Aromatics may be cause of valve seat recession after using unleaded fuel — General Aviation News

Oct 15, 2024

By General Aviation News Staff · April 16, 2024 · 6 Comments

Back in June 2023, the University of North Dakota (UND) Aviation program transitioned its training fleet from 100LL to UL94, a 94-octane unleaded fuel.

In October, after logging 46,000 hours using UL94, UND maintenance personnel identified abnormal valve seat recession in its Lycoming engines. Following a risk assessment, UND announced its decision to revert back to 100LL.

That led Lycoming Engines to conduct a “root cause analysis” of the issue, including further testing of UL94.

“This analysis has determined that the engine components and airframe components conform to specification and are not believed to contribute to valve seat recession,” Lycoming officials said in an April 12, 2024, press release. “However, testing has also demonstrated that, under certain conditions, use of UL94 may impact valve seat recession.”

Analysis indicates that aromatic concentration tolerances for UL94 may contribute to valve seat recession in certain flight profiles, Lycoming officials pointed out.

“Aromatic limits are not directly specified by ASTM D7547 and elevated aromatic concentration may result in slower flame speed, radiant heat from particulates, and particulate abrasiveness to valve seats that may contribute to valve seat recession,” they continued.

Lycoming officials noted they are working with the FAA to evaluate the sensitivity of piston engines to aromatics in fuels.

If the further analysis indicates it is needed, Lycoming officials said they will work with the industry to “further define” ASTM D7547 to “provide appropriate guidance to operators regarding how leaning techniques can impact valve seat recession with high aromatic fuels.”

Officials said that the company “remains committed” to the FAA’s Eliminate Aviation Gasoline Lead Emissions (EAGLE), an effort to find an unleaded fuel to replace 100LL by 2030.

Most fuel sold in the United States, including 100LL and both unleaded fuels now available to general aviation — from Swift Fuels and General Aviation Modifications Inc. (GAMI) — contain aromatics.