A loss of power to the No. 2 engine due to the failure of the accessory gearbox drivetrain. Also causal was the pilot’s inability to release the external longline due to the use of a safety wire across the coupling of the quick-disconnect mechanism, which interfered with his efforts to land. Contributing to the accident was the repair facility’s failure to maintain a clean environment, which resulted in the inadvertent introduction of a contaminant during the engine overhaul/repair process.