What was found
(CAN) DURING CLIMB OUT THE ENGINE CHIP LIGHT ILLUMINATED. PILOT INITIATED RETURN TO BASE, ON APPROACH A LOUD BANG WAS HEARD AND THE #1 POWER SECTION DECELERATED. AN EMERGENCY LANDING WAS CARRIED OUT. POST FLIGHT INVESTIGATION FOUND THE KEY BROKE OFF THE KEY WASHER. THIS ALLOWED THE MAIN INPUT SHAFT RETAINER TO LOOSEN WHICH ALLOWED RADIAL AND AXIAL PLAY OF THE SHAFT. THE POWER SECTION OUTPUT SHAFT DECOUPLED FROM THE GEARBOX ALLOWING THE POWER TURBINE ROTOR TO DROP AND CONTACT THE COMBUSTOR HOUSING. THE ROTOR DISINTEGRATED AND RESULTED IN SHUTDOWN OF THE POWER SECTION.