What happened
On September 9, 2019, a Bell 505 Jet Ranger X, registration HB-ZWR, was conducting a training flight in the Swiss Alps. After an initial landing at Clariden-Hüfifirn proved unsuitable due to deep snow, the crew decided to proceed to the Vorabgletscher landing site. The flight was operating under Visual Flight Rules (VFR) with a flight instructor, a student pilot, and three passengers on board.
During the approach to the glacier landing site, the instructor took control of the aircraft. As the pilot lowered the collective lever to touchdown, the left skid made contact with a snow ridge. While the aircraft briefly stabilized at a 5-degree right bank, it suddenly began an unexpected roll to the right. In an attempt to stop the movement, the pilot instinctively lowered the collective lever further. This action reduced the aerodynamic lift required to stabilize the aircraft, causing it to exceed the critical tilt angle and overturn onto its right side. There were no injuries among the five occupants, though the aircraft sustained heavy damage to the airframe and rotors.
The investigation
Investigators from the SUST examined statements from the flight instructor and student, as well as data from the aircraft's onboard avionics. The investigation focused on the aircraft's weight and balance, weather conditions, and the mechanics of the landing. The aircraft was equipped with 'bear paws' to prevent sinking in snow, and the landing mass was approximately 1573 kg. The investigation also analyzed the wind conditions, which were blowing from the southwest at 10 to 15 knots, creating an upward force against the terrain.
Findings
- The primary cause of the accident was a dynamic rollover caused by exceeding the critical tilt angle during touchdown.
- The incident was exacerbated by the approach to a landing site with an unfavorable and difficult-to-recognize surface consisting of wind-blown snow.
- The pilot's instinctive reduction of the collective lever during the roll actually accelerated the loss of stability by removing the aerodynamic force that was previously countering the roll.