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Gold at the end of the tunnel

When will G100UL come to a fuel pump near you?

George Braly, co-founder of General Aviation Modifications Inc. (GAMI), holds out a beaker and invites a visitor to waft it toward her nose. “Can you smell the difference between that and 100LL?” he says. “People say it’s got a sweeter smell.”


Photography by David Tulis George Braly, co-founder of General Aviation Modifications Inc., has been working on an unleaded aviation fuel solutoin since 2004. GAMI is well known for its GAMIjectors, after-market fuel injectors that improve fuel consumption and engine efficiency. GAMI co-founders Tim Roehl (left) and George Braly review engine performance data for a Continental engine running on GAMI’s G100UL fuel. The console they are seated at was originally used by NASA in Houston, Texas.

People are right. This unleaded aviation gasoline—gold in color, tagged with the name G100UL—does have a slightly different odor than the blue 100LL we’ve all come to know. It is the result of nearly 10 years of development, testing, and certification. In 2021 the FAA granted its developer supplemental type certificates that allow the fuel to be used on hundreds of piston aircraft—representing about 70 percent of the overall fleet. However, the remaining 30 percent of aircraft not covered by the STCs use 100 octane fuel and account for 80 percent of all avgas utilization.

According to Braly, the FAA’s Aircraft Certification Office in Wichita acknowledged via email that GAMI completed all certification requirements for the use of the fuel in all spark ignition engines and the aircraft that use them. Still required is an official FAA signature on an expanded available model list supplemental type certificate that would encompass the remaining aircraft engines. Airframes approved for pairing with the engines will be identified separately.

Is that all? one might wonder. This final step in the STC process has been a challenge, and it may yet take some time to obtain final FAA approval. Braly is hopeful that the announcement will come this summer. Once that hurdle is cleared, the next phase can begin—where the fuel is manufactured in bulk and distributed, and sold.

“It’s a chicken and egg thing,” Braly said. “We can’t produce the fuel until we’ve got the signed document from the FAA. We’ve got to get that done first.”

The goal of the testing, essentially, is to get the engine running on G100UL to destroy itself. That hasn’t happened in the years-long testing process.
GAMI, headquartered in Ada, Oklahoma, is a familiar name to many in aviation. Braly and his partner, Tim Roehl, founded the company in 1994. Perhaps its best-known product is GAMIjectors, fuel injectors that fix the imbalance on the fuel-air ratios from cylinder to cylinder in Continental engines, thus saving fuel and improving performance. More than 30,000 sets of GAMIjectors are operating on aircraft today. Braly is considered one of the main driving forces behind modern lean of peak operations. In 2012 GAMI was one of four companies that produced spar modifications for the Beechcraft T–34 Mentor. Those modifications kept the fleet of 400 aircraft flying after a fatal accident, airworthiness directives, and suggested (and expensive) spar inspections put the airplanes’ future in doubt.

GAMI’s most recent project include development of a supplemental alternator, an electronic turbocharger controller, and an STC for a turbonormalized Beech Baron. This follows STCd turbonormalizing systems for the Bonanza, Cessna 177, Cessna 185, and Cirrus SR22.

But the creation, testing, and certification of unleaded fuel has been on Braly’s radar and GAMI’s drawing board since October 2009. Because GAMI had both experience in detonation testing and a test stand, it made sense to Braly and Roehl that the company should develop its own fuel. By mid-December 2009 “we had put together some formulations that looked like they were promising, and we filed an STC application,” he said. Thus began the cycle of certification, interrupted by changes in FAA personnel.

Fast forward to today, as Braly shows visitors around the GAMI facility where the future of unleaded avgas may have been forged. Two test beds—engines on stands—are mounted inside walls covered with honeycomb soundproofing. The soundproofing was installed when Ada residents called the police during middle-of-the-night test runs. The honeycomb cut the noise level significantly. “Now we’re a good neighbor,” Braly said.

A Continental IO-550 turbonormalized engine that originally powered a Cirrus is fed the unleaded fuel and run at 415 horsepower. (Under normal operating conditions it would run at 310 horsepower.) Here, essentially, the goal is to get the engine running on G100UL to destroy itself. That hasn’t happened in the years-long testing process.

In a room next to the test bed, protected by a three-inch-thick wall, GAMI employees monitor engine performance on a bank of monitors mounted on a steel case. The case has the unmistakable appearance of a NASA control monitor—and indeed Braly acquired it as surplus from NASA in Houston, Texas.

A 10,000-gallon tank outside the test bed holds a batch of the gold stuff. It can be used as is, or can be mixed in any proportion with 100LL, Braly said. (It turns green if you mix it with 100LL.) There’s no need to purchase dedicated tanks.

What lies between the issue of the STC and the moment you get to fill the tanks of your airplane with G100UL? “Time, logistics,” said Braly. But he added that “this fuel is not hard to make. I can splash blend 10,000 gallons at this airport [Ada Municipal Airport]. Any competent refiner can make this fuel.”

He said GAMI has an agreement with Ann Arbor, Michigan-based fuel supplier AvFuel to manage the logistics and distribution of G100UL. “Our agreement is that any qualified refiner or blender that wants to make this fuel is they will be allowed to receive a license and they can make the fuel,” he said. Ideally, he’d like to see airports in California that are barred from selling high octane leaded avgas get the first batch as an initial rollout “and let it spread to the rest of the country.”

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Jill W. Tallman

Jill W. Tallman

AOPA Technical Editor
AOPA Technical Editor Jill W. Tallman is an instrument-rated private pilot who is part-owner of a Cessna 182Q.

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