Incident Overview
During an aerial application operation, the pilot was conducting east and west swath runs. After dispensing approximately half of the load, the pilot noted a vibration in the tail rotor pedals during a turn-around maneuver. The vibration progressively worsened over three subsequent swath runs. On the fourth turn-around, the tail rotor drive was lost.
Findings
Examination revealed that the tail rotor drive shaft aft universal joint had failed. The universal joint showed evidence of lubricant depletion, and the tail rotor drive shaft exhibited a blue discoloration, indicative of overheating due to friction. No log book entry was found to indicate that the universal joint had been lubricated in the 150 or more hours of operation since the last inspection. The manufacturer's recommended maintenance practices include lubrication of the universal joint after each 50 hours of operation.
Maintenance History
The absence of lubrication records for over 150 hours of operation, despite a manufacturer-specified lubrication interval of 50 hours, suggests that the universal joint was not maintained according to the recommended schedule.