Incident Overview
A commercial pilot was operating a single-engine helicopter shortly after departure from a platform in the Gulf of Mexico when a "high pitched winding noise" and a "pop" were heard, followed by a complete loss of engine power. The pilot performed an autorotation and landed the helicopter safely on the water, with the floats fully inflated. Shortly thereafter, a large wave struck the helicopter, breaking out the right windshield and rolling the aircraft inverted.
Rescue
Both the pilot and the passenger were able to exit the inverted helicopter and deploy one of the on-board life rafts. They were later rescued by a local boat and the United States Coast Guard (USCG).
Engine Examination
Examination of the engine revealed that the power turbine (PT) outer shaft was intact but displayed two spiral fractures. One spiral fracture was approximately 3 inches long, originating between a crack within one curvic tooth and the mid-shaft. At the mid-shaft location, where the shaft wall thickens, the spiral direction changed 90 degrees and continued for approximately another half-inch. The second spiral fracture was approximately 180 degrees from the first and was about half an inch in length, with one end starting at the root of a curvic tooth. The loaded faces of the curvic-coupling teeth were galled.
The fourth stage turbine connector nut was in its proper location and appeared undamaged; the deformable lock feature was still engaged. Removal of the nut was hampered by a piece of curvic tooth fragment wedged between the nut and the shaft. Once the fragment was removed, the nut rotated freely. The forward face of the shaft exhibited areas of polishing consistent with contact against the aft face of the pinion gear.
Metallurgical Findings
According to the Rolls-Royce metallurgical examination report, the fracture surfaces exhibited evidence of fatigue. The manufacturer noted that there have been 16 other instances since 1990 where the power turbine outer shaft had cracked; however, none resulted in an engine shutdown. In those cases, the only damage noted was fretting on the mating wheel. Each crack initiated in the curvic coupling, and fretting was found at the fracture origin. The fretting was attributed to a loss of clamp load, either from inadequate torque on the PT nut or raised metal on the curvics that prevented proper seating.
Manufacturer Actions
As a result of these 16 events and prior to this accident, Rolls Royce implemented several mitigating actions to prevent future occurrences, including changes to the assembly process and enhanced instructions in the overhaul manual on better handling practices to protect the curvic couplings.
Despite these measures, the source of the fractures in this accident could not be determined.