What was found
CIRCULATION OF SMOKE FUMES IN COCKPIT DURING CRUISE. PILOT OBSERVED USB CHARGING ADAPTER PLUGGED INTO CIGARETTE LIGHTER QUIT WORKING AROUND THE SAME TIME. AIRCRAFT RETURNED TO DEPARTURE AIRPORT AND MADE A PRECAUTIONARY LANDING WITHOUT INCIDENT. AMT INSPECTED AIRCRAFT FOR SOURCE OF FUMES AND IT WAS APPARENT THAT A USB CHARGING ADAPTER PLUGGED INTO THE AIRCRAFT CIGARETTE LIGHTER BASE WAS THE ORIGIN. CIGARETTE LIGHTER BASE, WIRING, CIRCUIT BREAKER, AND SURROUNDING STRUCTURE WERE ALL INSPECTED FOR DAMAGE AND FUNCTIONALITY, WITH NO DEFECTS NOTED. THE USB ADAPTER APPEARED TO HAVE FAILED INTERNALLY, CAUSING EXCESSIVE HEAT AND SUBSEQUENT EMISSION OF FUMES INTO THE COCKPIT.