100R fuel eligibility just became a piston-aircraft resale factor

Unleaded avgas has been a recurring topic in the aviation industry, but it has mostly been a regulatory discussion. Now, it may start to become a resale discussion. News Swift Fuels announced on July 14 that the FAA expanded its approved model list (AML) for its 100R unleaded avgas. The FAA issued certificates for 1,200 piston engines and added 1,600 airframe models to Swift Fuel’s AML. The AML identifies which engines and aircraft combinations have been approved under Swift’s certification program. Aircraft and engines with proper registered STCs can use the 100R avgas. Previously, the company had FAA approval for a limited set of around 840 engines. Swift now claims that the new approvals represent aircraft responsible for about 56% of current 100LL consumption. Context 100LL (low lead) is the avgas that piston aircraft burn today, and the lead is what lifts it to 100 octane. Engines built for high compression need that octane to avoid detonation, the uncontrolled knock that can wreck a cylinder. Unleaded fuels like UL94 already serve lower-compression engines, but that fuel is incompatible with the high-compression aircraft that consume most of the avgas sold. Most of the piston fleet uses engines that were designed before lead was identified as an environmental issue. With the inability to run on any unleaded fuel now in production, pulling 100LL before a true replacement exists would simply ground them. The FAA Reauthorization Act of 2024 directs the transition and requires airports that sold 100LL in 2022 to keep selling it until 2030, or until a certified unleaded replacement becomes available (2032 for Alaska). The FAA and its partners launched the Eliminate Aviation Gasoline Lead Emissions (EAGLE) initiative in 2022 with the goal of eliminating leaded aviation fuel in piston aircraft. It aims to identify at least one unleaded fuel acceptable for general aviation, with companies like Swift Fuels racing to be first. These potential fuels have to clear three separate stages: an ASTM specification, FAA fleet authorization, and STCs for the aircraft that will use them. An ASTM production specification means the fuel has an agreed industry recipe, so it can be made consistently and at volume. An STC authorizes it for a specific aircraft. And the FAA must grant PAFI fleet authorization for a fuel to be approved across the fleet. A fuel can hold an ASTM spec and still not be approved for certain aircraft. Swift Fuels’ 100R received an ASTM production specification in September 2025. Why eligibility matters now When shopping for a used aircraft, buyers already compare maintenance history, engine time, avionics and operating costs. This process provides an idea of how long they can keep an aircraft airworthy, even if they do not think about it in that way. Fuel eligibility may increasingly join that list as the end of avgas draws closer. If an aircraft has an engine that can only run on avgas, it may not have any fuel by 2030. Meanwhile, an aircraft that can operate on unleaded fuel has greater long-term operating certainty. Since only aircraft and engines with proper registered STC’s can use the 100R avgas, owners of eligible aircraft can purchase Swift Fuels’ Forever STC. Pilot’s who have already purchased a UL94 STC from Swift Fuels will automatically receive a 100R STC at no extra charge. Buyers may also consider if an aircraft already holds the necessary fuel-related approvals, as buying them could add to acquisition costs. The new question for piston buyers Fuel eligibility is not valued because of what it adds to an aircraft now, but it is valued because it removes uncertainty later. Buyers may now ask whether the engine and airframe are included in the current AML and whether the aircraft already holds the necessary approvals. Aircraft inside the approval window offer a clearer path into the unleaded transition, making it a positive line item. Aviation’s fuel transition may affect aircraft values gradually instead of dramatically. Buyers are unlikely to pay premium just because an aircraft qualifies for unleaded fuel. They may, however, start turning away aircraft that do not have a clear path forward as unleaded options become more common. Why it matters While leaded avgas may not be gone yet, the industry is now increasingly focused on the transition timeline. Things to keep an eye out for include additional AML expansions, future approval phases, airport adoption of unleaded and whether fuel eligibility begins appearing more frequently in aircraft listings. These factors could give an early sign of how the market evaluates aircraft during the transition to unleaded fuel.