University of North Dakota Stops UL94 Use Following Valve Recession Concerns

UND has gone back to 100LL from UL94 after citing an issue with "exhaust valve recession" in the Lycoming engines that power its fleet.

Meg Godlewski

Friday, November 10, 2023 at 06:15 PM ET

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UND switched to UL94 at the end of June and returned to 100LL in October 2023. [Shutterstock]

Key Takeaways:

The University of North Dakota (UND) reversed its transition to UL94 unleaded fuel for its training fleet, returning to 100LL due to detected "exhaust valve recession" in Lycoming engines.
This valve recession, where valves sit too low in their seats, was identified through UND's robust maintenance monitoring program, which found some cylinders exceeded specified minimum clearance limits.
UND is collaborating with Lycoming and Swift Fuels to investigate the cause of the valve recession, as a direct link between the issue and the lack of lead in UL94 has not yet been confirmed.
In contrast to UND's experience, other flight schools using UL94, particularly those operating under 100LL bans, have not reported similar engine issues.

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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 uncommanded loss of engine power and compression and, in worst cases, valve failure.