What was found
(CAN) DURING POWER APPLICATION, PILOT NOTICED AN ERRATIC READING ON THE MAIN ROTOR RPM GAUGE. ON FINAL APPROACH THE LOW ROTOR WARNING SYS ACTIVATED, ALARM IN HEADSET, HOWEVER, THE ROTOR RPM WHERE CORRECT. INVESTIGATION IN CONJUNCTION WITH MFG REVEALED THAT THE 6 BOLTS RETAINING THE MAST BEARING, WHICH IN TURN RETAIN THE PHONIC WHEEL IN PLACE WERE LOOSE AND NOT AT SPECIFIED TORQUE, CAUSING THE PHONIC WHEEL TO SPIN AND TRANSMIT FALSE INFORMATION TO THE SENSOR.