Accident Overview
On 26 May 2020, at approximately 15:01 local time, a Cabri G2 helicopter, registration HB-ZTT, operated by Helitrans AG, sustained serious damage during a hard landing on a communal road at Stiegelschwand, near Adelboden in the Bernese Oberland, Switzerland. The private flight, conducted under visual flight rules, had departed from and was to return to Thun Airport (LSZW). The pilot and sole passenger were uninjured.
Accident Sequence
The pilot had filed a local flight plan for a sightseeing flight with a passenger over Adelboden, Lenk, and Gstaad. Before boarding, he performed a 10-minute solo flight with three landings at Thun. His most recent flight prior to that had been his skill test for the Private Pilot Licence (Helicopter) on 11 December 2019.
At 14:44, the helicopter departed Thun and flew towards Spiez, then up the valley via Frutigen and Adelboden. While climbing at an indicated airspeed of approximately 50 knots and an estimated height of 200 metres above ground, the pilot perceived a power loss and heard an acoustic alarm. He observed that the low rotor rpm warning had activated and the rotor tachometer was in the yellow zone. The pilot could not describe what corrective actions he attempted during the subsequent descent, which lasted over one minute.
The helicopter descended straight ahead over a horizontal distance of about 1 kilometre, with an average sink rate of 2 m/s. The pilot did not initiate any corrective manoeuvre and could not prevent a hard landing on a paved communal road. Upon impact, the helicopter rotated clockwise on the ground. The emergency locator transmitter activated; no fire occurred.
After landing, the pilot noted the engine had stopped but the main rotor was still turning. He stopped the rotor to allow the passenger to egress unassisted, then switched off all switches and turned the fuel selector to off before exiting.
Aircraft and Pilot Information
The Cabri G2 is a two-seat helicopter with skid landing gear, a three-blade main rotor, and a fenestron tail rotor, with a maximum take-off mass of 700 kg. The last 25-hour inspection was performed on 23 May 2020, three days before the accident. The flight manual, Section 2 (Limitations), warns that exceeding power limits or entering the yellow zone when exceeding 30 kt IAS may accelerate engine and transmission degradation. Section 3 (Emergency Procedures) directs the pilot to lower the collective or increase throttle in response to a low rotor rpm (NR low) amber warning.
The pilot, a Swiss national born in 1957, held a Private Pilot Licence (Helicopter) issued by the Federal Office of Civil Aviation under EASA regulations. He had accumulated 75:26 total flight hours, all on the Cabri G2, with 0:36 hours in the last 90 days. His flight training began on 29 August 2018 and was conducted exclusively on the accident type. During his skill test on 11 December 2019, he had to repeat most emergency exercises because they were not initially successful. After a 5.5-month break from flying, he completed a 10-minute solo flight with three landings on the day of the accident.
Meteorological Conditions
A strong high-pressure system centred over the British Isles produced a moderate ground-level flow in Switzerland. In the pre-Alps and Alps, cloud cover ranged from 1/8 to 3/8 (occasionally 4/8 in eastern areas) with bases between 7000 and 9000 feet AMSL, and above 10 000 feet AMSL in inner-alpine regions. Visibility exceeded 8 km. Moderate bise turbulence was forecast for the afternoon up to flight level 060 over the Swiss Plateau and Jura, with light north wind turbulence along the Alpine main ridge between FL 080 and FL 140.
Technical Examination
A technical investigation of the helicopter found no evidence of pre-existing technical defects. Approximately 90 litres of fuel were drained from the tank. The earlier flight on the morning of the accident had revealed no anomalies. Both the main and tail rotors were freely rotatable; the left skid was bent. Ground marks indicated a clockwise rotation of about 90 degrees.
Analysis and Conclusions
The recorded flight data showed no anomalies until after overflying Adelboden. During the subsequent climb at 50 kt, the main rotor rpm dropped, activating both aural and visual warnings. The pilot had not been monitoring power parameters and was surprised by the alarms; he likely took no action to adjust power or attitude to counter the rotor rpm loss. No manoeuvre to reduce power demand was initiated. This lack of response is consistent with the pilot's low training level and limited ability to react to emergencies, as noted by his examiner during the skill test.
Consequently, the helicopter lost lift and descended. The hard landing on a small paved road between buildings was due to fortunate circumstances and was not a deliberate choice by the pilot. Owing to the low sink rate and good passive protection, the occupants were able to exit the wreckage uninjured.
The accident, during which the pilot lost control of the helicopter until ground impact, was caused by a main rotor rpm drop without corrective action by the pilot. Contributing factors were the pilot's low actual training and insufficient ability to react to emergency situations.