EDWALL, WA, USA
N6620Z
Cessna TU206G
The private pilot stated that during climb, about 10 minutes after takeoff, he felt a strong vibration in the engine, followed by an abrupt engine stoppage a few seconds later. During the subsequent forced landing onto rolling terrain that was surrounded by obstructions, the airplane struck a fence post, hit a ditch, and flipped over. An examination of the engine revealed that the crankshaft had fractured. The fracture was the result of fatigue cracking that initiated at the surface of the aft radius of the no. 2 main bearing journal. Heavy rubbing damage in the aft radius indicated that the bearing shifted aft and scored the radius resulting in fatigue initiation at the surface. A visual inspection of the crankcase mating surfaces revealed fretting, indicating that there was relative motion between the two crankcase halves. No evidence of heat distress or lack of lubrication was found. The engine, a Continental TSIO-520, had received a 'top' overhaul about 45 operating hours before the accident.
On August 18, 1997, about 1300 Pacific daylight time, N6620Z, a Cessna TU206G, operated by the owner/pilot, struck a fence and was substantially damaged during a forced landing near Edwall, Washington. The forced landing was precipitated by a total loss of engine power during climb. The private pilot and his pilot-rated passenger were not injured. Visual meteorological conditions prevailed and no flight plan had been filed. The personal flight departed from Spokane, Washington, and was destined for Moses Lake, Washington. The flight was conducted under 14 CFR 91. The pilot stated that he felt a "heavy vibration" from the engine about 10 minutes after takeoff during a gradual climb through about 4,000 feet above mean sea level. He stated that the propeller was rotating at 2,400 rpm and the manifold pressure was set at 27 inches at the time of the vibration. He also stated that he immediately reduced the throttle, scanned the engine gages, and noted that the gages were "all normal." The pilot further stated: "After about 6 seconds, the engine seized completely, coming to a very abrupt stop." The pilot stated that he "established best glide speed" and began looking for a suitable place to land. He stated that the terrain was "very rolling," so he decided to attempt a landing on a highway. The pilot further stated that during the approach, a strong headwind prevented him from landing on the highway. The airplane subsequently struck a fence post, hit a ditch, and flipped over. On August 27, 1997, an FAA aviation safety inspector from Spokane, Washington, acting on behalf of the Safety Board, disassembled and inspected the engine (excerpted copies of engine disassembly report attached) at an aircraft salvage facility in Deer Park, Washington. The inspection revealed that the crankshaft had fractured through the crankshaft cheek between the no. 2 connecting rod journal and the no. 2 main bearing journal. The no. 1 and no. 2 main bearings and their associated bearing saddles were severely damaged. A visual inspection of the mating surfaces of the crankcase halves revealed evidence of fretting (photographs attached). The engine examination also revealed that two nuts were found missing from the no. 2 cylinder exhaust stack. The separated crankshaft pieces and associated bearing fragments were subsequently examined at the Safety Board's Materials Laboratory Division in Washington, DC. According to the Metallurgist's Factual Report (attached) the following was noted: "The crankshaft separated through the crankcheek between the No. 2 connecting rod journal and the No. 2 main bearing journal... . The aft fracture face was totally obliterated by severe post fracture mechanical damage. Extensive damage had obliterated the fracture features on the periphery of the forward fracture face. However, the central portions of this fracture was relatively undamaged. The fracture face was cut from the remainder of the forward piece of the crankshaft for further examination. Examination of [this] fracture with the aid of a binocular microscope revealed ratchet marks and crack arrest positions, typical of fatigue cracking...." "The fatigue cracking initiated from two major origin areas separated by a large ratchet mark... . Both origin areas were located in the aft radius of the No. 2 main bearing journal. From these two origin areas, the fatigue cracking propagated.... Due to damage, the exact extent of the fatigue fracture was not determined. Further examination of the fracture surface disclosed that each origin area contacted a series [of] small ratchet marks, indicative of multiple fatigue initiations from the exterior surface of the crankshaft. The radius from which the fatigue cracking initiated contained what appeared to be heavy rubbing, apparently from contact with the bearing. Evidence of rubbing damage was also found on the No. 2 main bearing journal. Examination of the No. 2 and all other main and connecting rod journals revealed no evidence of heat discoloration that would be indicative of insufficient lubrication." According to maintenance record entries (excerpted copies attached) and manufacturer records, the engine, a Continental TSIO-520 (serial number 520431), was manufactured on June 5, 1981. The crankshaft was a forged component (forging no. 631649F) and was not manufactured under the newer vacuum air remelt (VAR) process. The engine manufacturer and the FAA had issued several service bulletins and airworthiness directives regarding subsurface fatigue cracking of non-VAR crankshafts such as the one installed in the accident engine. No evidence of subsurface fatigue cracking was found on the accident engine's crankshaft. An entry in the maintenance records, dated August 11, 1989, indicated that the engine underwent a major overhaul at Victor Aviation, Palo Alto, California, after accumulating 1,408 hours of operating time. The engine was then installed on the accident aircraft on August 24, 1989. An entry dated June 3, 1996, indicated that the engine underwent a top overhaul after accumulating 685 hours since the major engine overhaul, and 2,093 hours since it was manufactured. The entry was signed by a mechanic and pilot who was employed by the previous owner of the airplane. In an interview with the Safety Board (record of interview attached), the mechanic stated that he remembered performing the top overhaul on the accident airplane before it was sold to the new owner. He stated that the overhaul was performed while the engine remained attached to the airframe. He stated that he removed and replaced all six cylinder assemblies, and that he "absolutely" used a torque wrench to tighten down the through-bolts and hold-down studs. He stated that the airplane was placed in a hangar owned at an airport in Nevada at the time he performed the top overhaul. After the top overhaul, the mechanic remembered that he flew the airplane for about one and a half hours with no problems noted. The mechanic also remembered that the airplane "sat" after the test flight and did not fly often. He stated that he had performed numerous top overhauls on other airplanes prior to the one that he performed on the accident airplane. He stated that he would use the engine manufacturer's overhaul manual for guidance. The mechanic also stated that, while he performed the top overhaul, there were other mechanics that worked on the airplane. He also stated that after the engine underwent the major overhaul in 1989, it "vibrated." The accident pilot subsequently purchased the aircraft from the previous owner one year later in June of 1997. On July 6, 1997, after the engine had accumulated 3.7 hours of operating time since the top overhaul, it underwent an annual inspection with no discrepancies noted. The final entry in the logbook, dated July 9, 1997, indicated that the engine underwent an oil change after accumulating an additional 13 hours of operating time since the top overhaul. At the time of the accident, the engine had accumulated 45 hours since the top overhaul.
A fatigue failure of the engine crankshaft. Factors contributing to the accident were the rolling terrain, and the fence.
Source: NTSB Aviation Accident Database
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