Quick Takes: UND Flight School Reverts to 100LL After UL94 Trial

November 10, 2023 Written by; Aviation Seminars

100LL Fuel The University of North Dakota’s flight school, renowned for its aviation program, recently faced a challenge with its fuel choice, leading to a significant operational decision. Initially, in an effort to adopt a more environmentally friendly approach, the school began using Swift UL94 fuel in late June. However, after a period of rigorous testing and observation, the school has now reverted to using the conventional 100LL aviation fuel as of October 27.

This change back to 100LL was prompted by maintenance concerns observed during the trial period with UL94. The school, having logged 46,000 flight hours over four months with the new fuel, noticed a troubling trend of exhaust valve recession in several of their Lycoming-powered Piper Archers and Seminoles. Concerned about the potential impact on their fleet, the school’s director of maintenance, Dan Kasowski, reported these findings to AVweb. He also mentioned that a complete assessment of the affected aircraft is underway, and the data collected is being analyzed by Lycoming for a more detailed understanding.

Valve seat recession is not a novel issue in the realm of unleaded fuels, having historical precedents in the automotive industry since the 1970s. Lycoming had previously addressed this challenge in the 1990s, adapting their cylinders to be compatible with unleaded fuels. The current situation at UND has prompted Lycoming to undertake a proactive evaluation of the reported valve recession issues.

In contrast to the experiences at UND, Rabbit Aviation Services in San Carlos, California, has reported positive results from using UL94. CEO Dan DeMeo, overseeing maintenance and fueling for two flying clubs with almost 10,000 hours of flight per year, noted less spark plug fouling and cleaner oil with UL94. This differing experience highlights the variability in aviation fuel performance based on specific operational conditions.

Jeremy Roesler from UND shed light on the school’s decision-making process. When they initially switched to UL94, a comprehensive maintenance monitoring program was established to identify any potential issues. However, as problems began emerging, the school was forced to weigh the benefits of a less-polluting fuel against the operational and maintenance challenges it presented. Regular maintenance checks, including the unique “dry tappet” check, were crucial in identifying the valve recession problem. These checks revealed that the clearance between the rocker arm and the valve stem was diminishing in some aircraft, indicating valve seat recession.

Despite these challenges, Kasowski noted that after 46,000 flight hours with UL94, the rate of spark plug replacement and the level of spark plug fouling were comparable to what they experienced with 100LL. This observation adds a layer of complexity to the school’s decision to revert to 100LL, suggesting that the problems might not be solely attributable to the type of fuel used. UND has also initiated the process of sending both cylinders and engines for further analysis to gain deeper insights into the issue.


This case at the University of North Dakota’s flight school highlights the complexities involved in the aviation industry’s ongoing efforts to find more sustainable and environmentally friendly fuel options, while also ensuring the reliability and safety of their aircraft.

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