one lead supplier, three fuels that won't mix

Problem statement

Aviation gasoline (100LL) is, in the FAA's words, "the only transportation fuel in the United States that contains TetraEthyl Lead (TEL)"; as of summer 2026 about 222,000 spark-ignition piston aircraft are registered, roughly 175,000 of them active, and they burned an estimated 180 million gallons of 100LL in 2023. The FAA's July 2026 transition plan states plainly that "there is a single source TEL provider for the U.S. market. This provider has indicated an eventual end of production of the TEL additive. The only other potential sources for TEL could lead to reliance on sources such as China." Congress has set the end of 2030 (2032 for Alaska) as the date for eliminating leaded avgas, but the FAA "has determined that a drop-in replacement for 100LL is not feasible": three candidate unleaded fuels are moving through approval on different tracks, each may be mixed with 100LL in an aircraft tank, but "co-mingling of the unleaded fuels is not supported by any fuel offeror." The unsolved problem is the transition itself — how a fuel supply chain of refiners, distributors, and about 3,200 airports that sell 100LL (most with a single avgas tank) converts an entire national fleet to one of several mutually incompatible fuels, without stranding aircraft, fracturing the market regionally, or creating misfuelling accidents, before the sole lead supplier stops making the additive.

Why this matters

Piston aircraft carry out flight training, medevac, crop dusting, aerial survey, and transport to remote communities; NASEM records that "EPA reports that in 2017 piston-engine GA aircraft comprised the largest single source of lead air emissions in the United States," and the FAA plan names "concerns expressed by local communities about exposure to lead" as one of the two drivers of the transition. The other driver is supply fragility: a single TEL producer, fewer than 10 percent of some 120 North American refineries making 100LL, and only "an approximate 9 to 18-month supply of 100LL in the system at any given time," with Alaska "usually" receiving deliveries once per year. If TEL production ends before the unleaded transition is complete, or if the market fractures so that a pilot cannot find a compatible fuel along a route, the consequence is grounded aircraft and unsafe workarounds. As of summer 2026 only 212 U.S. airports offer any unleaded avgas variant.

What’s been tried and why it hasn’t worked

The search for a lead replacement is decades old: NASEM records that FAA and industry work such as the Coordinating Research Council's additive program "was unsuccessful in finding a replacement additive," and the Piston Aviation Fuels Initiative (PAFI, from 2013) sought a "drop-in" fuel meeting every property of the 100LL specification — a goal the FAA has now formally abandoned, so that "virtually every aircraft will require, at minimum, new placards, a handbook or flight manual supplement, and a logbook entry," and higher-compression or turbocharged engines may need hardware changes. Partial solutions ran into supply-chain economics: NASEM found that an unleaded fuel approved for only part of the fleet (UL94, usable by perhaps 57–68 percent of aircraft but not the high-consumption high-performance ones) "would require creating a second supply chain and fuel distribution system across the nation," with airport storage costs "significant and potentially prohibitive, especially for small airports"; automotive gasoline is no longer viable because ethanol blends are incompatible with almost all aircraft engines and refinery gasoline reaches its octane rating only after ethanol is added at terminals. The three current candidates are on divergent tracks — G100UL holds a 2022 fleetwide STC but has no ASTM consensus specification, which the FAA notes distributors, transport companies, airports and FBOs rely on; Swift 100R received an ASTM production specification in September 2025; and UL100 (marketed as UL100E) is completing PAFI fleet-authorization testing with an ASTM test specification (November 2025) — and the FAA's own barrier table lists co-mingling, additional storage, undisclosed fuel composition (offered only under NDA), untested compatibility with transport and storage materials, refineries that will not accept a replacement fuel, unknown price at scale, FBO liability insurance that "covers the cost of the fuel alone," and "distributor/FBO reluctance to transition." In-service surprises are already appearing: the University of North Dakota reported valve-seat recession with UL94 in Lycoming engines in October 2023.

What would unlock progress

The FAA's own framing is that Phase 1 (fuel authorizations and comparative testing, including co-mingling tests, projected to finish spring 2027) must resolve whether the unleaded fuels can be mixed; if they cannot, the National Transition (Phase 3) requires that "the transition to unleaded fuel cannot be gradual and must happen in a clear and controlled manner" at the majority of airports that can hold only one avgas, with each airport announcing its ultimate replacement fuel at a fixed milestone. What is missing is the operational science of that switch: network-level models of which fuel each airport should adopt given the fleet based there and the routes through it, sequencing rules that keep 100LL available exactly as long as needed and no longer (coordinated with the sole TEL producer's remaining output), and human-factors safeguards against misfuelling when several look-alike avgas grades coexist. Adjacent precedents are the unleaded automotive transition of the 1970s–90s (which had the luxury of dual pumps everywhere) and Alaska's once-a-year fuel logistics.

Entry points for student teams

An operations-research team could build a transition-sequencing model over the roughly 3,200 100LL airports — tank counts, based fleet, flight-school and medevac demand, distributor routes — that assigns fuels and switch dates to minimize stranded aircraft and detours under the "no co-mingling" constraint, and stress-tests it against an early end of TEL supply. A human-factors/design team could prototype a misfuelling-prevention system (nozzle, placard, color and labelling conventions, and a pilot-facing fuel-availability app) for airports carrying two or three avgas grades. A policy team could draft the 100LL phase-out trigger rules and the Alaska exception, using the FAA's Phase 3 milestone structure and the ACRP airport case studies the plan cites. Relevant skills: operations research, aviation regulation, supply-chain logistics, human factors.