What happened
On 6 May 2018, an Agusta AW139, registration G-MCSD, was performing a non-revenue commercial air transport flight near Aberdeen. During the climb phase following departure from Aberdeen Airport, the crew experienced an unusual vibration at approximately 2,500 feet. Although the aircraft reached its level altitude of 3,000 feet, the vibration persisted even as the speed was reduced.
Following the detection of the anomaly, the crew declared a PAN call and decided to return to the airfield. While the vibration intensity decreased as the aircraft slowed to 60 kt for a run-on landing, the vibration increased significantly once the helicopter was stationary on the ground. There were no injuries to the four crew members on board.
The investigation
Post-flight inspections revealed that a lightning protection bonding strip and a cable fairing had detached from one of the tail rotor blades. The missing components were not located. Further inspection of the remaining three tail rotor blades showed that two additional bonding strips had developed cracks.
Detailed microscopic analysis of the blades performed by the manufacturer determined that the fatigue cracks on all three strips originated from the same location. The investigation established that the loss of the cable fairing was a secondary result of the bonding strip failure.
Analysis of the Health and Usage Monitoring System (HUMS) data showed high acceleration values in the tail rotor gearbox during the flight, likely caused by the imbalance from the missing components. While metallic debris was found on the gearbox magnetic chip detector, it was not enough to trigger a cockpit caution. Disassembly of the gearbox revealed early-stage spalling on the output bearing, though this was determined to be unrelated to the blade component failure.
Findings
- The primary cause of the incident was the fatigue failure of the lightning protection bonding strips.
- The detachment of the cable fairing was a direct consequence of the bonding strip failure.
- The high vibration levels recorded in the tail rotor gearbox were induced by the loss of the rotor blade components.