What happened
On June 5, 2009, at 11:55 AM, a Czech Zenair CH 601 UL ultralight aircraft, identified as 68ADW, was performing a flight for travel purposes near the Tournus aerodrome in France. The pilot was attempting to land on runway 01, having previously requested landing permission from the DGAC, which had been denied due to the presence of trees in the visual approach paths. Despite the lack of authorization, the pilot proceeded with the landing plan to meet a contact residing near the airfield.
As the aircraft was on final approach, a ground witness observed the aircraft enter a sudden, asymmetric stall to the right. This led to a spin that caused the aircraft to impact the ground behind a line of trees, approximately 570 meters from the threshold of runway 01. The impact resulted in the death of the pilot and the total destruction of the aircraft.
The investigation
The investigation focused on the configuration of the aircraft's flight surfaces and the pilot's approach. The aircraft was equipped with electric "underside flaps" (volets d'intrados) located on the lower surface of the wing. Investigators examined the wreckage and determined that at the moment of impact, these surfaces were extended approximately 20 degrees on both sides of the wing.
Furthermore, the investigation looked into the manufacturer's involvement. While the aircraft was marketed as a Zenair model, the manufacturer stated they had never produced a CH 601 UL equipped with these specific flaps. It was determined that these modifications were likely performed by Czech Aircraft Works, a former licensee, without the involvement of the original designer. Additionally, it was found that owners of similar modified aircraft did not possess a specific flight manual detailing the unique aerodynamic characteristics of this flap system.
Findings
- The primary cause of the accident was the use of underside flaps at an insufficient airspeed to maintain necessary lift.
- The deployment of these surfaces, which function more like air brakes than high-lift devices, can cause a sudden increase in the sink rate and a potential loss of aileron effectiveness, especially in turbulent conditions.
- The pilot was operating without official landing authorization for the restricted-use aerodrome.
- There was a lack of specific flight documentation regarding the performance characteristics of the modified flap system.