What happened
On July 31, 2003, a Cosmos Bidulm ultralight, registration 59-FQ, was conducting a local discovery flight at the Vitry-en-Artois airfield. The flight was part of a community program providing free introductory flights for local children. Approximately ten minutes into the flight, while the pilot was performing maneuvers, the wing separated from the carriage in mid-air. The aircraft plummeted to the ground, resulting in two fatalities (the pilot and one passenger) and the total destruction of the aircraft.
The investigation
The investigation focused on the structural integrity of the aircraft and the sequence of the failure. Investigators examined the wreckage and found that the wing and the carriage had separated and landed 500 meters apart. The carriage structure, a triangular frame, had broken at three points.
Crucially, the investigation identified that the "torpedo" (a structural component connecting the front of the longitudinal spar to the mast) had failed due to buckling under compression. This failure was driven by a negative load factor and a violent projection of the trapeze bar against the torpedo during a pitch-up maneuver.
Investigators also discovered that the pilot had recently replaced the original anodized aluminum torpedo with a different, non-anodized version purchased just days before the accident. While the new component had a slightly higher breaking strength, its material specifications differed from the manufacturer's original design. Witnesses at the airfield reported seeing the aircraft performing numerous climbs, descents, and loops, and one passenger noted that the pilot had previously released the controls during a flight, causing the aircraft to enter a dive.
Findings
- The primary cause of the accident was the structural failure of the torpedo caused by excessive stress.
- This stress was generated by a combination of a negative load factor and the extreme movement of the trapeze bar during aggressive maneuvering.
- The pilot was performing highly demonstrative and abrupt maneuvers, including loops and tight turns, which induced a "tumbling" effect.
- The structural failure of the torpedo led to a chain reaction: the longitudinal spar lost its forward support, causing the mast to break and the engine block to be torn from its mounts.