What happened
On 7 April 2006, an Agusta A109S Grand, registration G-CGRI, was conducting a private flight near Liskeard, Cornwall. Approximately four minutes after takeoff, while cruising at 1,500 feet and a speed of 155 kt, the pilot experienced intense vibrations throughout the aircraft. Recognizing the danger, the pilot declared a Mayday and executed an immediate autorotation. The aircraft landed safely in a field, and the pilot was able to exit the helicopter without injury.
The investigation
An investigation into the incident revealed that one of the two flange caps on the tail-rotor trunnion assembly had separated. This separation caused significant damage to the vertical fin and one of the tail-rotor blades.
Examination of the components showed that the tail-rotor trunnion had suffered a fracture in the undercut of one of its threaded stubs. Metallurgical analysis determined that the failure was caused by an initial clockwise torsional overload, which was followed by a final axial tensile overload.
Prior to the flight, the aircraft had undergone maintenance that required the removal of the tail-rotor trunniion assembly. During this process, the maintenance engineer found that one flange cap could not be unscrewed by hand. Because the existing maintenance manual lacked specific torque instructions for the installation of these particular flange caps, the engineer used standard engineering practices to apply a torque of 70 lbf in during reassembly. While this value fell within the manufacturer's later-published limits, the investigation focused on how the initial over-torque occurred.
Findings
- The severe vibrations were a direct result of the loss of a tail-rotor trunnion flange cap.
- The failure of the trunnion stub was caused by an initial clockwise torsional overload followed by a tensile failure.
- The maintenance manual at the time did not provide specific torque loading figures for the installation of the threaded flange caps.
- The initial over-torque may have occurred either during the original manufacturing process or during the maintenance performed immediately prior to the flight.