UND Report Details Valve Issues Experienced With Swift Fuels 94UL

The University of North Dakota John D. Odegard School of Aerospace Sciences has released a two-page communication concluding that engines that had been operated on Swift Fuels 94UL unleaded fuel experienced more than 128 episodes of recessed valve seats or other valve problems. The report details the flight operation’s experience switching from 100LL avgas to Swift Fuels unleaded 94UL and then back to 100LL.

The paper, authored by Dean Robert Kraus and Director of Maintenance Dan Kasowski, relates the flight operation’s experience, with the school’s fleet of 76 Piper Archer singles, 14 Seminole twins, and five Robinson R44 piston helicopters. The reason for reinstating 100LL after about three months with 94UL as the exclusive fleet fuel supply was a cumulative total of 128 cylinders experiencing recessed valve seat events. Some of those events occurred after the switch back, but involved engines that had been run on 94UL for some time.

After the switch back to 100LL, the report says, “None of the replaced cylinders have shown exhaust valve seat recession while operating on 100LL Avgas.” UND is working with Swift Fuels and Lycoming to help assess the cause of the valve problems. The UND fleet logs a total of more than 100,000 hours of flight time annually “with some months seeing up to 13,000 hours.”

The first flight exclusively flown with 94UL was on June 22, 2023. The first sign of trouble was in late August, when a Seminole exhibited a 500-degree-low exhaust gas temperature reading. In the next two months, the report relates, 11 more valve seat recessions were discovered. It was then that, “a walkthrough of the entire fleet was performed and a quick check uncovered another 15 aircraft (17 cylinders) with possible compression issues.” 

UND then initiated a triage plan with Lycoming, including an immediate compression check and borescope of all 114 Lycoming engines before further flight. “That check revealed nine more cylinders with recessed seats.” Logbook checks showed that most of the engines that experienced valve problems had flown more than 400 hours on Swift Fuels 94UL.

The report concludes: “We are continuing work with Lycoming and Swift Fuels to identify the specific causes of damage experienced here. Unleaded fuel is the future of general aviation and UND fully supports the industry moving towards further adoption in a safe and cost-effective manner.” A PDF of the full report follows.

Russ Niles co-reported on this story.