The change from 100LL to UL94 unleaded fuel is not without some growing pains in the aviation fleet.
Lycoming Engines has released the results of a study undertaken after receiving reports from the University of North Dakota (UND) about abnormal valve seat recession in the company’s powerplants that were using UL94.
Throughout the country, several GA fleets have been converted so that they can use unleaded fuel. One of those fleets belongs to UND, which made the switch in June. The UND fleet consists of Piper PA-28-181 Archers and PA-44-180 Seminoles.
Over the summer, the fleet of 120 aircraft logged some 46,000 flight hours. Routine maintenance discovered what was described as “abnormal exhaust valve recession,” and in October the university switched back to 100LL.
According to a media statement from Lycoming Engines, a root cause analysis of the issue was undertaken, 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,” the statement read. “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.”
According to Lycoming, “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.”
What Is Valve Recession?
Exhaust valve recession occurs when the valve sits too low in the seat, according to Richard Scarbrough, an A&P mechanic and FLYING contributor. It results in valve discoloration—first red, green, then purple. It can also erode the guide. 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.
“The aircraft were flown to POH procedures and the UL94 fuel was on spec,” UND chief instructor Jeremy Roesler told FLYING in a previous interview.
When routine maintenance detected abnormal exhaust valve recession, Roesler said the decision was made to revert back to 100LL while the issue was investigated.
“We have been running 100LL since then,” Roesler said. “To date we have replaced 126 cylinders. We flew 46,000 hours on UL94. We are submitting a technical paper on our experience and findings.”
Roeslser added that the university flies approximately 107,000 hours a year.
Lycoming is working with the FAA to evaluate the sensitivity of piston engines to aromatics in fuels.
“If indicated by this further analysis, we will work with industry to further define ASTM D7547 and provide appropriate guidance to operators regarding how leaning techniques can impact valve seat recession with high aromatic fuels,” the company said.
Lycoming Engines said it remains committed to the FAA’s Eliminate Aviation Gasoline Lead Emissions (EAGLE) initiative, a cross-functional industry effort to identify and deploy a high-octane unleaded replacement for 100LL for piston-engine aircraft by the end of 2030.
The FAA did not respond to FLYING’s request for comment.
