Accident Overview
On 14 July 2015, an Airbus Helicopters AS 350 B3, registration HB-ZIS, was involved in an accident on the Guggigletscher near Lauterbrunnen, Switzerland. The helicopter was conducting an external load operation when the pilot lost control. The instability of the external load caused oscillations of the transport cable along the fuselage, resulting in damage to the main and tail rotor areas.
Causal and Contributing Factors
The Swiss Transportation Safety Investigation Board (STSB) determined that the accident was caused by the loss of control due to the instability of the external load, which led to cable oscillations and subsequent rotor damage. The identified causal factor was excessive speed with an unstable load. Contributing factors included insufficient fuel reserve, the air transport company's failure to evaluate the load attachment device, and the use of an inappropriate load attachment device for the mission.
Operational Context
The flight was part of a transport operation. The helicopter was equipped with an external load system. The flight crew comprised a pilot and three flight helpers. The accident occurred during the day under visual meteorological conditions. No injuries were reported among the occupants.
Investigation Findings
Examination of the wreckage and load system revealed that the load attachment device was not suitable for the task. The transport cable showed signs of oscillation damage. The fuel level at the time of the accident was below the required reserve for the mission profile. The investigation did not find any pre-existing technical faults with the helicopter or its engine.
Safety Recommendations
The STSB issued safety recommendations aimed at improving operational procedures for external load operations, including better assessment of load attachment devices and ensuring adequate fuel reserves. However, these recommendations are not part of the causal analysis.