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The University of North Dakota's aviation program initially switched to unleaded fuel for its aircraft fleet in 2022 but encountered engine damage and switched back to 100LL avgas in 2023. The decision was made after experiencing multiple incidents of exhaust valve seat recession, which led to an investigation of the entire fleet. The university conducted inspections and found that the issue persisted with the use of unleaded fuel but not with 100LL avgas. The cause of the exhaust valve recession is still unknown, but the university remains supportive of the industry's move towards unleaded fuel in a safe and cost-effective manner. [Extracted from the article]