100LL Replacement: Drop-in Or Not?

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As the FAA’s technical testing of unleaded avgas replacements grinds on, owners and manufacturers are still wondering what the approved fuels will look like and whether they will be true drop-ins. At a briefing last month at AirVenture, three members of the FAA Piston Aviation Fuel Initiative offered an overview that warned that the clean drop-in fuel isn’t necessarily in sight yet. AVweb was provided with copies of the briefing slides.

PAFI began its work in 2014 and is scheduled to issue test data by the end of 2018 that will inform ASTM in developing a new unleaded aviation fuel spec. The second phase of that testing selected two candidate fuels, one from Shell and one from Swift Fuel. These candidates have since undergone extensive testing, including test cell runs and continuing in 2017, flight tests.

On one slide presented at the briefing, the group said the second phase testing has revealed differences with potential impacts compared to 100LL and warned that “each fuel will have different impacts, different authorized fleets and different mitigations.” Mitigations means modifications, either to the aircraft fuel system or tanks, the operating limitations or even the ground fuel distribution system. It could include anything from O-ring changes to fuel bladder replacement.

It also suggests that one fuel might be suitable for one type of aircraft, but not another, meaning the product wouldn’t be a true drop-in. However, AOPA’s David Oord, who sits on the PAFI steering group, said these caveats overstate the case. “We might not have that many mitigations. I don’t think it’s the best term to use. It implies problems and I don’t think we have problems. We have differences,” Oord said.

The PAFI process allows the fuel providers to tweak the chemical makeup as the process moves forward and once PAFI’s test data is released in 2018, the ASTM process may have altered the formulations yet more. Even as PAFI continues its testing, the ASTM balloting process on a new fuel spec to replace the D910 standard for avgas is underway. Although it hasn’t been widely reported, at least three other companies are working on fuels outside of the PAFI process to meet the spec. Phillips 66/Afton and BP are among them. In addition, General Aviation Modifications Inc. continues its testing on its G100 unleaded fuel. GAMI is pursuing an STC approval for G100 and thus far hasn’t balloted in ASTM.

Despite the worries about a visible drop-in, Oord told us he’s confident there will be viable fuels to replace avgas. “I’m confident we’ll see a drop-in for the vast majority of the feel and I think we’re on the way there,” he told us in an interview after AirVenture.