Maintenance personnel’s failure to properly secure the engine cooling fan attachment bolts, leading to vibration of the cooling fan, the overextension of the tension belt actuator, the failure of the drive system vee-belts, and the loss of power to the rotor system. Contributing to the accident was the failure of the operator to incorporate the manufacturer’s service bulletin addressing the prevention of belt tension actuator overextension, and the failure of maintenance personnel and the pilot to observe the loose fanwheel bolts during the most recent 100-hour inspection and during the preflight inspection of the helicopter (respectively).