With on-going pressure from regulatory bodies and vocal climate activists, the entire aviation industry is working toward a clean air future.

For jet operators, the near-term solution is uplift of 100% sustainable aviation fuel (SAF), eliminating the 30-50% mix limits—by regulation—with conventional jet fuel. On the piston-engine side, the path forward has been the replacement of 100LL (low lead) aviation gasoline (avgas) with an unleaded product, rendering the operation of nearly all single and twin piston-engine aircraft completely lead-free.

Exhaust gas from 100LL has been cited as a health hazard, mainly for people living near airports with high volumes of general aviation traffic. In fact, according to a statement issued by the Environmental Protection Agency (EPA) Office of Public Affairs—at the request of the Aviation Business Journal—in October 2022, the EPA issued a proposed finding that emissions from aircraft engines operating on leaded fuel cause or contribute to air pollution that may endanger public health and welfare. If the final determination, to be issued by the fall of this year, concurs, the EPA will likely propose and promulgate regulatory standards for lead emissions from aircraft engines. The statement concludes that “the FAA would have a duty to prescribe standards for the composition, chemical, or physical properties of an aircraft fuel or fuel additive to control or eliminate aircraft lead emissions.”

Fortunately, unleaded avgas for use in many general aviation aircraft has been available nationwide since April 2015 from Swift Fuels. The West Lafayette, Indiana-based refiner is currently the sole source of that product, identified as UL94. According to Swift Fuels’ CEO Chris D’Acosta, UL94 is FAA-certified and meets ASTM (American Society for Testing and Materials) International Specification 7547 for premium unleaded gas.

“UL94 is OEM endorsed by Lycoming, Continental, Textron Aviation (Cessna/Beechcraft), Robinson Helicopter, and many more aerospace manufacturers,” D’Acosta explained. “It is completely lead-free, unlike 100LL avgas, which may contain as much as two grams of toxic lead per gallon. Due to local concerns about lead in avgas, we see our volumes growing.” He declined to provide data as to the privately held company’s annual production.

As D’Acosta explained, UL94 has a minimum octane rating of 94-MON (Motor Octane Number) and is approved for use in over 70% of the US piston-engine fleet, which includes 130,000 FAA certified aircraft. In contrast, less than 30%—some 50,000 FAA certificated piston aircraft—are required to use avgas with a minimum octane value of a 100-MON rating to properly suppress “detonation” (knocking) that can damage an engine.

Swift Fuels, D’Acosta reported, is going after the market for 100-MON with development of a product it refers to as 100R. “Avgas rated at 94-MON is a premium grade, all-hydrocarbon unleaded product with no tetraethyllead,” he noted. “100R is a similar premium grade fuel, but it incorporates an oxygenate specially designed for aviation use to improve the 100-MON octane rating. At the same time, it will have the cleanest combustion exhaust of any avgas product on a global scale. Proprietary FAA testing work to-date shows outstanding results.”

Swift Fuels, said D’Acosta, is, in fact, “in the midst” of the full-scale FAA certification program for 100R as a 100% 100LL replacement. Certification work toward the ASTM International fuel specification and corresponding OEM testing is slated for completion and introduction by the end of this year.

“We are working on a comprehensive roll-out plan with the FAA and industry,” he said. “Swift Fuels is teaching the market how to manage the transition to unleaded safely, with many of the safety protocols demonstrated for UL94. This is a major reason why we chose to use the STC (supplemental type certificate) protocol—to make sure each aircraft was reviewed by the FAA, confirming its certification and fuel-related documentation.”

Interestingly, an STC is only one means of approval for use of UL94 and the lower octane UL91, according to an FAA spokesman. “Some engine and aircraft manufacturers have also approved the use of UL94 and UL91,” said the spokesman adding that the FAA will be issuing a fleet authorization in the near future for use of this fuel. He also stated that UL94 can be made by any producer, provided the fuel meets ASTM International specification D7547 for UL91 and UL94 aviation gasolines and Swift Fuels licenses them the right to produce it in the US.

In the meantime, a 100-octane unleaded product is about to hit the market. Developed by Ada, Oklahoma-headquartered General Aviation Modifications, Inc. (GAMI), G100UL® avgas received STC approval on September 1, 2022, for every spark ignition engine and associated airplane in the FAA databases, according to GAMI Founder George Braly. Spark-ignition engines, he noted, represent 99% of the piston-powered fleet. The remaining few are powered by compression ignition engines, which use diesel or jet fuel.

Braly reported that “a large international producer of existing fuels” has agreed to produce G100UL starting in late summer of this year at its Houston refinery, with California as the initial distribution target and then the remainder of the West Coast. “Expansion to the rest of the US will require about 2-3 years,” he said.

Asked about the market for G100UL, Braly said it’s the same one for 100LL, which he reported is around 160 million gallons annually.

He added that existing 100LL infrastructure should be able to accommodate G100UL with minimal issues. “G100UL avgas is fully fungible with 100LL, so the existing tanks holding 100LL can be used for G100UL,” Braly stated. “Fundamentally, the existing distribution infrastructure should be able to adopt G100UL with minimal issues.”

In tandem with unleaded fuel developments has been the work of the Piston Aviation Fuels Initiative (PAFI) established by the FAA in 2014 to support the evaluation of candidate unleaded fuels to replace approved leaded avgas—with the objective of ultimately qualifying a fleet-wide solution with no STC required. Swift Fuels’ D’Acosta explained that once a candidate fuel formulation is qualified for PAFI testing, the FAA tests it using methods created through collaboration with industry.

“The collaborative process also includes the selection of materials, engines, and aircraft to test to ensure the full general aviation engine power range and aircraft operational spectrum are represented,” D’Acosta pointed out. “Company test data within PAFI are considered proprietary. The data collected contribute to FAA’s fleet eligibility determination.” He added that although no high-octane unleaded replacement fuel has yet been qualified under PAFI as a suitable drop-in candidate, approval of a qualified fuel would occur following the issuance of a new production specification/standard of safety and reliability.

An industry/government collaboration came together in March of last year with the first meeting of EAGLE (Eliminate Aviation Gasoline Lead Emissions). As former EAGLE Senior Coordinator Robert Olislagers explained, the goal is to eliminate lead emissions from avgas by the end of 2030 without adversely affecting the safe and efficient operation of the general aviation fleet. Participation in EAGLE includes the FAA, nine major general aviation trade associations, the American Petroleum Institute, and other stakeholders.

Olislagers noted that when EAGLE was founded, both PAFI and the STC programs to remove lead from avgas were already in place. “EAGLE bridges these programs but also looks holistically at the entire process from producing acceptable alternative fuels to refining the base fuels, blending the appropriate formulations, and testing and evaluating these blends for equipment compatibility,” he said. “Along with that is coordinate the transition to unleaded fuels while maintaining 100LL fuel until widespread availability of unleaded fuels has been established.”

Asked if the refining capacity is in place to meet anticipated demand for 100UL by 2030, Olislagers response was “absolutely.” He explained, “Current annual demand for 100LL averages about 160 million to 180 million gallons, which compared to the automotive market is only about 0.011% of the total volume of fuel sold in the US,” he said. “It is more of a boutique fuel in that regard, so the refining infrastructure is certainly there.”

Olislagers also stressed that the transition away from 100LL to 100UL will be most critical, but cautioned it is not realistic to think that the spigot for 100LL is turned off one day and 100UL will be available the next. “The transition will more likely be gradual, almost organic,” he said. One interesting fact, he added, is that the 30-32% of the piston-engine fleet that cannot use UL94 accounts for 70% of the avgas consumed today—further emphasizing the need to go to 100UL.

Olislagers also pointed to the progress industry has made toward fielding 100UL by the four major players—Swift Fuels, GAMI, Afton Chemical/Philips 66, and LyondellBasell/VP-Racing.

“GAMI received an AML-STC on September 1, 2022, and Swift Fuels’ 100R is not very far behind via the STC process. Afton Chemical/Phillips 66, as well as LyondellBasell, expect authorization possibly as early as 2024,” he said. “Making sure fuels have compatibility with equipment for assured safety, liability and warranty purposes will be critical, and remains the biggest challenge. Mechanisms such as ASTM approvals minimize risk but take time.”

Then there is the question of logistics and distribution. According to Olislagers, some 35-plus airport FBOs, dozens of private users, and 3 major university flight schools are carrying UL94 at this time. He reports this number is growing largely due to mounting community desire to see the conversion to unleaded fuel be escalated, and from community pressure stemming from the proposed EPA endangerment finding.

“Until there is a viable alternative for the entire fleet, it will remain somewhat of an open question, although I personally don’t see it as a huge issue. I do know that all the candidate unleaded fuel producers want to make one thing clear,” he remarked. “They will not provide fuel until they are sure product delivery is consistent and available at all times. Fuel supply and inventory control will be paramount, but I firmly believe the FBOs and distributors have the capacity to handle it.”

In spite of the progress made toward moving to greener fuels, some industry representatives tend to be cautious. Among them is Rob Lewis, Vice-President, Sales, Marketing and Business Development of EPIC Fuels, an Irving, Texas-based avgas distributor.

For starters, as Lewis explained, unleaded avgas fuels have completely different supply chains than leaded avgas, which he pointed out is refined from alkylates in a refinery, blended without chemicals, and then shipped by truck or rail to fuel storage terminals across the US. From there, the avgas is trucked again from the terminal to the airport.

In contrast, he noted, unleaded avgas products are not refined themselves, but are a ‘secret recipe blend’ of various specialty chemicals from multiple suppliers around the world. “They are blended at plants that can receive the specialty chemicals, blend the product, and then distribute it from there,” Lewis remarked. “Additionally, these products require STCs, and most airports and FBOs seeking to sell these fuels must invest in costly alternative methods for storing and dispensing them separately from 100LL. This will be problematic, as most only have one storage tank available for avgas.”

When asked if the EAGLE program’s goal of a total transition to unleaded avgas by 2030 is realistic, Lewis replied: “No, I don’t think so. I don’t see the EAGLE program coming up with any workable solutions in the foreseeable future.”

Al Pease, Director of Alternative Fuels for Ann Arbor, Michigan-based Avfuel, is more optimistic. “That (2030) goal is well within the reach of the initiative; however, the ultimate achievement of that goal depends on a multitude of factors and the contributions of numerous participants,” he stated. “Avfuel continues to engage with all the stakeholders in the EAGLE Initiative to commercialize an unleaded 100-octane fuel as soon as the market and infrastructure dynamics allow.”

Pease also stressed that, as with SAF, adoption of UL94 has progressed slowly, but early adopters have integrated the unleaded option into their offerings. “We continue to see a growing interest in unleaded solutions.”

Nonetheless, he acknowledged that UL94 is currently sourced by a single producer at one location, and GAMI’s 100 octane G100UL is not yet available in commercial quantities. With both fuels requiring an STC for their airframe and engine, Pease was asked if this could stop a lot of people from using the current unleaded product.

“Obtaining the STC is a relatively straight-forward process, and reportedly may be accomplished online,” said Pease, who also explained that the infrastructure to support the transition to unleaded avgas may not be that much of an issue.

“Excluding the two airports in California—Reid-Hillview (RHV) and San Martin (E16)—that have unilaterally banned the sale of 100LL, all other entities that sell 100LL and UL94 have been able to support both fuels, either via spare tanks or with dedicated fueling trucks on the field,” he said. “100LL and UL94 are miscible in any proportion for aircraft approved for UL94. 100LL and G100UL are miscible in any proportion for all listed aircraft.”

Pease said that in his opinion, the transition to unleaded avgas will be propelled by the EPA, which “will likely eventually issue an endangerment finding for lead with a subsequent phase-out program.”

The introduction of UL94 at Centennial Airport (APA) presents an interesting case study of unleaded avgas introduction. UL94, which came online at APA on May 3 of this year, is sold by jetCenters of Colorado, one of four full service FBOs at the Denver-area airport.

“The transition to UL94 began last fall when the surrounding community voiced its concerns about the health issues of leaded fuel,” noted Michael Fronapfel, APA’s Executive Director. “This was seen as an opportunity to get the conversation started and to see how the airport could help its based customers move toward UL94.”

To make this happen, the Arapaho County Public Airport Authority offered to pay for UL94 STCs for their based piston-engine tenants. The authority also offered to help the FBOs purchase new fuel tanks and fueler trucks. (As it turned out, jetCenters of Colorado was able to flush out an existing storage tank and fuel truck without need for a subsidy).

Fronapfel also said that a key piece of the APA transition to UL94 was proving to jetCenters of Colorado that their customers would buy the unleaded fuel, by showing that other locations had successfully made it available for sale. “Another point we made is that unleaded fuel means less engine maintenance, due to lower buildup of residue,” he remarked.

Because the price differential between UL94 and conventional 100LL avgas is about $1.00 per gallon, Fronapfel said that the airport authority will subsidize the price difference through the end of this year, but may extend it if budgets allow. He also reported that the same subsidy program could be applied to 100UL once it becomes available, and is hoping that the airport will be free of leaded avgas before the 2030 EAGLE Initiative timeframe.

“With our high amount of traffic, we can afford to provide financial assistance. However, there are a lot of smaller, low traffic airports that might not be financially able to offer a similar subsidy program for its based tenants and for infrastructure upgrades,” he said. “We would like to see more efforts by local and state governments to lead the transition to unleaded avgas. This would be especially helpful for the smaller airport.”

In addition, the “Securing Growth and Robust Leadership in American Aviation Act” passed by the US House of Representatives on July 20 not only reauthorizes the FAA, but requires the administrator to develop a plan to “safely expedite the transition” to unleaded fuel for the piston-engine fleet by 2030.

SAF Makes Inroads, But Output Still Small

Sustainable Aviation Fuel (SAF) has proven its viability as a drop-in replacement for conventional Jet A, but it is still limited to a maximum blend of 50% with the fossil fuel product. The goal, of course, is 100% Jet A replacement.

Currently, SAF production is still extremely small. According to data provided by Alton Aviation Consultancy, an international aviation advisory firm in New York, SAF represents approximately 0.1% of all aviation fuels, with the vast majority of purchases attributed to the commercial airlines, with business and general aviation operators and FBOs making smaller and shorter-term commitments. “That is likely to increase rapidly going forward,” said Adam Guthorn, Alton Aviation Consultancy Managing Director.

Looking at SAF alone, Guthorn cites International Air Transport Association (IATA) statistics, which estimate 2022 production at approximately 300-450 million liters—an increase of 3-4 times the 100 million liters produced in 2021, and over 10 times the 25 million liters in 2019. “The largest SAF producers are Neste and World Energy; however, there have been many announced investments in new or expanded SAF production by over a dozen other suppliers,” he reported.

Guthorn cautioned that getting to 100% SAF has its challenges, citing the fact that SAF is not a single fuel type, since it comes in a variety of chemical compositions.

“Many SAFs have fewer aromatic components—chemicals that help lubricate the engine and ensure the integrity of seals—and therefore require further research and standards development before they are viable at higher blends,” he pointed out. “Airlines, OEMs, and fuel suppliers are conducting trials with various forms of 100% SAF to demonstrate capabilities and confirm performance as full replacements for Jet A. Both Boeing and Airbus have run trials of 100% SAF flights on existing technology engines and aircraft with the goal of achieving 100% SAF for routine operations by 2030.”

Along with further research, Guthorn calls availability of feedstock “one of the biggest challenges” facing SAF mass production.

“There are continued debates about whether there is sufficient biomass available to meet growing demands for sustainable fuel,” he noted. “The US Department of Energy estimates that there are 1-billion dry tons of biomass which can be sustainably collected each year domestically—enough to produce 50-60 billion gallons of low-carbon fuels,” he explained. “Proposed EU legislation, however, excludes purpose-grown crops from SAF due to sustainability concerns. This could limit feedstock availability.”

Guthorn also cited Power-to-Liquid (PtL), which is a renewable SAF produced using green energy to synthesize fuel from hydrogen and captured CO2. “The technology is still relatively far from commercialization and is limited by input costs, although the process is highly scalable,” he noted.

To meet aviation’s long-term sustainability goals, a significant increase in SAF refining capacity is key.

“For SAF, the major challenges are the need to ramp up production and to reduce costs to near-parity with conventional jet fuel,” said Guthorn. “Currently announced SAF production facilities project to supply approximately 4% of the required aviation fuel by 2030, which is below targets set in Europe and North America. Additionally, nearly all the announced SAF facilities are located in the US and EU/UK owing to SAF policy requirements in the regions, so further investments in rest-of-world infrastructure will be necessary in the long-term to ensure global availability. Sustainable initiatives include operational considerations for logistics and ensuring that feedstocks are processed close to their source to minimize carbon impact.”

According to Keith Sawyer, Avfuel’s Manager-Alternative Fuels, between 15-16 million gallons of concentrated SAF were produced and distributed to all aviation segments in 2022. Of this amount, he noted, 12-13% were delivered into the general aviation supply chain. (Sawyer attributed that percentage to the General Aviation Manufacturers Association (GAMA) and other groups.)

While Sawyer acknowledged that the price of SAF is “at a premium compared to conventional jet fuel,” he expects that the “difference will narrow” as supply increases to meet demand over the next five years. “Today, that price is dependent on a number of factors, including—but not limited to—what the blend ratio of SAF to petroleum jet fuel is, how far the SAF had to be trucked to its final destination, and what state-level credits are available for operators who use SAF,” he explained.

Sawyer added that at this point in the US, SAF purchasers either operate for companies with admirable sustainability goals or are eco-conscious minded themselves. Those two factors, he said, have been the main drivers to date.

“The largest factors to drive SAF uptake, we believe, are legislative in nature, particularly in the U.S.—whether that’s financial incentives at the state or federal level for using SAF or adopting a blenders tax credit much like what is in place for renewable diesel,” he remarked. “Once those legislative initiatives spread and grow, they will support further SAF commitments, which, in turn, will spur greater production and widespread availability.”

In fact, a blenders tax credit (BTC) was included in the Inflation Reduction Act of 2022, which became effective in January of this year. Larry Schafer, CEO of Playmaker Strategies, a Washington, DC-based lobbying firm, explained that the BTC has a five-year window—2023-2027. However, he reported, the SAF industry wants to see this extended by at least another five years.

“Under the BTC, the fuel producer can claim a tax credit of $1.25 per gallon if the blend with conventional fuel results in a minimum 50% reduction in CO2 emissions. The benefit is obviously on the producer side, but the tax credit should result in savings for the consumer at the pump,” Schafer said. “Also, in theory, new blended fuel projects should be easier to finance now that the investment community is becoming more comfortable with SAF.”

Asked about any the existence of pending legislation on the Hill regarding sustainable fuels, Schafer said that while there isn’t any at the moment, the industry is working to make sure the BTC will do what it is supposed to. “Right now, our concern is what the rules will be and how they will be interpreted,” he stated. “Keep in mind, the Inflation Reduction Act has only been law for the past few months.”

Michael Sargeant, Vice President, Renewable Aviation Americas at Neste, reported that annual SAF production is poised to hit eight billion gallons of annual production by 2030, according to IATA. At that scale, he said, SAF will equal roughly 10% of total jet fuel demand—up from 0.1% today—but still not enough to meet the industry’s goal of net-zero carbon by 2050.

Headquartered in Finland, Neste is the global leader of SAF production, said Sargeant, and will support market demand with 515 million gallons at the start of 2024, reaching 750 million gallons by 2026 by expanding capacity at refineries in Singapore and Rotterdam.

But will the feedstock—the raw materials—be there, or could there be supply chain issues? Sargeant does not anticipate any and explained why.

“Neste’s SAF is made from sustainably sourced, 100% renewable waste and residue raw materials, such as used cooking oil and animal fat waste from all over the world,” he said. “Independent reports, such as the World Economic Forum/Clean Skies for Tomorrow report, confirm a global availability of waste and residues suited for our currently used refining technology of 40 Mton/a (Metric Tons Per Annum) globally by 2030, which is more than sufficient to support a significant ramp up in renewable fuels production.”

As more SAF is produced, will airports and FBOs need to be make substantial investments in new storage and dispensing systems? “Not necessarily,” said Dr. Patrick Gruber, CEO of SAF producer Gevo, based in Englewood, Colorado. “The fuel could be blended far away and put into the pipeline that feeds airports. To prove the point, we did exactly that for a Fly Green Day at ORD a few years ago. Everyone flew with a little of our SAF that day. Airports may want to install it in the future. But I think it will be current jet fuel suppliers who blend, certify, and deliver it through existing systems in the near term.”

Along with infrastructure, there is the question of how close the industry is to reaching the 100% SAF mark. “Technologies exist to make 100% and many of us have the ability to do it. But it isn’t relevant until the product value is sorted out. Airlines can’t afford it without help on carbon value from governments, purchasers of airline tickets, or both,” said Gruber. “No one wants to build projects that are at the mercy of government whim, so stable policy is necessary to attract investors for capacity buildout.”