Incident Overview
A student pilot was flying a helicopter in cruise flight at an altitude of 800 feet when a sudden loss of tail rotor thrust occurred. The pilot reported hearing a "sharp metallic-sounding squeal" coincident with the loss. In response, the student pilot executed an autorotation, which resulted in a hard landing. The helicopter sustained damage during the landing.
Examination Findings
Post-accident examination of the tail rotor drive shaft revealed smearing and rubbing on the forward fitting teeth, specifically in the middle of the tooth surfaces. Investigators noted that a wear/rubbing failure mechanism had been previously observed on other helicopters, attributed to insufficient lubrication. However, in this instance, a significant amount of grease was present on the forward fitting, making it impossible to confirm whether adequate lubrication had been consistently maintained throughout the component's service life. No significant manufacturing defects were identified.
Background
The helicopter model in question has been in service since the 1960s, with the tail rotor drive shaft design remaining essentially unaltered over that period. The shaft loading during normal operations was described as "relatively light," requiring only minimal lubrication. Despite over 3,000 helicopters of this type in service, only two prior similar events had been linked to insufficient lubrication, both occurring on the same helicopter model within the previous year. The drive shaft required an inspection every 300 flight hours; the most recent inspection had taken place 180 hours before the accident.