G100UL Flight Review: No Big Deal


The first FAA-approved 100 octane unleaded fuel is virtually indistinguishable from 100LL in operation and offers some important benefits beyond the environmental ones. Rick Durden, a regular contributor to our sister publication Aviation Consumer, logged 3.3 hours in a Beech C55 Baron with one engine running on General Aviation Modifications Inc.’s (GAMI’s) G100UL and the other running 100LL. “We didn’t have to check the phase of the moon before starting the engine, no modification to the engine was required to accept and burn the fuel safely and we didn’t have to have any special training before lighting off and running it,” Durden wrote. “Ho-hum, no big deal.”

That fuel is also about 3% more energy dense than 100LL, doesn’t leave deposits in the engine and on spark plugs and is being produced in commercial quantities for the first time, as we reported earlier this week. It should go on sale in California before summer. GAMI and AOPA, which is supplying the Baron, will be at the Buckeye Air Fair Friday to Sunday doing demonstration flights and answering questions about the fuel.

Russ Niles
Russ Niles is Editor-in-Chief of AVweb. He has been a pilot for 30 years and joined AVweb 22 years ago. He and his wife Marni live in southern British Columbia where they also operate a small winery.


  1. This seems like the first legitimate step towards an unleaded avgas future. It’s about time – but of course, that’s no fault of GAMI.

  2. GAMI played by the FAA rules–spent its own money, developed the product, and earned the FAA STC–yet couldn’t get its product out there–it’s been languishing out there for over a year while the FAA looks to develop its OWN alternative product–nd dithers and muses over MOSAIC.

    Those of us “of a certain age” recall the failure of Communist governments to even design and produce something as simple as a very basic automobile.

    Here’s wishing success to Braley and GAMI–to AOPA for supplying the twin engine test aircraft–and here’s hoping that the FAA will finally get out of the way of progress.

  3. 1. Where in California?
    2. Experimental… STC… Category error.
    3. $425 for the STC? Holy heck! Can we have UL94 now, pleeeeease?

  4. “While costs for the G100UL STC vary depending on aircraft engine and horsepower, GAMI says pricing will be roughly the cost to fill up their tanks with avgas.”

    “Braly estimated the cost of the STC would be similar to that paid for an STC to run mogas in small avgas engine. Petersen Aviation charges about $1.50 per horsepower for its mogas STC. So for a 160 horsepower Cessna 172, the G100UL STC would require a one-time fee of about $240”

    So it sounds like the G100UL STC one-time cost is about the same as the Mogas STC. As far as aviation expenses go, that is on the low side of costs. Certainly a lot cheaper than any engine mods that would be required to run UL94 on high-horsepower engines would be.

  5. It’s very interesting that $425 is a deal breaker for folks. Also, note that GAMI scales the STC’s cost based on make and model, much like EAA and Peterson Aviation does for their AutoGas STC.

    Using a Cessna 150F with a 100 HP Continental O-200 as an example:

    EAA AutoGas STC:
    $1.50/hp = $150

    Peterson Aviation STC:
    $2.00/hp = $200

    GAMI G100UL:
    Though I can’t find the formula for cost on their website, using the order form at G100UL.com provides me with a cost of $190 for the STC for the example airplane. That works out to $1.90/hp for comparison’s sake.

    I cannot decide if $190 is too much for a Cessna 150 owner, but I can conclude that the pricing seems to be inline with what other companies charge for a similar product. I can also decide that the $375 that I was charged for the GAMI STC for my own high-compression IO-360 on my Musketeer was worth it; that’s $1.875/hp.

    As a believer in free-market capitalism, I am more than happy to support American ingenuity with my wallet, especially when it’s a small-business. GAMI isn’t Shell, Boeing, or even Lycoming, y’all. This is the little guy we’re talking about, and the little guy did what others couldn’t.

    I suppose if I just wanted the government to take an individual’s privately-funded intellectual property and give it to me for free, I would consider moving to one of those property-is-theft sort of places. Seems that is what other people are proponents of.

  6. The lack of any issues between 100LL and G100UL is no surprise. GAMI had put the fuel through its paces in two big-bore engines in its state of the art test bed in Ada Oklahoma. They have more than a decade of experience with their formulation as they plodded through the FAA’s obstacle course for approval. The real test will come when the fuel is put in the hands of the general flying public for a year or two. Anyone who has ever worked on the development of a new product can tell you the general public can find ways to screw things up that the developers would never envision.

  7. FWIW, the Swift STC for UL94 and their 100R Unleaded 100 Octane fuel is $100 one time.. And the reasons for delay in finding a producer of GAMI’s fuel have more to do with liability attorney’s than chemistry.. (Who gets sued if there is a problem around a fuel with no pubished specs..) And the STC cost for GAMI is going to seem less onerous compared to the ~$2/gallon increase in price at the pump… That is going to cause some concerns every time!

  8. OK, ran my Mooney through the 100GL.com and came up with a one time $600 fee. Given that my airplane is one of the 30% that needs the high octane, I don’t see that as a big fee to use G100UL. Not that I like wasting money but $600 is not a barrier to keeping my bird flying.

    That being said, I did not hit the “Pay Now” button. I live on the east coast. Not even sure where I would find G100UL within flying distance. When it becomes available I will be OK paying for the STC. And maybe the price will come down by the time G100UL becomes wildly available.

  9. Swift Fuel 94UL in Florida:

    KAPF Naples – $7.05/gallon
    KSEF Sebring – $8.10/gallon
    KDED Deland – $8.25/gallon

    I suspect G100UL will close to these prices.

    • UND quit using UL94 in its entire fleet last October 27 due to valve recession. In previous touts of UL94, it was described as “100LL without the lead”. It started using UL94 exclusively on August 7. Only 3 months later, they switched back to 100LL.

      If it IS TRUE that 94UL (the stuff that wouldn’t work in the large aircraft fleet at UND IS REALLY 100 OCTANE WITHOUT THE LEAD–why is it higher priced?

      I’d LIKE to see unleaded fuel work out–no more leaded spark plugs in low compression engines–and it could be transported by pipeline, which SHOULD cut the price EVEN MORE than leaded 100 octane. It seems, however, that “WISHING is NOT AN OPTION”–what good is a product that damages engines?

  10. The FAA should be augmenting the cost of the STC. It’s in their best interest to remove all obstacles to get this stuff out there in large quantities. The city of Long Beach in California is currently covering the cost of the STC AND the cost differential for UL AVGAS at KLGB.

  11. I bought the STC a year ago. Just waiting for it to show up at my airport. The airport management has said that they will get it as soon as it’s available.

  12. GAMI deserve to be compensated for a ton of work they did on their own nickel. Municipalities that are trying to outlaw 100LL or are otherwise encouraging UL should help with the finances. Even if they don’t, though, let’s start evaluating the cost of the STC in units of “Fillups.” In the case of my T210, the STC is likely to cost a little less than one Fillup.

  13. I posted the $ numbers the other day in a similar AVWEB story line.
    For me: $190 for the Braly STC for a ’59 Cessna 150A straight tail.
    $8.21 cents per gallon for Braly GAMI 100UL.
    $213.46 for E to F to top of filler neck.

  14. As Stated Above:

    University of North Dakota Stops UL94 Use Following Valve Recession Concerns

    The school switched back to 100LL while it investigates the issue.

    While many flight schools across the country are making the transition to operations using 100LL aviation fuel to UL94, the University of North Dakota (UND) has reversed course. UND has returned to 100LL after citing an issue with “exhaust valve recession” in the Lycoming engines that power its fleet of Piper PA-28-181 Archers and PA-44-180 Seminoles.

    What Is Valve Recession?
    According to Richard Scarbrough, A&P mechanic and contributor to FLYING, “exhaust valve recession is when the valve sits too low in the seat.” If the valve is not properly seated, there can be “blow-by” that can result in an Un-commanded loss of engine power and compression and, in worst cases, valve failure.

    “Exhaust valve recession can result in valve discoloration—first red, green, then purple. It can also erode the guide,” said Scarbrough, adding that at this time no one has attributed exhaust valve recession to a lack of lead in the fuel.

    According to UND chief instructor Jeremy Roesler, the school switched the fleet back to unleaded fuel over the summer. UND boasts a robust flight training program, with the fleet of more than 120 aircraft logging more than 46,000 flight hours between June and October.

    “The aircraft were flown to POH procedures,” Roseler said. “The UL94 fuel was on spec.”

    When routine maintenance detected abnormal exhaust valve recession, Roesler said the decision was made to revert back to 100LL.

  15. My two comments still awaiting moderation…? Been a week. The links were to a recent AOPA article. A bit redundant now looks like…