SHERMAN, TX, USA
N298SH
Schweizer 269C
According to the pilot, the helicopter was on final approach at 20 feet agl and 40 knots when it 'abruptly yawed left.' The pilot noted that the engine RPM 'pegged off the dial.' The pilot lowered the collective and rolled the throttle to idle. Approximately 5 feet agl, the pilot increased the collective and applied 'slight' aft cyclic. The helicopter impacted the ground on the rear section of the right skid, bounced, and rolled over onto its left side. Examination of the helicopter at the accident site revealed that the clutch cable had separated. The clutch cable was examined and found to have failed as a result of high-cycle fatigue crack propagation on the individual wires. The initiation of the fatigue cracks correlated to an area of contact between the wires; however, there was no evidence of manufacturing or material defects in the cable assembly that could be associated with the failure. Examination of the failed cable revealed that it may have been difficult to notice the failing clutch cable wires without disassembly. According to the helicopter's maintenance manual , disassembly and inspection of the clutch cable and clutch assembly is to be completed every 400 hours. At the time of the accident, the helicopter had accumulated a total of 757.5 flight hours and 398.9 hours since the last 400-hour inspection.
On July 25, 2000, at 1215 central daylight time, a Schweizer 269C helicopter, N298SH, was substantially damaged when it impacted terrain during a forced landing at the Sherman Airport near Sherman, Texas. The helicopter was registered to and operated by Skylane Helicopters, Inc., of Decatur, Texas. The commercial pilot was not injured and his passenger received minor injuries. Visual meteorological conditions prevailed and a flight plan was not filed for the 14 Code of Federal Regulations Part 91 business flight. The local flight departed from the Sherman Airport at 1100, and was returning after visually inspecting power lines. According to a written statement provided by the pilot, the helicopter was on final approach to runway 13, approximately 20 feet agl and at 40 knots when it "abruptly yawed left." The pilot stated that the engine RPM was "pegged off the dial." The pilot lowered the collective and rolled the throttle to idle. When the helicopter was 5 feet agl, the pilot increased the collective and applied "slight" aft cyclic. He reported that the helicopter impacted on the rear section of the right skid, bounced and rolled over onto the left side, coming to rest on the runway. The engine was still running when the helicopter came to rest. The FAA inspector, who responded to the accident site, reported that both main rotor blades were damaged, the right skid was broken, the tail boom was severed, and the tail rotor gearbox was separated from the tail boom. The helicopter's clutch cable was found separated at the accident site. The clutch cable was routed from an electric linear actuator, around a single pulley, to the clutch control spring assembly. Attached to one end of the clutch cable was a swaged turnbuckle, which secured the cable to the linear actuator. On the other end of the cable was a swaged ball terminal. The ball terminal was normally seated within the spring clutch's inner spring guide with the cable passing through a hole on the bottom side of the guide. The linear actuator, when activated after engine start, pulled the cable tight, which compressed the clutch control spring assembly and pulled an idler pulley support. As the idler pulley support was pulled, the idler pulley applied a tension force on, and expanded a belt set. The belt set was wrapped around a lower pulley, which rotated with the engine crankshaft, and an upper pulley, which extended into the main rotor transmission. With the belt set pulled tight, the lower pulley drove the upper pulley. The clutch cable was removed and was taken to Materials Analysis, Inc., of Dallas, Texas, on September 13, 2000, for examination. According to a report supplied by Materials Analysis, the cable's stainless steel wires were separated at the ball terminal. The majority of the separations were located within the sleeve portion of the swaged ball terminal. Examination of the wires under a scanning electron microscope revealed that the wires displayed flat fracture surfaces and beach marks indicative of "high-cycle fatigue crack propagation." The fatigue cracks "initiated at the outer surfaces of the wires and appeared to coincide with an area of contact between two wires." There were no apparent metallurgical defects associated with the fatigue crack origins. Hardness tests and chemical analysis on the wires revealed that the cable wires met manufacturing specifications. Dimensional measurements of the swaged ball terminal revealed that it also met manufacturing specifications. No misalignment of the swaged ball terminal and the cable was noted. According to Schweizer Aircraft Corporation, this is the third known failure of the clutch cable. The other two failed clutch cables, which did not result in accidents and were therefore not investigated by the NTSB, were also examined by Materials Analysis. The three failed clutch cables were identical in location and mode of failure. The total time of the other two clutch cables is unknown. According to the Schweizer 269 Maintenance and Inspection Manual, under the section titled, "Periodic Inspection of Belt Drive Clutch Control Engagement Cable," the clutch cable and attaching fittings are to be examined "closely" at every 50-hour inspection. In part, the instructions state to inspect the first 8 inches of cable directly below the clutch spring assembly for evidence of corrosion and broken cable strands. According to the maintenance instructions, no broken cable strands are acceptable. Mechanics are also instructed to "Perform disassembled inspection (step e.) and end terminal inspection (step f.) at intervals specified in HMI Appendix B." According to Appendix B, this inspection is to be completed every 400 hours. During the disassembled inspection, the mechanic is instructed to remove the ball terminal from the spring assembly and inspect the area adjacent to the swaged ends with a 10X magnifying glass. In addition to the scheduled maintenance instructions, the pilot's preflight checklist in the Schweizer 269 Pilot's Flight Manual calls for the visual inspection of the clutch control cable, the spring assembly, and the lower end of the spring retainer tube with the clutch engaged and disengaged. The aircraft was manufactured in 1998, and was issued a standard airworthiness certificate on February 26, 1998. Review of the aircraft's maintenance records revealed that the last 50-hour inspection was completed on June 19, 2000, at an aircraft total time of 713.6 hours. On March 9, 2000, at an aircraft total time of 666.8 hours, the drive belt set was replaced with a new set. The aircraft records indicated that the 400-hour inspection had been completed at an aircraft total time of 358.6 hours. At the time of the accident, the helicopter had accumulated a total of 757.5 flight hours. Examination of the failed clutch cable revealed that the failed cable strands may not have been visible during a preflight or maintenance inspection. Disassembly of the clutch cable from the spring assembly (as called out at the 400-hour inspection) may have been the only way to detect the failing cable strands.
the total failure of the clutch cable during landing, which resulted from fatigue cracking of the cable's individual wires.
Source: NTSB Aviation Accident Database
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