CEN12LA242

The pilot reported that, shortly after departure, the helicopter yawed left and that he heard a "loud howling/grinding sound" emanating from the upper area of the helicopter. He subsequently reduced power and performed a minimum power descent to an open area. During the landing, the helicopter touched down hard. A postimpact examination of the engine found that two third-stage turbine wheel blades had cracked and separated due to high-cycle fatigue at the trailing edge during operation of one or more natural frequencies of the wheel. Four additional blades displayed fatigue cracks. Metallurgy examinations revealed that the blades had no evidence of material anomalies and met the material hardness and composition specifications. The observed high-cycle fatigue initiation and propagation characteristics were consistent with operating in a condition conducive to vibration. The engine manufacturer plans to release design changes that will increase the operating speed natural frequency margin and improve the vibration margin.

On April 14, 2012, approximately 0255 central daylight time, a Bell 407 helicopter, N509MT, sustained substantial damage during a forced landing near Aberdeen, South Dakota. The commercial pilot, the sole occupant, sustained minor injuries. The helicopter was registered to and operated by Med-Trans Corp, Tucson, Arizona under the provisions of 14 Code of Federal Regulations Part 91 as a positioning flight. Visual meteorological conditions prevailed for the flight. The flight originated from the Aberdeen Regional Airport (KABR), Aberdeen, South Dakota, about 0250.According to a statement provided by the pilot, shortly after departing KABR, the helicopter began a left yaw and emanated a loud howling/grinding sound from the upper area of the helicopter. Suspecting a failed or impending failure of the main drive shaft or transmission, the pilot reduced power and scanned for any caution lights. With no caution light illuminated, the pilot began a minimum power descent to an open area. During the landing, the helicopter touched down hard resulting in spread skids, a severed tailboom, and damage to the main rotor blades. The pilot egressed the helicopter and notified with communication center of the accident at 0258. An eyewitness reported that he heard a helicopter departing the airport and the engine did not sound normal. The engine made a noise similar to a back fire and he saw sparks and flames coming out of the exhaust. He could see flames from the helicopter as it descended. A postaccident examination of the helicopter found internal damage to the engine. The engine was removed and transported to Rolls-Royce for an examination. On May 16, 2012, an engine exam was conducted under the auspices of the Federal Aviation Administration. The examination revealed that two blades from the third stage turbine wheel had fatigue cracked and separated. One other blade also display signatures of fatigue cracks. Metallurgy confirmed that the blades had no evidence of material anomalies, and met the proper values for material hardness and composition. Rolls-Royce engineering has determined that the third stage turbine wheel blades fractured in high cycle fatigue at the trailing edge location due to operation on one or more natural frequencies of the wheel. The observed high cycle fatigue initiation and propagation characteristics are consistent with operating in a condition that is conducive to vibration. Rolls-Royce plans to release two design changes that will increase the operating speed natural frequency margin on the initial change and further increase the frequency margin and also improve the vibration margin on the second design change.