Background
On 24 November 2009, an Agusta A109K2 helicopter, registered HB-XWM, was involved in an accident on the former Interlaken airfield in the canton of Bern, Switzerland. The Swiss Accident Investigation Board (SUST) conducted the investigation and published its final report (No. 2153).
Accident Sequence
During the flight, two bolts securing the swashplate (plateau cyclique) fractured. This allowed the swashplate to be displaced by the main rotor, rendering the helicopter nearly uncontrollable. The pilot lost control of the aircraft, leading to the accident.
Investigation Findings
Metallurgical examination of the failed bolts revealed that the fractures originated from fatigue cracks. The investigation traced the root cause to incomplete maintenance documentation provided by the manufacturer, AgustaWestland, and used by the maintenance organization, Agusta Aerospace Services SA (AAS). The documentation lacked critical instructions, which led to improper maintenance procedures that initiated the bolt failures.
Causes
According to the investigation, the accident was caused by the fracture of two swashplate attachment bolts, resulting in a loss of control. The incomplete maintenance documentation published by the manufacturer and used by the maintenance organization was at the origin of the bolt failure.