What happened
On October 6, 2010, a pilot was performing external load operations near Gotzenalm, Germany, using an Eurocopter AS 350 B3. The mission involved transporting a 20-meter-long log to a designated drop zone. As the pilot approached the landing site, the aircraft experienced sudden vibrations. Because the pilot could no longer see the load in the external mirrors, they immediately unhooked the load to mitigate the issue.
However, the vibrations intensified, and the helicopter began to rotate around its vertical axis. Despite attempts to stabilize the aircraft by reducing power and increasing airspeed, the helicopter entered a spin and crashed. During the approach, a witness observed the log striking the ground several times before a loud bang occurred, followed by the helicopter's tail boom buckling and the aircraft striking the terrain.
The investigation
The BFU examined the wreckage, the external load equipment, and the flight data recorded on the aircraft's VEMD. The investigation focused on the integrity of the textile sling and the sequence of events leading to the loss of tail rotor drive. Laboratory tests conducted by the WIWeB institute revealed that the sling failed due to an overload caused by an impact load. The investigation also analyzed the terrain, which was uneven and sloped in the direction of the approach.
Findings
- The primary cause of the accident was a massive impact load occurring when the log struck the ground, which caused the external load sling to snap.
- Upon breaking, the tensioned sling recoiled upward, striking both the main rotor and the tail fin, which subsequently severed the tail rotor drive shaft.
- The pilot was operating without a ground marker (signalman) at the drop zone, which could have provided critical warnings regarding the load's altitude.
- The uneven, undulating, and downward-sloping terrain made it difficult for the pilot to maintain adequate clearance for the long load.
- The specific method of attaching the log (vertically) increased the risk of high-impact loads during ground contact compared to a horizontal attachment.
- The use of the specific sling for logging purposes was not permitted by the manufacturer's documentation.