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While the aviation industry is trying to move away from leaded avgas, not every aircraft has received approval to run on an unleaded alternative. How these aircraft will operate in an unleaded industry is unclear. What happened Swift Fuels announced on July 16 that it entered a Memorandum of Understanding (MoU) with the FAA Technical Center to test its 100R unleaded aviation fuel on seven high-performance Continental and Lycoming engines. The goal of the tests is to expand the company's available engine testing results and eventually receive fleetwide FAA approval for the 100R. The seven models are: Continental IO-550D Continental TSIO-520-VB Continental O-470-U Continental W670-6N Lycoming IO-540-K1A5 Lycoming TIO-540-J2B Lycoming TIO-540-AJ1A The FAA expanded Swift Fuels' approved model list (AML) for its 100R unleaded avgas on July 14, adding over 1,200 piston engines and 1,600 airframes. Aircraft covered under the AML may operate on 100R after obtaining the appropriate STC authorization. What makes these engines different from the rest is that they are higher compression engines, which have additional testing criteria. Swift Fuel claims to have tested these same engines privately. The company states that joining the Tech Center in testing is the best and fastest way for the two groups to judge the fuel performance and determine if it follows FAA engine protocols. Why the difficult engines matter The company claims that 55% of the U.S. piston fleet is covered by its AML. Many of the 45% not approved are higher-performance engines that have larger fuel demands and consume a significant amount of avgas. If approved, these engines would give a larger portion of the U.S. piston fleet a clearer pathway through the unleaded-fuel transition. What it means for aircraft owners Owners of higher performance piston aircraft are still waiting to see if 100R will be an option in the future. This testing may determine if they will have the same path to unleaded power as already approved models, or if they will have to find alternative approved fuels in the future. Until then, their long-term fuel options are uncertain. Buyers may now ask whether the engine and airframe are included in the current AML and whether the aircraft already holds the necessary approvals. While unlikely buyers will pay a premium just because an aircraft qualifies for unleaded fuel, they may start turning away aircraft that do not have a clear path forward as unleaded options become more common. These approvals seem to be moving from broad approvals to smaller edge cases for aircraft that are harder to approve. Future announcements are likely to be more significant for specific aircraft and their owners rather than the fleet as a whole. Why it matters The FAA tests could affect future AML expansions and high-performance engine approvals. The results could help determine if fuel eligibility eventually becomes a positive line item on the piston market.