What happened
On July 14, 2015, at approximately 08:22, an Airbus Helicopters AS 350 B3, registration HB-ZIS, crashed on the Guggigletscher in the Lauterbrunnen region. The aircraft was performing a specialized transport mission, moving a 600 kg wooden roof structure from the Sphinx Observatory to the Eigergletscher station.
During the flight, the pilot accelerated the aircraft to a ground speed of 50 knots. At this speed, the external load became unstable, causing the 30-meter transport cable to swing violently. The cable struck the helicopter, impacting both the main and tail rotor systems. This impact caused significant structural damage, including the twisting of the tail boom and the severing of the tail rotor drive shaft. The resulting imbalance led to the failure of the main rotor drive and the loss of control, resulting in a high-impact crash and subsequent fire. The pilot was killed in the accident.
The investigation
SUST examined the wreckage, flight data from the collision warning and map display units, and various video recordings of the flight. The investigation focused on the technical state of the HB-ZIS, the stability of the external load, and the operational procedures of the air operator. Investigators also reviewed the fuel levels and the configuration of the attachment points used to secure the roof structure.
Findings
- The primary cause of the accident was the loss of control resulting from an unstable external load, which triggered a cable whip effect.
- The excessive flight speed while carrying an unstable load was the direct causal factor.
- The attachment method used for the roof structure was inappropriate for this specific transport mission.
- The air operator had not properly evaluated the load attachment system for this operation.
- The aircraft was operating with an insufficient fuel reserve, with only 7-4 liters remaining at the time of the incident.
- The aircraft was operating near its performance limits during a hover out of ground effect (HOGE) maneuver.