What happened
While operating in cruise flight at 800 feet, a student pilot experienced a sudden loss of tail rotor thrust accompanied by a sharp, metallic-sounding squeal. In response to the loss of thrust, the pilot performed an autorotation, which resulted in a hard landing.
The investigation
Investigators examined the tail rotor drive shaft and its forward fitting teeth. They discovered smearing and rubbing on the middle of the tooth surfaces. While the forward fitting contained a significant amount of grease at the time of examination, it could not be determined if lubrication had been consistently maintained. No significant manufacturing defects were identified in the components. The investigation noted that the shaft loading was relatively light and the design has remained essentially unchanged since the 1960s. The subject shaft required inspection every 300 hours and had been last inspected 180 hours prior to the accident.
Findings
- A wear/rubbing failure mechanism was identified on the forward fitting teeth.
- This specific failure mechanism, resulting from insufficient lubrication, had been observed on two other helicopters of the same model within the previous year.
- Although grease was present during the inspection, the continuity of lubrication could not be confirmed.