Accident Details
On April 14, 2012, at approximately 0255 central daylight time, a Bell 407 helicopter (registration N509MT) sustained substantial damage during a forced landing near Aberdeen, South Dakota. The commercial pilot, who was the sole occupant, sustained minor injuries. The helicopter was registered to and operated by Med-Trans Corp of Tucson, Arizona, under 14 Code of Federal Regulations Part 91 as a positioning flight. Visual meteorological conditions prevailed. The flight originated from Aberdeen Regional Airport (KABR) at about 0250.
According to the pilot's statement, shortly after departure, the helicopter began a left yaw and emitted a loud howling and grinding sound from the upper area. Suspecting a main drive shaft or transmission failure, the pilot reduced power and scanned for caution lights. With no caution lights illuminated, the pilot initiated a minimum power descent to an open area. During landing, the helicopter touched down hard, resulting in spread skids, a severed tailboom, and damage to the main rotor blades. The pilot exited the helicopter and notified the communication center at 0258.
Eyewitness Account
An eyewitness reported hearing the helicopter depart the airport with an abnormal engine sound, similar to a backfire. Sparks and flames were observed coming from the exhaust, and flames were visible as the helicopter descended.
Postaccident Examination
A postaccident examination revealed internal damage to the engine. The engine was removed and transported to Rolls-Royce for examination, which took place on May 16, 2012, under the auspices of the Federal Aviation Administration. The examination found that two blades from the third stage turbine wheel had fractured due to fatigue cracking, and one other blade showed signatures of fatigue cracks. Metallurgical analysis confirmed no material anomalies and proper hardness and composition. Rolls-Royce engineering determined that the third stage turbine wheel blades fractured in high cycle fatigue at the trailing edge due to operation on one or more natural frequencies of the wheel. Rolls-Royce plans to introduce two design changes to increase the operating speed natural frequency margin and improve vibration margin.