Background
The helicopter, a Eurocopter AS350B2 registered as HB-XJC, was owned and operated by Lions Air AG and chartered by Air Zermatt AG. On the day of the accident, it was scheduled to be flown from Zermatt to Raron. The pilot, a Swiss citizen born in 1950, held a commercial pilot license for helicopters (CPL(H)) and had accumulated 9,026 total flight hours, including 147 hours on the accident type. The helicopter had been in service for approximately three months before the planned transfer flight.
Accident Sequence
The pilot arrived at Zermatt Heliport around 16:00 local time and took over the helicopter. He was informed by a mechanic about a peculiarity during the starting procedure of this machine. After engine start, as the pilot prepared for departure, he experienced strong vibrations accompanied by ground resonance. Witnesses reported hearing a loud engine surge. The helicopter lifted off uncontrollably. At about one meter above ground, the pilot reduced power, and the helicopter fell back to the ground, rotating approximately 90 degrees to the right before landing hard on its skids. Visible damage was observed at the junction of the fuselage and tail boom.
The pilot stated that he did not notice any auditory or visual warnings and was focused on instruments during engine start. He could not recall whether the collective lever had become unlocked while manipulating the fuel flow control (debit) during the vibrations. Witnesses described the helicopter shaking violently.
Investigation Findings
Technical examination revealed that the depth of the locking notch on the collective lever arresting head was 0.8 mm. The helicopter manufacturer later issued Alert Service Bulletin No. 67.00.27 on 25 September 2003, requiring a minimum locking notch depth of 0.7 mm. The engine, a Turbomeca Arriel 1D1, showed several discrepancies: the freewheel assembly had damage consistent with a hard landing, centrifugal compressor blades had rubbed against the cover, and turbine components exhibited cracking and coating alteration. The engine's condition suggested it had operated beyond normal temperatures. The fuel control unit (FCU) displayed functional discrepancies due to an internal leak in the non-return valve, with acceleration fuel flow too low and response times out of tolerance. The FCU's calendar limit had been exceeded by two years. No anomalies were found in the fuel analysis, and the main and tail rotor systems showed no signs of abnormal vibration. The helicopter's weight and center of gravity were within limits at the time of the accident.
Probable Cause
The investigation concluded that the accident was attributable to uncontrolled lifting of the collective lever during the acceleration process. It remained an open question why the collective did not remain locked or was not locked. The following circumstances contributed: technical defects in the turbine caused a particular start-up behavior that demanded increased attention from the pilot.