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Phillips 66 has paused the testing of its 100M unleaded aviation fuel developed with Afton Chemical, according to the company and federal officials. The fuel, designed to provide a more environmentally sustainable alternative to aircraft powered by piston engines than traditional 100LL fuel, had been undergoing evaluation under the Piston Aviation Fuels Initiative (PAFI) program.The program, launched by the FAA in 2014, was later folded into the agency's larger EAGLE Initiative that aims to eliminate the use of leaded fuel in aircraft by 2030.An FAA representative told GlobalAir.com that Phillips 66 paused the PAFI evaluation "due to issues encountered during durability testing" and referred further questions to Phillips 66. "Phillips 66 is committed to its vision of developing an unleaded aviation fuel offering and is currently evaluating this product's development and all viable alternative options," Al Ortiz, a senior advisor for the company, told GlobalAir.com by email on Tuesday morning.AvWeb cited an unnamed source with the company who reportedly said the testing was paused due to an issue with the failure of a Lycoming engine during testing, caused by a build-up of manganese deposits.According to the Phillips 66 website, the company considers PAFI as a "crucial component" in the EAGLE Initiative and has played an integral part in development, fuel evaluation and authorization since 2013.RELATED STORIES:First unleaded fuel candidate passes PAFI testing, advances to next stage What's next for G100UL and when might we really get the lead out of aviation? The FAA announced in August 2022 when it shared that Afton and Phillips 66, as well as LyondellBasell Industries and VP Racing, were the two fuel development programs undergoing tests at the FAA's William J. Hughes Technical Center under the PAFI program. LyondellBasell and VP Racing had successfully passed the 150-hour engine durability test phase in November, according to the agency, and is moving on to the next stage in testing. In September 2022, the FAA approved Supplemental Type Certificates (STCs) for use of a 100-octane unleaded fuel known as G100UL produced by GAMI in many piston-powered planes.