What happened
On May 26, 2020, a Cabri G2 helicopter, registration HB-ZTT, was conducting a private VFR flight from Thun toward Adelboden with one passenger on board. While climbing at approximately 50 knots and an altitude of 200 meters above ground level, the pilot experienced a sudden loss of engine power and an audible alarm. The low rotor RPM alarm was triggered, indicating the main rotor speed had dropped into the yellow zone.
Following the alarm, the aircraft entered a descent lasting over one minute. The helicopter maintained a level flight path for approximately one kilometer, descending at an average rate of 2 m/s. The pilot did not initiate any corrective maneuvers to address the power loss or the rotor speed decay. Consequently, the aircraft struck a paved municipal road near a sawmill in the Adelboden area. The impact caused significant damage to the aircraft, including a buckled left skid, but the pilot and passenger escaped the wreckage without injury.
The investigation
Investigators from the SUST examined the wreckage, flight data from the onboard Flarm 4 system, and eyewitness accounts. The technical examination of the Cabri G2 revealed no pre-existing mechanical defects or technical failures that could have caused the incident. The aircraft had undergone a 25-hour inspection only three days prior to the accident.
Data from the flight path showed that the aircraft's altitude and speed remained relatively stable until the period of rotor decay. The investigation also reviewed the pilot's training history, noting that during a previous skill test, the pilot had struggled to successfully complete several emergency procedures.