Bell Helicopter Textron Canada 206 L4 N55SL accident near Beverly, Massachusetts

Probable cause

Maintenance personnel’s inadequate inspection of the engine’s outer combustion case, which resulted in the failure of the case due to fatigue cracks that initiated on the inside surface of the case and the subsequent total loss of engine power.

NTSB analysis

The pilot reported that, shortly after the helicopter lifted off, the engine abruptly lost all power. The pilot attempted an autorotation to a taxiway but chose to land in the grass adjacent to it when it became apparent that the helicopter would not reach the taxiway. During the flare and landing, the main rotor blades contacted the tail boom, severing it just aft of the horizontal stabilizer. The helicopter then came to rest on its skids.

An initial examination of the engine revealed that the outer combustion case was ruptured in one of the two elbow areas. Subsequent examination revealed that the combustion case failed due to fatigue cracks that had initiated on the inside surface of the case along an aft horizontal butt weld line. The fractured weld line was located where a wire mesh reinforcement patch was brazed to the case. The fracture extended outward along the weld line toward the neighboring flanges and progressed along a portion of the flange joints; the fracture surface outside the case butt weld exhibited overstress tearing.

A review of maintenance log entries revealed that the most recent 150-hour inspection, which included a procedure to detect outer combustion case cracks, was performed on the engine about 41.5 hours (engine operating time) before the accident when the engine had 11,142 total flight hours, and no anomalies were noted. It is likely that maintenance personnel did not adequately inspect the engine at that time, which resulted in their failure to detect the fatigue cracks in the combustion case.

Text from the NTSB. Open the NTSB docket for ERA14LA248 for the full record.