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Devil in the details

Why is it so difficult to come up with an unleaded avgas?

By Paul Millner

Oh no! I can hear you thinking. Not another article about unleaded avgas. Haven’t we been hearing about this “coming soon” since the Reagan administration?

Illustration by Leigh Caulfield

Well, yes. Lead was phased out of mogas (automotive gasoline) in the 1980s and early ’90s because of environmental toxicity and its adverse impact on catalytic converters. At the time, the industry wasn’t sure how to meet all the requirements for a safe aviation fuel and achieve the 100-aviation octane required for avgas without lead. (That would equate to a 105-pump octane at your corner service station.) So, the industry launched the Coordinating Research Council (CRC) Unleaded Avgas Development Group in 1991. That group of oil companies, aviation companies, the FAA, and alphabet groups like AOPA worked hard for 20 years looking for a solution that, other than lead content, would meet the existing avgas specification.

Unfortunately, no easy answer was identified. The FAA then launched the Unleaded Avgas Transition Aviation Rulemaking Committee (UAT ARC), which in turn, produced PAFI (Piston Aviation Fuels Initiative) phases one and two, and now has announced EAGLE (Eliminate Aviation Gasoline Lead Emissions). Let’s look at the pillars of EAGLE to better understand the challenge of unleaded avgas.

  • “Support Research & Development and Technology Innovations: The FAA and industry stakeholders will support research and testing of piston engine modifications and/or engine retrofits that may be necessary for unleaded fuel operations.”
    We hope this is not required. The five companies that have fielded candidate fuels so far see no need for engine modifications or retrofits. (Phillips 66 has suggested an improved spark plug to resist fouling by its proposed MMT additive, but this would be an optional enhancement to minimize maintenance expense.)
  • “Develop Unleaded Fuels Infrastructure and Assess Commercial Viability: Industry stakeholders will coordinate the production of commercially viable unleaded fuels and create the necessary infrastructure and distribution channels to support widespread usage of these fuels.”
    The blending and distribution infrastructure will be similar to today’s for leaded avgas. If there are new points of supply, adjustments will have to be made—but that’s standard practice for the energy industry. The petroleum industry has smoothly managed the shutdown of more than 800 refineries over the past 45 years in the United States alone (leaving some 130 in operation), and gasoline has continued to be relatively economically available.
  • “Continue to Evaluate and Authorize Safe Unleaded Fuels: The FAA will address fleet-wide authorization of unleaded aviation fuels of different octane levels. The Piston Aviation Fuels Initiative will continue to evaluate, test, and qualify high-octane aviation unleaded fuels with the objective to ultimately transition the fleet to unleaded aviation fuel.”
    It’s a little concerning that the FAA is interested in fuels of different octane levels, because the rest of the industry is not focused there at all—avgas is already a very small production product. Splitting up that volume among different products is uneconomical.
    Through PAFI, the FAA’s Technical Center in Atlantic City, New Jersey, will continue to work with the FAA-chosen PAFI participants to qualify their fuels. Swift Fuels and GAMI will continue their own efforts to obtain supplemental type certificates (STCs) for their fuels, with the support of industry participants. Of course, to the extent that the engine and airframe manufacturers continue to revise their approvals for fuels as Lycoming, Cessna, and Continental have already done, STCs may not ultimately be required for every aircraft using these fuels. Either the PAFI or STC process could lead to approval for the entire general aviation piston fleet, with varying degrees of paperwork for the aircraft owner.
  • “Establish Any Necessary Policies: The EPA is evaluating whether emissions from piston-engine aircraft operating with leaded fuel contribute to air pollution that endangers public health. The EPA plans to issue a proposal for public review and comment in 2022 and take final action in 2023, which can lead to EPA regulation of lead emissions from piston-engine aircraft. The FAA will subsequently publish regulations that certify piston engine modifications and new piston engines that do not require leaded aviation fuel, and regulate fuel components for aviation fuels.”
    That description doesn’t quite capture the core issue: The FAA acknowledges that it’s not quite certain just yet how the PAFI fuels will be approved by the administration. That obviously has to be worked out, but the more recent indications from FAA headquarters make the PAFI approval look a lot like implementing an STC approval. Note also that no one seems to be seeking FAA help certifying engine modifications for UL100, certifying new engines for UL100, or regulating fuel components. The existing process and fleet are just fine for this transition. But the FAA does need to figure out how to approve the PAFI fuels, and share that with GA aircraft owners. Even better, the FAA has been asked to make an online, expedited process available for filing the necessary Form 337s and STCs to expedite the transition. For example, did you land at an airport with only unleaded avgas, but you don’t have the STC yet? Go online via FBO computer or your smartphone, and both acquire the STCs and file them with Oklahoma City. The FAA Office of Innovation says that’s a great idea—but the devil is in the implementation details.

Develop unleaded fuel

To understand the 100 unleaded avgas challenge, you need to understand a little bit about avgas formulation (put on the lab coats). Generally, avgas has four components: aviation alkylate for octane and energy, light ends for vapor pressure to assist winter starting, aromatics for octane, and an octane booster to get the octane to the 100 specification. For leaded avgas, that booster is lead. For the unleaded avgas candidates, the proposed solutions vary.

The solutions proposed by the five contenders for a new formulation affect the makeup of all but the light ends part of this formula: substitution of novel aromatics and octane boosters compared to the components we’ve been using for the past 90 years in avgas, and tightened quality requirements on aviation alkylate for some of the formulations.

These are not minor changes—these are major reformulations of avgas. Fortunately, most of the hard work is already behind us: aside from the Shell fuel that has been sidelined, the formulations appear not to adversely interact with materials in either our aircraft or our engines; they offer the same or better detonation protection in operation than the existing 100LL; and the manufacturing and handling issues appear to be tractable. But it’s not sufficient for these issues to appear to be solved, they must be proven to be solved to the FAA and industry’s satisfaction, and that’s what is taking the time right now.

The next steps involve determining who and where the new fuel(s) will be produced, and how the existing distribution infrastructure can be adapted or augmented to get these fuels to market. None of these are horribly difficult processes, but they occur stepwise, and require time and resources. For instance, it’s difficult to convince a major oil company to commit to producing an unleaded fuel until that fuel is FAA approved, and there is some operating experience to define any possible requirements that may differ from 100LL. Once there is a commitment, facilities must be dedicated and/or modified to blend the fuel, fuel component supply chains have to be established, and initial operating experiences have to be validated to assure there are no surprises in the product integrity of avgas.

The most hopeful sign from the most recent initiative, EAGLE, is that it is timeframe focused, 2030 or before. All involved hope that before is much before 2030. Up until now, the focus has been on achieving the goal, not on when the goal will be achieved. We’re far enough along now to focus on that finish line. Stay tuned as we keep you updated on developments, and what positive benefits you might expect from the new fuel aside from the environmental improvement.

Paul Millner, a pilot and co-founder of the Cardinal Flyers type club, spent 38 years as an engineer for a major oil company, where he directed unleaded avgas development efforts for a decade.



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