What happened
On June 12, 2020, a Reims Aviation S.A., F152, registration HB-CXV, was performing a training flight at Beromünster Aerodrome (LSZO). The flight, involving an instructor and a student, was intended as a trial flight. Due to strong Föhn winds earlier in the day, the crew had delayed the departure to wait for more favorable conditions.
At approximately 19:05, the crew initiated a takeoff from runway 33 using a maximum performance technique. The aircraft was operating with a tailwind component of approximately 5 to 10 knots. As the aircraft accelerated, the instructor utilized a specific decision-making method known as the "70/50 rule" to monitor progress. However, as the aircraft reached the end of the available runway length, it failed to achieve the necessary rotation speed. The aircraft lifted off at a speed significantly lower than the 50 knots required by the Aircraft Flight Manual, causing the tail to strike the ground. Unable to maintain a climb or accelerate, the instructor executed an emergency landing in a field of tall grass. The aircraft struck the ground on its main gear and nose gear, subsequently overturning. There were no fatalities or serious injuries, though the instructor sustained one light injury.
The investigation
SUST examined the flight data from the Flarm system, weather records, and the aircraft's maintenance history. The investigation focused on the takeoff performance calculations and the environmental conditions at the time of the accident. Investigators analyzed the available runway length, which was restricted to 400 meters due to the aerodrome's configuration that day, and compared it against the aircraft's performance requirements under tailwind conditions.
Findings
- The crew performed the takeoff based on an optimistic calculation of the wind component, assuming only 3 knots of tailwind when actual conditions were between 5 and 10 knots.
- The aircraft lifted off at an airspeed between 40 and 45 knots, which was below the prescribed 50-knot rotation speed.
- The available runway length was insufficient for the actual wind conditions.
- The use of the "70/50 rule" as a primary decision criterion provided a false sense of security, as meeting the threshold at the runway midpoint did not guarantee sufficient performance for the remainder of the roll.
Safety action
Following the accident, the aerodrome operator updated regulations for runway 33, prohibiting takeoffs with a tailwind when the aircraft is near its maximum takeoff weight. Additionally, the instructor implemented more conservative decision "gates," aiming for 80% of rotation speed at the runway midpoint and applying a 30% safety margin to performance calculations on grass runways.