When Charles Lindbergh and his wife, Anne Morrow Lindbergh, embarked on their ambitious 1933 flight to survey over-water routes for airlines, they packed a secret weapon. The famous couple headed to some of the most isolated areas of the North and South Atlantic because that’s where the most direct routes from North America to Europe and South America lay.
Technology had come a long way since Lindbergh’s most famous flight from New York to Paris just six years earlier, and the heart of their new Lockheed Model 8 Sirius (named Tingmissartoq by a young Inuit boy in Greenland) was a brand-new Wright Cyclone SR-1820 radial engine, a state-of-the-art powerhouse that made 710 hp and set the standard for reliable performance at a time when powerplant technology was rapidly developing.
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The Wright had an Achilles heel for this particular mission, however. To turn that blur of pistons, rods, and valves into that familiar snarl of thrust, it needed high-octane fuel, a relatively exotic concoction not available in places like Greenland, Labrador, Africa, and the east coast of South America. Fortunately, regular automotive gasoline had reached those areas in reliable quantities, but its use in the gleaming Wright would have rendered it a shattered hulk of steel and aluminum in seconds because of a phenomenon known as detonation, or more commonly, engine knock. The answer was an innovation by the Ethyl Gasoline Corp.
The company, owned by General Motors, discovered that a terribly toxic and volatile chemical called tetraethyl lead changed the way gasoline burned in the extreme temperatures and pressures created by those polished pistons and cylinders. Instead of exploding like a firecracker, the fuel-air mixture ignited in a sustained whoosh of smooth power that could be comfortably contained by the cylinders. Tucked in the Lindbergh’s baggage were .918 liter (about a quart) cans of TEL that was added to the car gasoline they were able to find along the 30,000-mile route. Without it, the astonishingly audacious trip, most of it over water, would not have been possible.
As war clouds gathered over Europe, the modest contribution of aircraft in World War I had been leveraged into a fundamental role for air power in future conflicts. Any thoughts of a resumption of the medieval tactics still clung to by military traditionalists were shattered by the scream of constant speed props attached to miracles of mechanical technology that gave them hundreds of horsepower to pull and push ever more aerodynamic designs to speeds that were previously unthinkable.
Those airframes could carry thousands of pounds of increasingly specialized munitions designed to create unprecedented mayhem where they fell or were shot. Throughout World War II, research and development of more exotically powerful engines became the foundation of ever-faster, better performing airplanes.
None of it could be accomplished without TEL, the almost magical additive that allowed all the explosive power of gasoline to be managed in a way that the highly refined engines survived.
Toward the end of the war, turbine power, which allowed hydrocarbons to be burned in a much less restrictive environment, opened the door to more power and speed, and development of piston engines virtually stopped, but their utility did not. Thousands of WWII surplus aircraft continued in civilian service, and through the 1950s there were spectacular piston designs that made it into service. Their days were numbered. Jet engines became more reliable and efficient, and they set the standard for a new age in aviation that continues today. The most current jets are twice as efficient as those from 50 years ago and are the foundation of an unprecedented era of safety and reliability in commercial aviation.
But a jet doesn’t work for all applications, and piston power remains a vital resource, particularly in general aviation. Despite innumerable advancements in materials, production standards, manufacturing tolerances and design, a modern internal combustion engine is only as good as the fuel it gets. And like all things aviation, it’s all about compromise.
Exotic concoctions push the unlimited class aircraft at the National Championship Air Races to jet speeds but within calculated limits that account for the tremendous forces working to tear those engines to pieces. The boost, temperatures, and mixtures are all painstakingly set to run the engine on the ragged edge of destruction for just long enough to finish the race.
The rest of the world doesn’t work that way.
Above all, what pilots the world over need is reliability, consistency, and predictability which, in sum, equate to safety. For more than 100 years GA has enjoyed that privilege. For the past 30 years the universal piston engine fuel has been 100LL, and it works in virtually everything from the 65 hp Continentals in legacy Cubs to the 2,250 hp behemoths that pull B-29s through the air.
Seals, bladders, and other fuel system components seem relatively immune to 100LL’s inherent solvent action (all gasolines are solvents), and it has been a solution to powering an increasingly niche market. Those days are within a few years of being over. Even the most strident defenders of GA recognize that even the relatively small amount of TEL in 100LL (about 0.56 grams per liter) is something we have to learn to live without.
That’s proving to be a bigger challenge than most people thought.
The earnest development of unleaded aviation fuel began about 40 years ago as leaded automotive gasoline was being phased out. Progress was slow, however, since there was neither political nor commercial pressure to do so. The 100LL was universally available and proven to work well in almost all engines.
It wasn’t until about 2014 that public pressure began to build from environmental groups to eliminate lead from avgas. In that year, the Center for Environmental Health won a landmark settlement agreement with 26 FBOs and four fuel distributors, compelling them to sell an unleaded fuel as soon as one became “commercially available.” At press time, a California judge is weighing arguments on whether to force those entities—now 18 because of takeovers and closures—to sell General Aviation Modifications Inc.’s G100UL, which is now being sold at two California airports.
But the real impetus for change came in 2023 when the Environmental Protection Agency issued an “endangerment finding” that said lead in aviation gasoline is an environmental and health hazard. The 100LL’s days were officially numbered through that formal finding, and that led to the formation of Eliminate Aviation Gasoline Lead Emissions (EAGLE), a joint FAA and industry task force that has set a deadline of the end of 2030 to have a technically feasible and universally available unleaded replacement fuel for 100LL.
Ideally the new fuel will be a drop-in replacement, but it seems more likely that some performance and technical adjustments will be necessary to accommodate the lack of lead. There are additives that will provide most of the benefits of lead in raising octane and providing protection from detonation, factors especially vital for the high-horsepower engines that power the light twins and complex singles used in commercial operations.
It turns out those additives have their own issues as the three companies now vying to provide the 100LL replacement are finding out through hard-won experience. The three so-called candidate fuels are at various stages of evaluation and, in some cases, limited sales, but none have reached the level of availability and acceptance needed to meet the EAGLE mandate.
There are three companies putting forward 100-octane unleaded fuels to become the replacement for 100LL, and all are pursuing different pathways toward that goal. It’s somewhat complicated by the fact that while the FAA has the final say on the approval of a new fuel, its regulations allow two distinct methods of obtaining that approval. It can issue a “fleet approval” for a fuel that basically anoints it as a fuel for all aircraft (potentially with restrictions), or companies can obtain a supplemental type certificate for their fuel and that requires each aircraft to be individually certified to legally use that fuel.
Adding to the layers of evaluation is the common, but not mandatory, practice of obtaining a fuel specification through ASTM International, a private industry consensus standards organization. The three companies have each chosen a regulatory pathway that uses different combinations of those options.
The most current jets are twice as efficient as those from 50 years ago. [Credit: Smithsonian Institute]
GAMI G100UL
GAMI co-founder George Braly and his staff began developing an unleaded 100 octane fuel in 2012 and did not accept any half measures.
To evaluate and test the results of their alchemy, they created one of the most advanced engine labs in the world. The fuel GAMI came up with, like 100LL, works seamlessly in all engines under all circumstances. That was confirmed by the FAA in 2022 with an STC that covers all engines in certified aircraft and all certified airframes except helicopters.
All lab and field testing has shown that from an engine performance and health perspective there have been virtually no issues. But the additive responsible for that performance, toluidine, is an aggressive solvent, and that has possibly caused some concerns in the commercial introduction of the fuel in California.
The fuel seems to have aggravated the impacts of preexisting fuel leaks in some of those aircraft. There are also some suggestions that it affects commonly used fuel system lubricants, and that in turn has led to temporary fouling of downstream components like fuel injectors.
Braly said he has been unable to duplicate any of those results and says the vast majority of aircraft using G100UL in the field have had no issues at all. He also notes that toluene, from which toludine is produced, is frequently used in 100LL at refineries whose base gasoline stock needs that octane boost, and airplanes often have fuel seepage when using that fuel.
GAMI is relying entirely on the STC process for approval of its fuel. Braly refuses to obtain an ASTM spec because he doesn’t trust the integrity of the ASTM process.
Swift Fuels 100R
Like GAMI, Swift has gone the STC route for FAA approval for its fuel, but it is also in the process of getting an ASTM International spec.
Swift CEO Chris D’Acosta says the ASTM spec is essential for commercial acceptance of the fuel since distributors and sellers rely on those specs to ensure the fuels are compatible with their equipment. Swift got its first very limited STC on 100R last year when the FAA gave approval for late model Cessna 172R and 172S with fuel-injected Lycoming engines to use the fuel.
But in recent public statements, D’Acosta has said the list of approved aircraft will grow. He also plans to replace the 94UL unleaded gas his company has been selling for almost a decade with the 100R. That will make it available to about two-thirds of piston aircraft in the U.S.
The company will also begin testing 100R in a large 6-cylinder, turbocharged engine this year as part of the bid to become a universal replacement for 100LL. The big engines account for about 25 percent of installations but use about 75 percent of the 100LL because those engines are usually installed on planes in commercial service.
So far there have been no reports of service difficulties in the small number of aircraft burning 100R. One issue that may arise is its lack of fungibility with G100UL, and that could stem from D’Acosta’s confident assertions that his will be the last fuel standing.
“…We believe the way the market works [that there] is really only room for one unleaded fuel,” D’Acosta said at the Sun ’n Fun Aerospace Expo in Lakeland, Florida. “We think that one unleaded fuel will be 100R. We thought that all along. We continue to think that.”
LyondellBasell Industries 100E
This is the only fuel going through the full PAFI/FAA testing and approval process. Not much has been revealed about the composition of the fuel, but LyondellBasell Industries wanted to manage expectations for it and other fuels right out of the gate.
At EAA AirVenture 2024, company spokesman Dan Pourreau told a forum it was not possible to make an unleaded drop-in replacement for 100LL without performance or technical changes to some engines.
In a later statement he said: “Based on the information available, it is safe to say that neither UL100E nor other unleaded fuels available today have sufficient detonation resistance for all general aviation piston engines and aircraft’s operating envelopes. The reason is that most engines and aircraft in operation today were certificated using leaded fuel with an average MON approaching 104. Current STC’d unleaded fuels and candidate fuels have MONs in the 99 to 101 range. This 3-5 number octane deficit will necessarily require modifications to aircraft operating envelopes, as described in current pilot operating handbooks (POHs) and engine operating manuals, to achieve acceptable detonation margins when flying on unleaded fuels.”
Porreau said the FAA-conducted tests at its own facilities in New Jersey are the best option to identify any shortcomings and develop strategies to deal with them in the field. PAFI periodically reports on the progress of testing and in February said the evaluation of 100E is, overall, about 28.8 percent complete.
PAFI is using 10 different engines and nine aircraft along with numerous aircraft parts, sealants, and coatings to conduct the tests. It has apparently performed well in those tests.
“So far, so good,” Porreau said.
What Is Tetraethyl Lead?
In chemistry parlance, it’s a single atom of lead (Pb) bonded to four ethyl groups (CH2CH3) through a carbon atom. When it hits the blast furnace conditions of an engine cylinder, the bonds between the lead atom and the ethyl groups break, and when combustion occurs, the lead forms lead oxide.
The lead oxide has a dampening effect on the combustion and “prevents fractions of the fuel mixture from burning too quickly and causing a highly undesirable ‘engine knock’” according to Britannica. Controlling that combustion allows the use of tools like high compression and turbocharging to get more power from engines. TEL was first identified in the mid-1800s, but nobody had a use for it.
For whatever reason, General Motors scientists discovered its nearly magical effect on engine performance in the early 1920s, and they quickly learned to treat it with extreme care. At least 17 workers died of lead exposure at the first plants built to make commercial quantities of the chemical. Safeguards were put in place, and the companies using TEL were able to later convince the government that the small amounts of lead pushed out of engine exhausts was harmless.
Decades of research proved that wrong, and most countries banned leaded gasoline in the 1980s and ’90s, except for aviation gasoline. Assuming the current regulatory path is upheld, the U.S. will be the first country to ban leaded avgas in 2030.
Meanwhile, only one company, Innospec in the U.K., still legally makes TEL and it wants out of the business. It has pledged to make enough to supply the world’s 100LL refiners until 2030 but makes no guarantees after that. There are unconfirmed reports that some Chinese refineries are making TEL illegally.
This feature first appeared in the June Ultimate Issue 959 of the FLYING print edition.
