Accident Overview
A helicopter operated by a 19,000-hour pilot was transporting four passengers to an offshore oil platform when it experienced a catastrophic engine failure. The pilot performed an autorotation into the open ocean waters of the Gulf of Mexico. Within seconds of landing on the water, the helicopter rolled inverted. The pilot and passengers exited the aircraft and inflated their personal life vests, waiting for rescue.
Emergency Response
The pilot and one passenger drowned before rescue personnel arrived approximately two hours after the accident. Surviving passengers stated they were not aware of an emergency lift raft on board, and the skid-mounted emergency float system was not inflated prior to landing. Rescue personnel reported high winds and rough seas at the accident site.
Wreckage Examination
Examination of the wreckage revealed that the float "ARM" switch was found in the disarmed position with its cover closed. The skid-mounted emergency floats were found inside their protective bags. The float system was tested and found functional, with no anomalies detected during airframe component examinations.
Engine Examination
Downloaded data from the ECU showed that engine performance prior to the loss of power was normal, and the engine was operating in a steady state condition prior to the initial deterioration of NG. Detailed inspection of the engine revealed progressive turbine wheel damage throughout the power turbine. The damage varied from approximately 95% of airfoil material missing on the first stage wheel to approximately 10% missing on the fourth stage wheel. The damage observed in the gas producer turbine section was consistent with the separation of one or more first stage wheel airfoils. Most fracture surfaces were obscured, typical of elevated turbine temperatures (in excess of material limits according to the manufacturer). All four turbine wheels showed evidence of solutioning, and incipient melting was observed at the tips of the airfoil remnants. Fracture surfaces of the first stage wheel airfoils did not reveal fatigue.
Metallurgical Findings
Detailed metallurgical examination revealed the presence of sulfides on the first and second stage turbine wheel surfaces. According to the manufacturer, the presence of sulfides indicates that sulfidation has occurred. Damage on the concave surface adjacent to the fractures near the leading edges of the airfoils was consistent with type 1 hot corrosion (sulfidation) damage. Examination of radial cracks at the trailing edges of the airfoils revealed heavy oxidation consistent with thermal fatigue. According to Rolls Royce, thermal fatigue cracking at the airfoil base is not uncommon. Evidence of EPS 10649 (S1 Aluminide, a protective coating applied during manufacturing) was confirmed adjacent to the corrosive damage found on the wheels.