What happened
On July 20, 2006, an AS350B3 helicopter, registration HB-ZDY, operated by Heli Bernina AG, was performing a commercial VFR flight near Morteratsch, Switzerland. After completing several rotations, the pilot arrived at a gravel path near the Morteratsch parking area to deliver a transport net.
Because the intended landing spot in the parking lot was occupied by vehicles, the pilot and a ground assistant coordinated a landing on the adjacent gravel path, intending to position the aircraft perpendicularly to the path. During the landing phase, the ground assistant noticed the aircraft was at a 4/5 angle relative to the path and radioed an instruction to the pilot to move the tail to the right ("Heck nach rechts").
As the pilot was already in the final stages of the descent, the aircraft's tail rotor struck a post from the parking lot fence. The pilot felt the vibration and immediately moved the aircraft forward to land on the path. There were no injuries to the crew, though the tail rotor blades and their assembly sustained damage, and the fence post was also broken.
The investigation
The investigation examined the operational sequence and the communication between the crew and the ground assistant. Investigators established that the pilot and the assistant were communicating via radio in German. The investigation also reviewed the pilot's professional credentials, noting a valid commercial helicopter license and a recent annual flight review. Technical inspections of the AS350B3 revealed no mechanical failures contributing to the event, and meteorological conditions were clear with high visibility.
Findings
- The primary cause of the accident was a misunderstanding regarding directional instructions between the ground assistant and the pilot.
- The ground assistant issued the command to move the tail to the right based on his own perspective on the ground, rather than the pilot's perspective in the cockpit.
- The pilot interpreted the instruction relative to his own position, resulting in a counter-clockwise rotation that directed the tail into the fence post.
- The landing maneuver was likely progressing too rapidly for the ground assistant to correct the instruction once it was transmitted.