What happened
On September 13, 2006, a factory-new Flight Design CTSW 2006 ultralight aircraft was conducting a technical acceptance flight near Eggersdorf, Germany. The flight was intended to verify the aircraft's condition prior to delivery to a new owner. The pilot, an experienced engineer involved in the development of the CT series, was performing maneuvers in the airspace near Jahns and Marxdorf.
During the final phase of the flight, the aircraft was traveling at a high speed of approximately 287 km/h over the ground. Shortly after crossing the B1 highway, the aircraft entered a steep, nose-down attitude, which a witness described as being "upside down." The aircraft then underwent three to four rapid rolls. The aircraft subsequently struck a wooded area west of Jahnsfelde, resulting in the death of the pilot and the destruction of the aircraft.
The investigation
The BFU investigation focused on the mechanical state of the aircraft's trim system and the sequence of aerodynamic events. Investigators examined the wreckage, which showed significant destruction to the cockpit and the left horizontal stabilizer. The investigation also analyzed radar data, which confirmed the aircraft's high speed and the sudden termination of the radar track at a low altitude.
Laboratory tests conducted by the manufacturer, Flight Design GmbH, were critical to the investigation. These tests replicated a phenomenon where the spring within the trim system could become jammed or "blocked." When this tension is suddenly released, it causes a significant change in the trim force, leading to a sudden longitudinal movement of the aircraft.
Findings
The investigation established that the accident was caused by the interplay of a trim system malfunction, an unsecured radio, and low altitude.
It is highly probable that the trim spring suddenly released from a blocked state, causing a sharp pitching motion. This high positive acceleration caused a portable radio, which was inadequately secured on the co-pilot seat, to be thrown through the cockpit roof window. The sudden influx of airflow through the resulting openings in the roof and luggage compartments created massive drag, significantly decelerating the aircraft. This deceleration, combined with the pilot's attempt to recover from the steep dive, led to an aerodynamic stall and a subsequent spin. Due to the low altitude, the pilot was unable to recover before the aircraft impacted the trees.
Safety action
Following the investigation, Flight Design GmbH implemented a "Trim Tab Spring Modification" for the CTSW 2006. This modification introduced a telescopic guide for the spring to prevent it from jamming. This update was applied to all aircraft in production and made available to existing owners by the end of 2006.