Monmouth, OR, USA
N785M
Enstrom F-28F
The pilot reported that he was spraying Christmas trees, when the engine experienced an over speed condition and he "lost all power to the transmission." The pilot initiated a forced landing to a field. During the forced landing, the helicopter touched down hard and bent the tail boom. The pilot stated that the engine was producing power, but it wasn't being delivered to the rotor blades. Examination of the helicopter revealed the shaft of the main drive belt tension assembly was fractured, which resulted in the main drive belt disengaging from the transmission. Materials examination of the shaft showed that the shaft fractured due to high cycle, low stress fatigue. The fatigue initiated from multiple origins at the thread root. No material defects specific to the origin were noted, but overall hardness values suggested that the shaft strength was less than typical strength values for the specified material. It remained undetermined whether the strength was above the design minimum since no material specifications for the shaft could be produced by the manufacturer.
On March 6, 2007, about 1100 Pacific standard time, an Enstrom F-28F single-engine helicopter, N785M, was substantially damaged during a hard landing near Monmouth, Oregon. The helicopter was operated by a private individual, doing business as, Willamette Valley Helicopters, Newport, Oregon, under the provisions of 14 CFR Part 137. Visual meteorological conditions prevailed, and no flight plan had been filed. The commercial pilot was not injured. The local aerial application flight departed a dirt pad near Monmouth about 1050, with a planned destination of Corvalis, Oregon. The pilot reported that he was spraying Christmas trees, when the engine experienced an over speed condition and he "lost all power to the transmission." The pilot initiated a forced landing to a field. During the forced landing, the helicopter touched down hard and bent the tail boom. The pilot stated that "the engine was producing power, but it wasn't getting delivered to the rotor blades." Examination of the helicopter revealed the shaft of the main drive belt tension assembly was fractured. The shaft fracture resulted in the main drive belt disengaging from the transmission. The helicopter underwent its most recent annual inspection on January 30, 2007. At the time of the accident, the helicopter accumulated 2,393.2 total hours. The main drive belt tension assembly was examined by the National Transportation Safety Board Materials Laboratory. Examination of the shaft revealed the fracture was located in the threads above the upper end of the spring housing, 1.58 inches below the lower end of the end fitting, and 2 to 3 threads above the upper face of the jam nut. The thread roots of the shaft were covered in a substance that appeared to be caked grease. The fracture had relatively smooth features and was in a flat plane perpendicular to the shaft axis, features that are consistent with fatigue. Ratchet marks were observed at one side of the fracture identified as the origin area, and fatigue features covered more than 95 percent of the fracture surface. About 1/3 of the surface, including the origin area, appeared relatively darker than the remainder of the surface. The fracture was examined using scanning electron microscopy (SEM). The SEM examination showed smooth faceted features. The faceted features were consistent with fatigue on crystallographic planes associated with high cycle low stress fatigue in metals such as austenitic stainless steels. The fracture origin was located where the flank of the thread intersected the thread root. Circumferential machining marks were also visible on the thread surfaces. According to revision A to C of the engineering drawing, the shaft could be manufactured from type 416 or 18-8 stainless steel bar. The shaft was analyzed using energy dispersive x-ray spectroscopy (EDS). The EDS spectrum was not consistent with type 416 stainless steel, but was consistent with an 18-8 stainless steel, such as a type 302 stainless steel. No strength requirements were present on the engineering drawing, and a manufacturer representative was unable to produce a material specification that included a hardness or strength requirement. According to the Aerospace Structural Metals Handbook, the typical hardness of type 302 stainless steel cold drawn bar is 212 BHN, which corresponds to an approximate tensile strength of 114,000 pounds per square inch. The failed shaft hardness was 186 BHN, which corresponded to a tensile strength of approximately 89,000 pounds per square inch.
The failure of the main drive belt tension assembly shaft due to high cycle, low stress fatigue.
Source: NTSB Aviation Accident Database
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