What happened
On July 6, 2014, a Klemm KL 25 historical aircraft was conducting a series of training flights at the Großenhain airfield (EDAK) in Germany. The day's activities involved several pilots performing solo and instructional flights. During the fourth flight of the day, the pilot, seated in the rear cockpit, was accompanied by one passenger.
After departing from the grass runway 11, the aircraft performed a series of maneuvers, including a turn to the east and several full circles and figure-eels at an estimated altitude of 50 meters. While crossing the concrete runway 11 on a northern heading, the pilot entered a left-hand turn. During this maneuver, the aircraft's bank angle increased significantly, and the nose began to drop. The aircraft subsequently stalled from an altitude of approximately 30 meters, with the left wing striking the ground.
The investigation
The BFU investigation examined the aircraft, the flight parameters, and the environmental conditions. The aircraft, a wooden-construction low-wing monoplane, was found to have sustained heavy damage. The engine was torn from its mounts, and the fuselage and wings suffered structural failures, including broken wooden joints and broken plywood skinning. No technical defects were identified in the aircraft's systems or engine.
Investigators analyzed video footage recorded by the passenger on a mobile phone. This footage showed the airspeed indicator in the front cockpit reading approximately 47 mph (75 km/h) just one second before the stall. The investigation also considered the aerodynamic effects of the aircraft's design, noting that for aircraft with large wingspans and low flight speeds, a significant difference in airflow velocity can occur between the inner and outer wingtips during turns.
Findings
- The pilot and passenger sustained two minor injuries (cuts) and were treated at a hospital.
- The primary cause of the accident was an aerodynamic stall during a steep left-hand turn.
- The aircraft was flying at a low airspeed of approximately 75 km/h, which, combined with the increasing bank angle, led to the loss of lift on the inner wing.
- The aircraft sustained heavy damage, and the engine was displaced from its mounting.