What happened
On 14 March 2004, an Agusta A109E, registration G-HIMJ, was performing a private flight departing from Colney Park Heliport at Leeds Bradford Airport. Shortly after lifting into a hover and departing into the wind, the pilot heard a whining noise followed by a loud bang at approximately 300 feet. Although the pilot maintained control of the aircraft, the loss of the tail rotor drive system became apparent.
To mitigate the situation, the pilot elected to perform an immediate landing in a nearby field. During the landing flare and application of the collective, the fuselage began to spin around the rotor axis. The aircraft completed two or three rotations before coming to a stop. There were no fatalities and no injuries to the two occupants, though the aircraft sustained significant damage, including a failed tail rotor drive shaft and structural damage to the landing gear and fuselage.
The investigation
Investigators examined the wreckage and found that the forward section of the tail-rotor drive shaft had failed. This failure caused the engine output drive shaft to come into contact with the adjacent bulkhead, resulting in severe heat damage and cracking of the titanium alloy tail-rotor shaft tunnel.
Further inspection of the main rotor gearbox revealed that the lower attachment of the rear left brace assembly had failed. Specifically, four bolts securing the steel attachment bracket to the aircraft structure had fractured. Analysis of the fracture faces showed that two adjacent pairs of bolts had failed due to fatigue, with the remaining bolts failing under overload.
Testing of the corresponding rear right brace assembly revealed that at least one bolt had been installed with a significantly lower torque value than required. Because the aircraft was nearly new and had no record of the joint being dismantled, investigators focused on the initial assembly process.
Findings
- The loss of tail rotor drive was caused by the misalignment of the drive shafts, which allowed the rotating shaft to strike the titanium structural tunnel.
- This misalignment was a direct result of the failure of the rear left brace assembly attachment bolts.
- The investigation concluded that the bolts had been incorrectly torque tightened during manufacture, leading to insufficient pre-load.
- The lack of proper tension caused an uneven distribution of cyclic loads among the bolts, initiating rapid fatigue failure in the joint.