What happened
On May 9, 1997, a Bücker Bü 131 biplane, registration HB-UVF, departed from Kägiswil airfield for a scheduled 20-minute aerobatic training flight. The flight, involving a flight instructor and a student, initially proceeded under good weather conditions with a cloud ceiling above 2,000 meters. During the maneuvers, the instructor monitored the weather toward Sarnersee and determined that returning to the airfield was still possible.
However, the weather deteriorated with extreme speed. As the crew attempted to navigate toward various exits from the valley—including Entlebuch and Flühli—incoming clouds rapidly closed off all available routes. The aircraft eventually became trapped between several mountain peaks and ridges. Faced with the imminent loss of visual flight rules (VMC) conditions and the danger of flying into clouds, the instructor decided to execute a forced landing in the rugged terrain of Alp Grossrisch. The aircraft struck the ground heavily, resulting in a capsize, but both the pilot and the student were able to exit the aircraft with minor injuries.
The investigation
The investigation examined the meteorological conditions, the flight path, and the crew's decision-making process. Investigators reviewed data from the Swiss Meteorological Institute, which confirmed that a cold front associated with a low-pressure system from the UK was crossing Switzerland. This system brought rapidly dropping cloud bases, decreasing visibility, and increasing wind turbulence.
The investigation also looked into the pre-flight preparations. It was noted that because the flight was intended to be very short, the crew did not consult the latest meteorological information via the AIS Met Information Environment (AMIE). The technical condition of the Bücker Bü 131 was found to be airworthy, with no mechanical failures contributing to the event.
Findings
- The crew failed to recognize the rapid deterioration of the weather in a timely manner.
- The flight instructor and student assessed the weather situation too optimistically.
- The rapid onset of clouds effectively trapped the aircraft, leaving no viable exits under VMC conditions.
- The decision to perform a forced landing in unsuitable terrain was considered the correct course of action to avoid the much higher risks of flying into clouds or attempting an uncontrolled turn in restricted airspace.