As California works to phase out leaded fuel at its airports, the first three tarmacs in the state, all in the Bay Area, started offering 100-octane unleaded fuel last fall. Contra Costa County officials are also looking at making the switch, joining San Jose, San Carlos and Watsonville. But the challenges that aviation operators are running into in Concord illustrate why the state and federal government’s plan to permanently ground leaded gas by 2031 is not as simple as it might seem.