuel - the jump from 87 to 100 octane produced measurable performance advantages at altitude. After the war, high-octane leaded avgas became the certified standard for the entire piston fleet.

Automotive gasoline phased out lead through the 1970s and 1980s. Aviation did not follow, because the certification environment and the specific octane demands of high-compression piston engines created a technical problem that had no simple solution.

The Two Pressures Forcing Change

Two separate forces are now converging on 100LL, and each one alone would be enough to drive action.

The first is regulatory. Studies consistently found elevated lead concentrations in soil, air, and blood samples near busy general aviation airports. In 2023, the EPA formally determined that lead emissions from piston aircraft engines may reasonably be anticipated to endanger public health and welfare. That determination triggers a regulatory process with no reverse gear.

The second pressure gets less attention but may be more immediately dangerous. Tetraethyl lead for aviation is produced by one company on Earth: Innospec (formerly Associated Octel), operating from facilities in the United Kingdom. A plant incident, a UK regulatory action, a business decision to exit a shrinking niche market - any of these stops 100LL production globally. Not gradually. Abruptly. There is no backup supplier. The FAA and industry insiders have treated this single-source vulnerability as a genuine strategic risk for years, even when public discussion has focused on the environmental angle.

The FAA’s PAFI Program and Why It Fell Short

In 2012, the FAA launched the Piston Aviation Fuels Initiative (PAFI), a coordinated industry effort to identify a drop-in unleaded replacement capable of serving the entire certified piston fleet. Two candidates advanced to full evaluation: formulations from Swift Fuels and Shell Aviation. Testing covered engine dynamometer runs, flight testing in actual aircraft, and materials compatibility across thousands of seal types, bladder tanks, and fuel system components found across decades of certified airframes.

Around 2018, the FAA announced that neither candidate had passed all requirements to advance as a universal fleet-wide replacement. PAFI did not deliver the solution it was designed to find.

What GAMI Did Differently

General Aviation Modifications, Incorporated (GAMI), based in Ada, Oklahoma, had been developing an unleaded avgas in parallel with PAFI. Most pilots know GAMI from their GAMIjectors, the fuel injection nozzle technology that balances fuel distribution across cylinders for efficient lean-of-peak operation. The fuel they developed is called G100UL: G for GAMI, 100 for octane rating, UL for unleaded.

GAMI’s regulatory approach was structurally different. Rather than pursuing amendments to individual type certificates - which would have required each engine and airframe manufacturer to separately validate the new fuel - GAMI applied for a fleet-wide Supplemental Type Certificate (STC). One approval covering any aircraft currently certified for 100LL.

Earning that STC required proving G100UL matched 100LL across every relevant dimension: octane rating, volatility, water contamination behavior, thermal stability, and materials compatibility with the enormous diversity of seal and elastomer types found throughout the certified piston fleet.

One specific engineering challenge involved exhaust valve seat recession. In older engine designs, combustion deposits from tetraethyl lead provided trace lubrication at the exhaust valve seat interface. Remove the lead and that lubrication mechanism disappears, potentially accelerating valve wear in older engines. GAMI built the solution into the G100UL chemistry itself rather than addressing it after the fact in an operator bulletin.

In September 2022, the FAA approved the STC. For the first time in the history of certified piston aviation, a 100-octane unleaded avgas with fleet-wide approval existed. Any aircraft certified for 100LL could legally burn it.

The Current Fuel Landscape for Piston Pilots

Two unleaded options are now in the market, covering different segments of the fleet.

Swift Fuels (Terre Haute, Indiana) produces UL94, a 94-octane unleaded avgas that has been commercially available for several years. It is approved for normally aspirated engines - the powerplants in Cessna 172s, Piper Cherokees, and Diamond DA40s - that operate at compression ratios which do not require 100 octane. At airports that stock it, UL94 is a legal, available, lead-free option today.

The gap is in the high-performance segment. Turbocharged and high-compression engines - the big-bore Lycoming and Continental powerplants in Beechcraft Bonanzas, Mooneys, Piper Senecas, and Cessna 206s - were designed for 100 octane. Running them on 94 octane can introduce detonation risk or require operational restrictions that significantly limit aircraft capability. That portion of the fleet requires G100UL. And at many airports, particularly smaller rural fields, 100LL remains the only option on the fuel farm.

Why Distribution Is the Remaining Problem

Certification solved the technical problem. Distribution is where the friction lives now.

A fuel producer must license the G100UL formulation from GAMI and manufacture it to specification. That fuel then moves through infrastructure built entirely around 100LL. FBOs face real capital decisions: separate storage tanks to prevent cross-contamination, separate fuel trucks with clear labeling, updated line personnel procedures. Those investments require a demand signal from pilots and airports that makes them commercially justifiable.

Engine manufacturers Lycoming and Continental are both engaged in this process. Their technical guidance matters when pilots are making decisions about engine overhauls or new aircraft purchases. Both have been updating their positions as testing data accumulates, and understanding where they stand on fuel compatibility is part of the homework for any affected owner.

What Piston Pilots Should Do Now

The transition is happening on a decade-scale timeline, not next spring. But the EPA finding is on record, the Innospec supply risk is documented, and the GAMI STC is active and being used. Waiting for the change to arrive is not the same as being prepared for it.

Know your aircraft. Review your pilot’s operating handbook and type certificate data sheet. Understand which fuels your specific airframe and engine combination is approved for. If the G100UL STC applies to your aircraft and you intend to use it, the STC documentation must be in your aircraft records before the first gallon goes in.

Ask your FBO. Does your home airport stock UL94 or G100UL? If not, ask what it would take to add it. FBO managers track those conversations as demand signals. Flight schools making commitments to transition training fleets to UL94 at airports that stock it shift the economics in a direction that makes additional investment easier to justify.

Understand the supply risk. If Innospec’s UK operation faces any disruption - regulatory, operational, or commercial - the global 100LL supply stops. That is not a remote scenario being managed by regulators in the abstract. It is a documented single point of failure that applies to every piston aircraft currently dependent on 100LL.

Key Takeaways

• 100LL’s days are numbered on two fronts: a 2023 EPA endangerment finding and a global supply chain that runs through a single manufacturer in the United Kingdom
• GAMI’s G100UL STC, approved by the FAA in September 2022, is the first certified 100-octane unleaded avgas with fleet-wide approval for aircraft currently certified for 100LL
• Swift Fuels’ UL94 is already available at some airports and legally approved for normally aspirated engines; high-performance engines require G100UL
• The technical problem is solved - distribution, adoption, and commercial inertia are what remain
• Pilots can accelerate the transition by asking their FBOs about unleaded availability and ensuring STC documentation is in order before switching fuels