Factual Information
The accident involved a Eurocopter AS 350 B3 helicopter, registration HB-ZLN, operated by Heli-Linth AG, based at Mollis aerodrome. The pilot, a Swiss citizen born in 1968, held a commercial pilot licence for helicopters (CPL(H)) and had accumulated 10,027 total flying hours, of which 6,291 were on the type. The flight was conducted under visual flight rules (VFR) as a commercial aerial work operation.
On 21 December 2013, the helicopter departed Mollis shortly after 08:00 local time. The crew, consisting of the pilot and two flight assistants, had a schedule of three missions. Two passengers (the partner and seven-year-old son of one flight assistant) accompanied the crew throughout the day. After completing initial transport flights, the helicopter and crew returned to Alp Loch for wood-felling work. At approximately 12:07, the helicopter took off from Alp Loch (elevation approx. 1360 m AMSL) to return to Ober Erlen (approx. 860 m AMSL).
History of the Flight
The pilot initially climbed slightly, then descended and increased speed. The helicopter crossed the Sernftal valley at a groundspeed of approximately 95 knots and an average rate of descent of about 2000 ft/min. During a right turn onto the base leg of the approach to Ober Erlen, the helicopter suddenly pitched nose-down sharply without pilot input, according to the pilot's statement. The precise bank angle was not recorded. The helicopter struck the ground with high vertical and forward speed. The accident occurred at approximately 12:08 local time at coordinates 731 820 / 201 160 (Swiss grid), elevation 860 m AMSL.
Injuries and Damage
Of the six occupants (pilot, two flight assistants, three passengers), five sustained serious injuries (three crew members and two passengers). One passenger was uninjured. The helicopter was destroyed, and minor terrain damage resulted from fuel leakage.
Cause
The Swiss Transportation Safety Investigation Board (STSB) determined that the accident was caused by the helicopter striking the ground during a landing approach with high vertical and considerable forward speed. The exact cause could not be fully clarified, but it was highly likely attributable to operational factors.