What happened
On June 5, 2010, a pilot and a passenger were conducting a sightseeing flight from the Kamen-Heeren airfield in a Flight Design CTSW. While flying north near Drensteinfurt, the pilot initiated a 180-degree descending turn. During this maneuver, the aircraft's speed increased to approximately 313 km/h, significantly exceeding the maximum permissible speed (VNE).
As the aircraft accelerated, the pilot heard a loud bang and experienced elevator flutter, which caused the aircraft to pitch up twice. Despite the structural damage, the pilot was able to maintain control and decided to return to the departure airfield. The flight continued to Kamen-Heeren without further incident, though the landing was performed at an increased approach speed.
The investigation
The BFU investigation focused on the structural failure of the horizontal stabilizer and the circumstances leading to the overspeed. Investigators examined the aircraft's maintenance history and found that a required technical bulletin regarding the parachute recovery system had not been implemented. Additionally, required placards regarding weight limits and prohibited maneuvers were missing from the cockpit.
Post-accident inspections revealed that the right elevator had broken approximately 30 cm from the vertical stabilizer. The investigation also identified previous repairs on the leading edge of the horizontal stabilizer and a crack in the left main landing gear attachment.
Following the accident, the manufacturer conducted ground vibration tests. While these tests initially showed similar frequency behavior across different elevator variants, a subsequent workshop flight revealed that trim tab flutter could still occur at speeds of approximately 275 km/h.
Findings
- The primary cause of the structural damage was the exceedance of the maximum permissible speed (VNE) during a descending turn.
- The high-speed maneuver induced aerodynamic flutter, resulting in the breakage of the elevator.
- The aircraft was operating at a speed of 313 km/h, well beyond its design limits.
Safety action
Following the investigation, the German Aero Club (DAeC) issued airworthiness directives to mitigate the risk of similar occurrences. These measures included:
- Restricting the maximum permissible speed (VNE) to 225 km/h.
- Requiring the installation of a warning placard next to the airspeed indicator.
- Implementing trim tab modifications as specified in manufacturer service instructions.