Phoenix, AZ, USA
N7041X
ROBINSON HELICOPTER R22 BETA II
The private pilot reported that he was practicing for the commercial pilot helicopter examination and was operating in the airport traffic pattern. The pilot stated that, while entering downwind, he climbed the helicopter to 1,900 ft and was accelerating it to 75 knots when he felt an "abnormal vibration" and smelled "something…burning." The low rotor rpm light illuminated, and the low rotor rpm horn sounded. He subsequently entered an autorotation, made an emergency call, and landed successfully off the runway on dirt. Postaccident examination of the helicopter revealed that the fanshaft had fractured circumferentially; the helicopter had a total time of 337 hours. Examination of the fanshaft revealed a crack in the fanwheel's base, which had beach marks that ran from the fanshaft's bearing surface out and down to the mounting surface, consistent with fatigue. The roller bearings and the races bearing surfaces had minor surface damage. The bearings and races were cut away from the fan shaft, which revealed a v-shaped fracture surface that had two cracks emanating from it. This area had sustained damage from the two halves contacting each other with every rotation of the crankshaft, and some material was missing. The fracture origin could not be determined due to the damage. The only anomaly identified during the examination was that the fanwheel was out of balance; however, this likely resulted from the fanshaft failure.
HISTORY OF FLIGHTOn December 29, 2014, about 1900 mountain standard time, a Robinson Helicopter Company R22 Beta, N7041X, made an autorotation landing at the Phoenix-Mesa Gateway Airport, Phoenix, Arizona. Quantum Helicopters was operating the helicopter under the provisions of 14 Code of Federal Regulations (CFR) Part 91. The private pilot was not injured, and the helicopter was not damaged. The local instructional flight departed Chandler, Arizona, at an undetermined time. Visual meteorological conditions, and no flight plan had been filed. The pilot was practicing for the commercial pilot helicopter examination and was operating in the taxiway Charlie pattern. While entering downwind for runway 30, the pilot stated that he climbed to 1,900 ft and was accelerating the helicopter to 75 knots when he felt an "abnormal vibration" and smelled "something...burning." The low rotor rpm light illuminated, and the low rotor rpm horn sounded. He entered an autorotation, made an emergency call, and landed successfully off the runway in the dirt. An examination on site determined that the fanshaft fractured circumferentially. The helicopter had a total time of 337 hours. TESTS AND RESEARCHFanshaft Examination The roll pin alignment mark remained aligned, which indicated no movement between the shaft and the fanwheel. The fanwheel to fanshaft mating surfaces exhibited no galling or evidence of slipping. Investigators checked the fanwheel balance. The forward face registered 10.2 grams (maximum limit was 0.5), and the aft face registered 6.42 grams (maximum limit was 0.50). There were no indications of modifications or repairs since the part was new. A crack was observed in the base of the fanwheel. Cutting the base in a non-affected area exposed both surfaces of the crack. A visible beach marks appeared to run from the bearing surface of the shaft out/down to the mounting surface. The roller bearings and the bearing surfaces of the races had minor surface damage. After cutting the bearing and race away from the fanshaft, a v-shaped fracture surface that had two cracks emanating from it was observed. This area sustained damage from the two halves contacting each other with every rotation of the crankshaft, and some material was missing. A definite origin could not be determined. The bearing surface on the fanshaft and the inner diameter of the inner bearing race were discolored (reddish/brown), and the inner race had an area of material buildup adjacent to the fractures in the shaft.
The failure of the fanshaft due to fatigue.
Source: NTSB Aviation Accident Database
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