ACCORDING TO THE PILOT-IN-COMMAND, WHO WAS DEMONSTRATING HER COMPETENCY TO THE DESIGNATED PILOT EXAMINER FOR THE PURPOSE OF OBTAINING HER MULTIENGINE FLIGHT INSTRUCTOR CERTIFICATE, NO ANOMALIES OCCURRED WITH THE AIRPLANE DURING THE FLIGHT. THE LANDING APPEARED NORMAL EXCEPT THAT DURING ROLLOUT THE GEAR DOWN AND LOCKED LIGHT TURNED OFF. THE GEAR WARNING HORN SOUNDED AND THE LEFT GEAR BROKE OFF THE AIRPLANE. THE PILOT LOST CONTROL OF THE AIRPLANE, IT VEER OFF THE RUNWAY, AND THEN COLLIDED WITH A SIGN. EXAM OF THE LEFT MAIN LANDING GEAR REVEALED IT POSSESSED A PREEXISTING FATIGUE CRACK ON THE TRUNNION'S OUTER DIAMETER SURFACE IMMEDIATELY ADJACENT TO THE LOWER FILLET RADIUS OF THE WEB. THE CRACK HAD ENLARGED TO A WIDTH OF 1.25 INCHES BEFORE THE TRUNNION FINALLY SEPARATED. THE PILOT REPORTED THAT THE AIRPLANE'S LAST ANNUAL INSPECTION HAD BEEN PERFORMED ABOUT 53 FLIGHT HOURS AND ABOUT 1.5 MONTHS PRIOR TO THE ACCIDENT FLIGHT.