Incident Details
On December 1, 2011, at approximately 1005 central standard time, a Bell 407 helicopter, registration N435PH, experienced a loss of engine power while maneuvering near a platform in the Gulf of Mexico. The helicopter was operated by PHI, Inc., under 14 CFR Part 135 as a non-scheduled air-taxi flight. Visual meteorological conditions prevailed and a company flight plan was filed. The flight originated from platform EC278-C and was en route to platform EC261, both located in the Gulf of Mexico.
Emergency Landing
According to the pilot, while en route to EC261, the engine chip light illuminated. The flight was abeam platform EC278-B at the time, and the pilot elected to divert for a precautionary landing. While on base leg to EC278-B, the pilot heard a loud whining noise, followed by a loud popping noise, and the helicopter began to yaw. The pilot immediately entered an autorotation, called mayday, inflated the floats, and performed a successful water landing. Neither the commercial pilot nor the sole passenger was injured, and the helicopter was not damaged during the landing. The occupants exited the helicopter unassisted into a life raft and were rescued within 10 minutes. The helicopter remained upright for approximately 20 minutes before overturning.
Engine Examination
The engine was removed from the helicopter and examined at the PHI facility in Lafayette, Louisiana, under the supervision of a Federal Aviation Administration inspector. Examination and disassembly revealed that the No. 2 bearing was fractured, with bearing components located throughout the engine. These components were retained for metallurgical examination. The engine was a Rolls-Royce Model 250-C74B gas turbine engine, serial number CAE 847428. According to the operator's maintenance records, the engine had undergone a 150/300 hour inspection and an overhaul to the compressor section at a total time of 7,475.6 hours. The engine was installed on the helicopter 2.7 hours prior to the incident.
Metallurgical Analysis
Metallurgical examination of the bearing and other components was completed by Rolls-Royce Corporation in Indianapolis, Indiana. According to Rolls-Royce, the No. 2 bearing outer ring cracked in fatigue, initiating from a spalled area on the raceway surface. The microstructure, hardness, and chemistry of the outer ring conformed to engineering drawing requirements. The damage associated with other compressor and turbine components examined was consistent with damage that would occur after the No. 2 bearing failure.
The failed bearing was then sent to FAG Aerospace, Inc., in Stratford, Ontario, Canada, for evaluation. Results of the evaluation showed the failure occurred due to false brinnelling, a condition where the bearing is stationary but subjected to ambient vibration or shock during transport. The bearing balls were free to vibrate within the bearing, causing relative microscopic motion. The spacing between each set of brinnelling marks equaled the ball spacing within the bearing. False brinnelling increased sufficient load during bearing operation, which caused premature spalling. One spalling area was severe enough to propagate mechanical stress in the outer ring, causing the ring to fracture.