Accident Sequence
On 22 July 1990, a helicopter operated by Heli-Linth AG was being prepared for a rescue operation at Mollis airfield. The pilot and flight assistant began preparations while waiting for the doctor. The helicopter, an AS 350 B2, registration HB-XUK, was positioned on a rolling platform in front of the hangar. The pilot started the turbine. Approximately three minutes later, the doctor arrived and boarded the helicopter.
Immediately after the helicopter lifted off from the platform, the turbine speed decreased. The pilot managed to land the helicopter next to the platform. However, upon ground contact, the helicopter lifted off again and moved toward the hangar. The main rotor struck the building, causing the helicopter to pivot and crash against the hangar. The three occupants were uninjured. The helicopter was destroyed, and the hangar sustained damage.
Investigation Findings
The pilot held a valid professional pilot license and was authorized to conduct the flight. The helicopter had a valid airworthiness certificate. The turbine was examined by the manufacturer and no pre-existing defects were found.
Analysis indicated that during the start sequence, the pilot moved the fuel flow lever from rear to front until it engaged in the "flight" position under normal circumstances. However, if the lever is not properly engaged, the turbine speed is not automatically regulated. When the pilot increased collective pitch to take off, the turbine speed and power dropped because the fuel control was not synchronized. After landing, the power demand decreased, causing the turbine speed to rise again, and the helicopter became airborne again unless the pilot lowered the collective.
Since the turbine showed no defects, it was determined that the fuel flow lever was not properly engaged at the time of the accident takeoff.
Cause
The accident was attributed to the pilot's loss of control of the helicopter. A contributing factor was that the pilot omitted to synchronize the turbine control with the main rotor during the start procedure.