Aspen, CO, USA
N47BC
PIPER PA-46-310P
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.
On December 26, 2007, approximately 1000 mountain standard time, a Piper PA-46-310P single-engine airplane, N47BC, piloted by an airline transport pilot, experienced an engine crankshaft failure and subsequent propeller separation, approximately 12.5 miles east of the Aspen/Pitkin County Airport (ASE), Aspen, Colorado. The airplane was registered to Bretton International Ltd., Springfield, Missouri, and operated by the pilot. The personal flight was being operated on an instrument flight rules flight plan from ASE to Denver, Colorado, under the provisions of Title 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed. The pilot and three passengers were not injured and the airplane sustained minor damage. The flight departed ASE approximately 0945. The pilot said that on climb out passing 16,000 feet, he began to notice oil droplets forming on his windscreen. On seeing the oil, he contacted Aspen Approach, declared an emergency, and began a standard rate, 180-degree turn, back to the airport. He said the turn took about a minute. Just as he'd 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 pilot said everything on the airplane continued to work. He had electrics, could navigate and extended the landing gear. The pilot said because he knew the Aspen area well, he was able to navigate and set up for a dead stick landing. He said he could see the runway through the oil on his windscreen as a dark blur in the surrounding snow. He said he kept his speed up as he didn't want to land short. Once he got close to the runway, he looked out the side windows to judge the flare and touchdown. He said he just wanted to keep the airplane in the center of the runway. He rolled out to the end of the runway and turned off on to the taxiway before stopping. The pilot said it was only when he was on the ground and one of the responding firemen informed him that he realized he didn't have a propeller. 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. No other damage to the airplane was found. The engine was retained for further examination. A review of the aircraft logbooks revealed the Teledyne Continental Motors (TCM) TSIO-550-C engine, serial number 814584-R, and a overhauled propeller were installed on the airframe on May 14, 2007. At the time of the failure, the engine had accumulated 147 hours. From the time of the engine installation to the incident, the engine underwent 4 oil changes with no anomalies noted. On March 13, 2008, at the facilities of TCM, Mobile, Alabama, under the supervision of the NTSB investigator-in-charge, the engine and crankshaft were examined. The crankshaft was fractured through, aft of the propeller flange. The forward (flange) portion of the crankshaft was not returned with engine, and as of this report, is still missing in the mountainous terrain. The crankshaft and counterweight assembly exhibited lubrication distress and thermal damage at the number 1 main bearing journal and all 6 connecting rod journals. The forward section of the crankshaft, which contained the fracture surface, was sectioned from the crankshaft and submitted for metallurgical examination. No additional anomalies were noted during the engine examination that would have precluded engine operation. The NTSB and TCM material laboratories examined the fractured crankshaft. Visual examination and bench binocular microscope examination of the forward portion of the crankshaft revealed clamshell marks typical of fatigue cracking that emanated from multiple origins at the aft fillet radius of the propeller flange. The fracture face at the origin of the fatigue crack contained mechanical damage. The damage obliterated the fracture features in the origin area of the fatigue crack. The curvature of the crack arrest positions near the origin area suggested crack initiation from multiple locations at the exterior surface. The surface of the fillet radius in the general area of the fatigue origin area showed no evidence of mechanical damage, i.e., gouge or corrosion damage. The fatigue crack propagated inboard through the wall of the flange, then extended circumferentially on each side to nearly the diametrically opposite side of the origin of the fatigue crack, and terminated in the general area. Beyond the terminus of the fatigue crack, the fracture was on a slant plane consistent with overstress separation. The hardness of the core material and the surface hardness of the nitrided layer for the incident crankshaft were within the specified range. The depth of the nitride layer also was within the specified range. On August 19, 2008, two hikers located the propeller which had the forward part of the crankshaft still attached. The assembly was recovered and forwarded to the NTSB Materials Laboratory, Washington DC, for further examination. Examination of the assembly revealed the forward portion of the spinner was not attached to the propeller assembly, and the aft portion of the spinner on one side was deformed aft. One blade contained a dent at the leading edge and tip of the same blade showed curled deformation. The curl deformation was aft and toward the trailing edge. The alternator puller and crankshaft propeller flange were attached to the propeller hub by six lugs and corresponding nuts. The alternator pulley was installed aft of the propeller flange portion of the crankshaft. The alternator pulley was disassembled from the propeller flange portion of the crankshaft with a torque wrench. According to the Maintenance Manual for the Piper PA-46-301P, at installation the specified preload torque for an attachment nut was to be between 58.5 and 71.5 foot pounds (ft-lbs). The release torque for 5 attachment nuts was within the specified preload torque range for an installed nut, and the release torque for the sixth attachment nut was 55 ft-lbs. After disassembly, examination of the contact faces between the propeller flange portion of the crankshaft and the alternator pulley, and between the faces of the attachment nuts for the lugs and alternator pulley, revealed no evidence of fretting damage. The length of the blades from tip-to-tip measured approximately 80 inches, which was the nominal tip-to-tip blade length. Visual and bench binocular microscope examination of the forward portion of the crankshaft revealed clamshell marks typical of fatigue cracking that emanated from multiple origins at the aft fillet radius of the propeller flange. The fatigue crack origin area was located in line (parallel) to the axis of the propeller blades relative to the propeller flange portion of the crankshaft. The initial portion of the fatigue crack was on a plane that was largely tangential to the disk of the flange, indicative of initiation and initial propagation under primarily bending loads introduced at the stud location. The face of the fracture at the origin of the fatigue crack contained mechanical damage. The damage obliterated the fracture features at the origin area of the fatigue crack. The curvature of crack arrest positions near the origin area suggested crack initiation from multiple locations at the exterior surface. The surface of the fillet radius in the general area of the fatigue origin area showed no evidence of mechanical damage. The fatigue crack propagated inboard and forward through the wall of the flange and terminated in the dowel pin hole. Beyond the terminus of the fatigue crack, the fracture was on a slant plane consistent with overstress separation.
The failure of the crankshaft due to fatigue. The fatigue crack initiation was not determined.
Source: NTSB Aviation Accident Database
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