What happened
On August 23, 2024, an Arcus M motor glider, registration OE-9SB, departed from Birrfeld Airport (LSZF) for a multi-day cross-country flight intended to reach Grenoble, France. The aircraft was operated by two experienced pilots. After an initial period of powered flight, the engine was shut down near the Balmhorn, and the crew transitioned to unpowered soaring along the southern slopes of the Rhone Valley.
While performing circuit patterns (eight) near Chistehorn, the aircraft was flying at approximately 2,355 meters. During a right-hand turn, the crew failed to complete the maneuver, leading to an aerodynamic stall. The aircraft entered an uncontrolled, nearly vertical spiral dive, impacting the steep terrain with the nose of the fuselage. Both occupants sustained fatal injuries upon impact.
The investigation
Investigators from the SUST examined flight data from both the LX9000 flight data recorder and the PowerFlarm Core system. The analysis confirmed that the aircraft's altitude was rapidly decreasing in the moments before impact. The investigation also reviewed the aircraft's configuration, noting that the engine had been used for approximately 46 minutes during the flight and that the aircraft was not carrying water ballast in the wing tanks. Medical examinations and toxicology reports confirmed that both pilots were sober and had no medical conditions that could have contributed to the accident.
Findings
- The primary cause of the accident was a loss of control in flight resulting from the aircraft exceeding its critical angle of attack.
- A contributing factor was the potentially aft center of gravity position, which may have influenced the aircraft's handling during the maneuver.
- The aircraft entered a steep spiral dive from which recovery was impossible due to the low altitude and terrain proximity.