NTSB Identification: DFW07IA184.
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Nonscheduled 14 CFR
Incident occurred Thursday, August 16, 2007 in Gulf of Mexico, GM
Probable Cause Approval Date: 09/26/2008
Aircraft: Bell 407, registration: N433PH
Injuries: 1 Minor,1 Uninjured.
NTSB investigators used data provided by various sources and may not have traveled in support of this investigation to prepare this aircraft incident report.
Shortly after departing in a single-engine helicopter from a platform in the Gulf of Mexico, the commercial pilot heard a "high pitched winding noise" and a "pop" followed by a loss of engine power . The pilot landed the helicopter safely on the water with the floats fully inflated. Shortly thereafter, a large wave impacted the helicopter and broke out the right windshield and rolled the helicopter inverted. The pilot and the passenger were able to exit the helicopter and deploy one of the on-board life rafts. They were later rescued by a local boat and the USCG. Examination of the engine revealed the power turbine (PT) outer shaft was intact; however, there were two spiral fractures observed on the shaft. One spiral fracture was approximately 3-inches-long and originated between a crack within one curvic tooth and the mid-shaft. At the mid-shaft location, as 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 approximately one half-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 to be undamaged. The deformable lock feature was still engaged with the nut. The easy removal of the nut from the shaft was hampered by the presence of a piece of curvic tooth fragment that was wedged between the nut and the shaft. Once the tooth fragment was removed, the nut rotated freely and was removed. The ring described by the forward face of the shaft, exhibited areas of polishing consistent with contact against the ring described by the aft face of the pinion gear. According to the Rolls-Royce metallurgical examination report, the fracture surfaces exhibited evidence of fatigue. The manufacturer also stated that there have been 16 other instances where the power turbine outer shaft had cracked since 1990; however, none of these resulted in an engine shut down. The only damage noted in these cases was fretting on the mating wheel. Each of the cracks 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. As a result of these 16 events, and prior to the PHI accident, Rolls Royce implemented several mitigating actions to prevent a future occurrence. These included changes to the assembly process, enhanced instructions in the overhaul manual on better handling practices to better protect the curvic couplings. The source of the fractures could not be determined.
The National Transportation Safety Board determines the probable cause(s) of this incident to be: Fatigue failure of the power turbine outer shaft for undetermined reasons. Full narrative available
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