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. 

Comments (9)

I do not trust Government to do what is truly going to protect our GA engines. They are happy to cost you $15,000 in modifications to your aircraft and call it a success. Only Government can make an issue out of a non-issue and cost the nation untold billions to do it. As usual, the little guy is getting squeezed.

Posted by: bruce postlethwait | August 8, 2017 8:22 PM    Report this comment

" '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."

Here we go already with the goalpost-moving (read: "screwing").

And THIS is why my next airplane will burn kerosene.

Posted by: Tom Yarsley | August 9, 2017 5:55 AM    Report this comment

Meanwhile most of the Rotax 912s in the country, along with many Lycoming O-235s, are having to suffer from lead deposits that could be mitigated by giving us access to a lead-free, ethanol-free fuel. But no, they wait. The O-235s have waited for 40 years, so far.

Posted by: Thomas Boyle | August 9, 2017 6:52 AM    Report this comment

What chaps my hyde is that GAMI already has, and has had, a suitable drop-in replacement for years now with hundreds of hours of testing behind it. That's it - they've done it...A drop-in replacement already exists. The problem is the PAFI process essentially recreates the specs of existing 100LL, so GAMI's fuel doesn't meet this restrictive specification. So even though a replacement fuel already exists, the government people "in charge" refuse to consider it. - So stupid.

Posted by: JOHN EWALD | August 9, 2017 8:53 AM    Report this comment

Very little has been said about the cost of 100UL aircraft fuels. GAMI estimates about $5 to $6 per gallon. GAMI and the rest of the folks in the hunt are not saying anything. We now pay from $4 to $5 (on average) for 100LL. For folks with an STC, 94 octane non-ethanol fuel is about $3.50 or less. The 100LL fuel is 94 octane without the lead, and virtually 75% of the fleet could use it, as is. The rest of the fleet that needs tetra-ethyl lead could add it at refueling time. I sent a suggestion to the FAA suggesting this and that refueling trucks add a tetra-ethyl lead injector metered directly by the gallon of 100UL 94 octane sold. I did not get an answer. Swift Fuels is selling 100UL of their own design and providing STCs for aircraft that did not already have a MOGAS STC. (I could not price this fuel) It seem that an answer is staring us (and FAA) in the face and we (and FAA) are turning a blind eye to it.

Michael Winthrop (N9777W)

Posted by: Michael winthrop | August 9, 2017 1:16 PM    Report this comment

Correction: Swift is selling 94UL with the STC.

Posted by: Michael winthrop | August 9, 2017 1:20 PM    Report this comment

The issue is detonation and the problem of developing a new fuel that will meet the FAA and engine manufacturers certification regarding detonation prevention.

Posted by: greg wyatt | August 9, 2017 2:36 PM    Report this comment


Actually THE issue is that the EPA has a hair across their collective asses regarding all things leaded. If that were not the case, this looking-for-mister-goodbar odyssey never would have manifested in the first place. The problem isn't finding a high-octane fuel - we already have one (100LL). The problem is finding a fungible, miscible, and politically-correct high-octane fuel. Apparently, that's much more easily said than done. Thus the goalpost-moving. With apologies to Captain Renault, "I'm shocked!"

Posted by: Tom Yarsley | August 9, 2017 4:04 PM    Report this comment

I support getting lead out of avfuels asap and I am willing to pay more for it.

Posted by: STEFAN KING | August 12, 2017 4:59 PM    Report this comment

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