By Paul Bertorelli.
University of North Dakota’s flight school has decided to abandon Swift UL94 and return to 100LL after exhaust valve recession in Lycoming-powered aircraft. The school conducted an extensive trial using the new unleaded fuel almost exclusively in its Lycoming-powered Piper Archers and Seminoles. After recording exhaust valve recession, the school opted to return to 100LL in October.
Partly because students were interested in a less-polluting fuel, the school switched to Swift Fuel’s ASTM-spec UL94 in late June. In four months of flying totaling 46,000 hours, the school found evidence of significant valve recession in some of the Archers. The school’s director of maintenance, Dan Kasowski, told AVweb Wednesday (Nov. 8) that the total number of aircraft impacted isn’t known yet because the data is still being collected. The data is being forwarded to Lycoming for analysis. “The fuel is on-spec, so that’s not a problem. I don’t want to speculate on what it could be. We’re waiting for Lycoming to tell us,” Kasowski said.
Valve seat recession was a known problem when unleaded fuels were introduced in the automotive market starting in the 1970s. As explained in this AVweb video, the precise mechanism is debatable, but hardened valve seats and cylinder heads tamped down recession damage. Lycoming addressed this issue in its cylinders during the 1990s and was believed to have valve seats and guides suited for unleaded fuels. Lycoming said it was “proactively evaluating” the data received from UND and would provide appropriate guidance based on its analysis. As of today’s deadline, Swift’s Chris D’Acosta confirmed that Lycoming is looking at materials, pilot operating methods, flight telemetry and additional data to understand the findings at UND. “The comment attributed to equivalent spark plug fouling and replacement vs. 100LL is generally not consistent with our own experience with UL94. This may be an…