What happened
On April 14, 2012, at approximately 02:55 CDT, a Bell 4/07 helicopter, registration N509MT, performed a forced landing near Aberdeen, South Dakota. The flight, operating as a positioning flight under 14 CFR Part 91, departed from Aberdeen Regional Airport (KABR) at approximately 02:50.
Shortly after departure, the pilot reported the aircraft began a left yaw accompanied by a loud grinding or howling sound from the upper section of the helicopter. An eyewitness observed the engine making a noise similar to a backfire, noting visible sparks and flames exiting the exhaust during the descent. Suspecting a failure in the transmission or main drive shaft, the pilot reduced power and initiated a minimum power descent into an open area. The subsequent hard landing caused the skids to spread, the tailboom to sever, and the main rotor blades to sustain damage. The pilot, the sole occupant, sustained one minor injury and was not killed.
The investigation
Post-accident examination of the engine revealed internal damage. A subsequent examination conducted by Rolls-Royce under FAA auspices on May 16, 2012, determined that two blades from the third-stage turbine wheel had separated due to fatigue cracking. An additional blade also showed signs of fatigue cracks.
Metallurgical analysis confirmed that the blades met all specifications for material hardness and composition, with no evidence of material anomalies. Engineers determined the fractures occurred due to high-cycle fatigue at the trailing edge, caused by the engine operating at one or more natural frequencies of the wheel, which created conditions conducive to vibration.