Back when most of our GA aircraft were built, pilots were accustomed to and trained on selecting the correct grade of avgas when refueling. But for the past couple decades, that hasn’t been an issue—the choice was only 100LL, what we all know as avgas.
Swift Fuels introduced its 94UL unleaded avgas in 2015, and it’s appearing at more and more airports nationwide. But not all pilots are certain whether their aircraft can operate on 94UL legally, and more importantly, safely. GAMI has FAA supplemental type certificate approval for its G100UL unleaded avgas on a large number of engines and airframes, with approval for every piston engine in the fleet expected soon, but production is still months away. Swift Fuels, for its part, is continuing certification work on its 100R unleaded avgas, and the company forecasts approval in a year or two. And both Phillips 66 and Lyondell Chemical (think ARCO) have unleaded avgas projects underway through the FAA’s Piston Aviation Fuels Initiative (PAFI), although those may be five years away. So over time, you may see a variety of aviation gasolines on offer as you fly from place to place.
Fueling safety requires that you know what fuel your aircraft can safely use, which fuels it can’t use, and where to find them in the face of potential bans on certain grades of avgas. Is it as simple as looking at the aircraft/engine paperwork? Unfortunately, no. Our aircraft paperwork references aviation gasoline grades long ago abandoned or made unavailable. At the same time, new gasoline options are appearing at the airport. What does a pilot need to know to avoid making an expensive or dangerous mistake?
Most of our GA aircraft have engines originally certified to operate on 80/87, 91/96, or 100/130 octane avgas. The first number in each case is the aviation lean octane rating. The second number in each avgas octane pair is the aviation rich rating, tested under conditions typically found in highly boosted engines at very rich mixtures.
91/96 avgas went away 60 years ago as the airlines transitioned from gasoline-burning piston aircraft to kerosene-burning turbojets. 80/87 avgas went away 20 years ago as 100LL was adopted as a compromise: enough octane for engines requiring 100 but less lead than 100/130, to minimize lead fouling of lower-compression 80/87 engines designed for much lower lead additization levels. But aircraft continued to be built and sold with 80/87 and 91/96 avgas listed as the required fuel, because of certification inertia.
In 2016, the FAA sought to clarify the requirements by issuing a Special Airworthiness Information Bulletin (SAIB) HQ-16-05, now in revision 1, “Fuel– Grade UL94 Unleaded Aviation Gasoline.” The bulletin seems to say that any aircraft or engine certified for either 80/87 avgas or 91/96 avgas is perfectly safe and legal to use grade UL94 avgas. The FAA similarly argues that since the UL91 fuel (found mostly in Europe, made by Total and Hjelmco) meets almost all the same requirements as 100LL, except for octane and lead content, then that fuel too is acceptable for use in certified aircraft with octane requirements of 91 or less.
The FAA explains, “These aviation fuel operating limitations may be listed in the product’s Type Certificate Data Sheet (TCDS), installation manual, service instructions, or as limitations associated with a Supplemental Type Certificate (STC).” Lycoming has been active in revising its fuel service instruction SI1070 to reflect approved fuels.
So, where does that leave us? Start by checking the type certificate data sheet (available on the FAA’s website, or your type club’s website) for fuel requirements. Make sure to consider any STCed changes that may have been imposed by modifications.
For instance, most higher-compression piston mods revise the minimum grade from 80/87 or 91/96 to 100. Similarly, turbocharger retrofits typically require the higher-octane fuel. If the TCDS or STC calls for 80/87 or 91/96, then according to the FAA SAIB, you can use avgas rated at 91 or 94 octane. (This does not extend to mogas, whose octane ratings are inflated relative to avgas by less severe octane test procedures.)
If the TCDS or STC requires 100, 100/130, or 100LL, then 100 unleaded fuels should all work. Everything the developers are saying now points to those fuels being completely fungible (intermixable) with 100/130 or 100LL. Just fuel and go, safely. Paperwork may be required.
If you want to use mogas, then determine if EAA or Petersen Aviation have STCs for both the engine and airframe of interest. Note that mogas STCs can require modifications because properties other than octane are affected: Some low-wing airframes require boost pumps, or fuel line re-routing, for example, because of mogas’ higher vapor pressure.
Fueling safety requires that you know what fuel your aircraft can safely use, which fuels it can’t use, and where to find them in the face of potential bans on certain grades of avgas.
We’ve asked the FAA to clarify its guidance on UL94. If Lycoming or Continental service information list your engine as being suitable for UL94, then you’re likely good to go. Technically, you also need guidance from the airframe manufacturer, since the airframe TCDS also prescribes a fuel. But since the only salient properties that are different on the currently proposed unleaded avgas is octane, there’s no general airframe implication.
What about legality? If both your airframe and engine manufacturer have revised their service literature to reflect approval of the fuel you have available, then you’re legal. If they have not, then you may need to purchase an STC to cover both the engine and the airframe. Swift Fuels is selling a “forever” STC that covers both engine and airframe for $100, and applies both to their current 94UL (for eligible engines/aircraft) and to their eventual 100R when it is FAA approved. GAMI says it will sell STCs for both engine and airframe, and price them competitive to the existing mogas STCs, or $2 or less per horsepower. Phillips and Lyondell hope to achieve “fleetwide approval” via the FAA’s PAFI process. How exactly that will work is unclear, as the FAA has offered three different explanations in the past six months.
GAMI officials say they hope to make their STC available online, via smartphone or FBO computer, and even hopefully file the STC in the aircraft’s FAA records electronically. Otherwise, there are unlikely to be STC police at the self-fuel (although one airport hints they plan to do just that).
The future of aircraft fueling promises to be more complicated than the past couple decades, and look more like the 1950s when three or four grades of avgas were commonly available. Armed with a basic understanding of what makes a fuel safe and legal for your aircraft, you’ll be prepared to work your way through the oncoming proliferation of avgas grades. Let us know if you have questions at [email protected]
Paul Millner was an engineer for a major oil company, where he directed unleaded avgas development efforts for a decade.