What was found
DURING STRAIGHT AND LEVEL FLIGHT BACK TO THE AIRPORT, THE HVAC SYSTEM STOPPED FUNCTIONING. ATTEMPTED TO CYCLE POWER ON THE CONTROL UNIT, WHICH DID NOT RESTORE FUNCTION. ABOUT A MINUTE LATER, A CREWMEMBER IN THE REAR CABIN REPORTED A BURNING SMELL. CHECKED THE CIRCUIT BREAKER PANEL, AND FOUND A POPPED BREAKER. AT THIS POINT, TURNING FINAL TO THE AIRPORT. WHILST ON FINAL, THE BURNING SMELL PENETRATED THE COCKPIT. NO SMOKE WAS OBSERVED. A NORMAL LANDING WAS MADE, FOLLOWED BY A NORMAL SHUT DOWN. AIRCRAFT INSPECTED, COULD NOT DUPLICATE THE ISSUE.