What was found
(CAN) VIBRATION NOTICEABLE ON COLLECTIVE CONTROL PWR CHANGES. FURTHER INVESTIGATION REVEALED THAT THE MAIN ROTOR MAST HAD .2500 INCH VERTICAL PLAY. ON THE REMOVAL PROCEDURE OF THE MAIN ROTOR MAST FROM THE GEARBOX IT WAS NOTED THAT THE MAST NUT HAD NO TORQUE. A THOROUGH INSPECTION OF THE ACCESSIBLE GEARBOX COMPONENTS WAS CONDUCTED WITH NO FAULTS DETECTED, THE MAIN ROTOR MAST WAS REINSTALLED AND THE ACFT FLOWN WITH NO FURTHER VIBRATION DETECTED.