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) reverted its flight school fleet from unleaded UL94 fuel back to 100LL due to concerns over "exhaust valve recession" detected in its Lycoming engines.
UND had a proactive maintenance monitoring program that identified the valve recession, which can lead to engine power loss if valves do not seat properly, prompting the switch while the issue is investigated.
UND is collaborating with Lycoming and Swift Fuels to analyze data and determine the cause of the recession, although no one has yet definitively attributed it to the lack of lead in the fuel.
In contrast, other flight schools, such as those at Reid-Hillview Airport which are mandated to use unleaded fuel due to a 100LL ban, have not reported similar issues with exhaust valve recession.

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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.