On March 6, 2007, an Enstrom F-28F (registration N785M) was involved in an aviation accident near Monmouth, OR. Investigators recorded the probable cause as: The failure of the main drive belt tension assembly shaft due to high cycle, low stress fatigue. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
A helicopter suffered a main drive belt disengagement due to a fractured shaft, leading to a hard forced landing that bent the tail boom.
Accident Description
The pilot of a helicopter reported conducting aerial spraying operations over Christmas trees when the engine experienced an overspeed condition. Subsequently, the pilot lost all power to the transmission and initiated a forced landing to a field. During the landing, the helicopter touched down hard, resulting in a bent tail boom. The pilot noted that the engine was producing power, but it was not being delivered to the rotor blades.
Examination Findings
Post-accident examination of the helicopter revealed that the shaft of the main drive belt tension assembly had fractured. This fracture caused the main drive belt to disengage from the transmission. Materials analysis determined that the shaft failed due to high cycle, low stress fatigue, with fatigue initiating from multiple origins at the thread root. No material defects were identified at the initiation sites, but overall hardness values indicated that the shaft's strength was lower than typical values for the specified material. It remained undetermined whether the strength met the design minimum, as the manufacturer could not provide material specifications for the shaft.