NTSB Identification: DEN08IA044
14 CFR Part 91: General Aviation
Incident occurred Wednesday, December 26, 2007 in Aspen, CO
Probable Cause Approval Date: 12/24/2008
Aircraft: PIPER PA-46-310P, registration: N47BC
Injuries: 4 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.
During initial climb at 16,000 feet, the pilot noticed oil droplets forming on his windscreen. On seeing the oil, he contacted air traffic control, declared an emergency, and began a standard rate, 180-degree turn, back to the airport. As he completed the turn, he heard a "boom," felt a shutter, and then the windscreen was covered with oil. The pilot said he may have noted a pitch up with the bang, but he immediately got on the best glide speed, 90 knots, and began retracing his ground track via the global positioning system (GPS) back to the airport. The airplane landed uneventfully back to the airport. An examination of the airplane showed minor damage to the front of the cowling, and the engine showed the crankshaft fractured aft of the flange. The propeller and spinner, and the forward part of the crankshaft were missing. Several months after the incident, the separated propeller assembly and crankshaft were located. Examination of the engine revealed the crankshaft was fractured through, aft of the propeller flange. The fracture of the crankshaft was caused by a fatigue crack that stemmed from the aft relief radius of the propeller flange. The surface of the aft radius contained no evidence of mechanical damage and the fatigue crack emanated from multiple origins over a wide area, indicating that a great amount of stress was necessary to initiate the fatigue crack. The reason for the crack initiation could not be determined. The fatigue crack origin area was located in the relief radius in line with the axis of the propeller blades, consistent with forward and aft bending stresses imposed by the blades during flight. The location indicates a possibility that the blades were out of balance while rotating or that the crankshaft was somehow damaged in the past. No evidence of fretting was noted between the faces of the crankshaft propeller flange and the alternator pulley. The propeller assembly was attached firmly to the crankshaft.
The National Transportation Safety Board determines the probable cause(s) of this incident to be: The failure of the crankshaft due to fatigue. The fatigue crack initiation was not determined. Full narrative available
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