What happened
On July 20, 2021, a Bücker Bü 131B aircraft, registration HB-UVC, was engaged in a private VFR flight from Langenthal. During the approach to the airfield, the pilot handed control of the aircraft to a passenger to allow the pilot to prepare necessary documents on his kneeboard. Following this handover, the passenger increased engine power, descended, and then pulled the aircraft into a steep climb. This maneuver caused the aircraft to enter an inverted state, leading to a stall and a subsequent left-hand spin.
The pilot attempted to recover from the spin using theoretical knowledge, but the recovery was unsuccessful. During the spin, the engine was observed to be producing increased power rather than idling. The aircraft eventually struck a field near Subingen at an altitude of approximately 411 meters. The impact resulted in one fatality (the passenger) and one serious injury (the pilot). The aircraft was destroyed.
The investigation
SUST examined the aircraft, the flight controls, and the medical histories of those on board. The investigation confirmed that the aircraft's weight and center of gravity were within permissible limits and that no mechanical failures contributed to the accident. The flight controls, including the elevators and rudder, were found to be functional and were not damaged by anything other than the impact.
Medical examinations revealed that the passenger suffered from a neurodegenerative disease that had resulted in the loss of his medical certification in 2019. While the exact impact of this condition on his behavior during the flight could not be definitively proven, it was a known factor. The pilot, while experienced in motor and glider flight, had no formal training in aerobatics or Upset Recovery Training.
Findings
- The pilot handed control of the aircraft to a passenger who was not medically fit to fly.
- The passenger initiated a maneuver that caused the aircraft to enter a spin.
- The pilot lacked the necessary training to recognize, prevent, or recover from unusual flight attitudes.
- The engine was not at idle power during the spin, which likely hindered the recovery process.