What happened
On August 23, 2011, a Stemme S 10-VT, registration HB-2403, departed from the Speck-Fehraltorf airfield for a planned flight through the Alps. Shortly after takeoff from runway 12, the pilot and passenger noticed that the aircraft was unable to gain altitude despite the engine operating at full power.
In an attempt to avoid flying low over the nearby village of Pfäffikon, the pilot initiated a gentle left turn to follow the standard traffic pattern. However, the aircraft's climb performance was insufficient to clear obstacles. The right wing struck trees, causing the aircraft to rotate and crash into a meadow. The impact destroyed the aircraft, specifically shattering the nose, breaking the tail, and tearing off the canopy. Both occupants sustained serious injuries.
The investigation
Investigators examined the engine, the aircraft's flight data, and the mechanical state of the flight controls. The engine was tested on a test bench and found to be functioning correctly, with the turbocharger providing the expected boost pressure.
Technical analysis revealed that the spoilers (air brakes) had deployed during the initial climb. While the pilot had followed the checklist and retracted the spoilers, the mechanism had not been fully locked into the retracted position. Due to the aerodynamic suction created by the airflow over the wing, the unlatched spoilers were pulled into the extended position. Additionally, the investigation found that the landing gear remained extended throughout the flight, and the aircraft's takeoff weight was approximately 5% above the maximum allowable limit.
Findings
- The primary cause of the collision was insufficient climb performance resulting from the unexpected deployment of the spoilers.
- The spoilers deployed because they were not properly locked in the retracted position after takeoff.
- The aircraft's takeoff mass exceeded the maximum allowable limit by approximately 5%.
- The extended landing gear contributed to increased aerodynamic drag, further reducing climb capability.
- The pilot's recent training experience on this specific aircraft type was low, which may have hindered the recognition of the configuration error.