What happened
On July 4, 2012, a Sunwheel ultralight aircraft departed from Dingolfing airfield for a local flight. The pilot and one passenger were on board. Shortly after takeoff, once the aircraft had reached an altitude of approximately 40 to 50 meters, witnesses observed the aircraft entering a high angle of attack and subsequently entering a descent. The aircraft entered a right-hand turn and stalled, impacting the ground vertically with a high longitudinal pitch. The impact caused the aircraft to catch fire, and both occupants sustained fatal injuries.
The investigation
The BFU investigation examined the aircraft, the engine, and the propeller. The engine, a Rotax-912, was disassembled by the manufacturer, and no mechanical failures were found that could have caused an engine outage. However, the investigation of the newly installed composite propeller revealed a critical discrepancy: the pitch of the two blades was set incorrectly. One blade had a pitch of +10.7 degrees, while the other was set to -11.2 degrees. Field tests confirmed that such an unbalanced setting caused extreme vibrations.
Additionally, the investigation found that the aircraft had undergone several unauthorized modifications, including the installation of an extra fuel tank behind the pilot's seat and the use of a non-approved propeller. These changes contributed to an estimated takeoff weight of at least 419.8 kg, which exceeded the maximum allowable takeoff mass of 400 kg by at least 19.8 kg.
Findings
- The pilot initiated a turning maneuver at a low altitude while experiencing engine performance issues.
- The aircraft entered an uncontrolled flight state due to an excessive angle of attack and a loss of airspeed during the turn.
- The aircraft was significantly overweight, exceeding its maximum takeoff mass.
- The propeller pitch was improperly adjusted, creating asymmetrical blade performance.
- Unauthorized modifications, including an additional fuel tank, altered the aircraft's center of gravity and weight limits.