Aerospatiale AS 355F1 N101MZ accident near Makakilo City, Hawaii

Probable cause

A partial loss of engine power while maneuvering over rough/uneven terrain that was precipitated by the failure of a 3rd stage turbine blade as a result of high-cycle fatigue.

NTSB analysis

During external load operations, the right (No. 2) engine experienced a loss of engine power; the associated engine chip light illuminated. The pilot executed a running landing to a nearby landing zone. During the landing, the helicopter encountered rough terrain, which resulted in deformation of the tail boom skin. Postaccident examination of the No. 2 engine revealed that a turbine blade had separated from the 3rd stage turbine wheel. Metallurgical examination of the blade determined that it had failed as a result of high-cycle fatigue probably due to an intermittent vibratory state. The engine manufacturer issued a service bulletin in 2006 and a revised bulletin in 2009 that systematically removed the 3rd stage turbine wheel that was susceptible to this failure mode from the fleet of Model 250 Series II engines. The service bulletin required that the 3rd stage turbine wheel must be replaced at overhaul or if the helicopter experienced an one-engine inoperable (OEI) condition. The turbine wheel had not been replaced, and it could not be determined if the helicopter had experienced an OEI condition.

Text from the NTSB. Open the NTSB docket for WPR12LA125 for the full record.