What was found
(CAN) THE PILOT HAD JUST DEMONSTRATED A SUCCESSFUL AUTO-ROTATION FOR THE EXAMINER, WHEN SITTING ON THE GROUND AT IDLE, WHEN SUDDENLY THE HELICOPTER BEGUN TO VIBRATE. THE VIBRATION CEASED FOR 5 SECONDS, THEN CAME BACK STRONG, AT THIS TIME, THE ENGINE WAS IMMEDIATELY SHUTDOWN. A FEW SECONDS LATER THE MAIN ROTOR SEPARATED FROM THE MAST. SINCE THE ENGINE WAS ALREADY SHUTDOWN, NO SUDDEN ACCELERATION OCCURRED. NONE OF THE PILOTS SUSTAINED ANY PHYSICAL INJURIES.