What happened
On May 4, 1997, during a competition flight organized by the German Dragon Flying League, a Bautek Twister hang glider entered an uncontrolled flight attitude, resulting in the structural disintegration of the aircraft. The pilot was flying a scheduled task from Kandel toward Heimeck, via Oberprechtal.
After gaining altitude over the Tafelbühl ridge, the pilot flew north, crossing the valley east of Yach. In an attempt to reach the more favorable wind conditions on the western flank of the Hohestein ridge, the pilot crossed the crest. An observer flying at a higher altitude witnessed the hang glider pitching around its transverse axis, reaching a longitudinal pitch of more than 90 degrees. Shortly thereafter, at an altitude of approximately 25 meters, the airframe broke apart, and the pilot struck the forested hillside, sustaining fatal injuries.
The investigation
The BFU investigation utilized GPS data, witness statements, and on-site wreckage analysis to reconstruct the flight path. Technical examinations of the Bautek Twender revealed that the seam of the forward attachment strap of the keel slider had separated, and the fitting of the left spreader bar had torn away, while the right fitting showed signs of tearing.
Laboratory testing at the University of Stuttgart focused on the construction of the keel slider. Investigators found that the webbing was secured with box stitching alongside a stiff plastic film intended to improve gliding performance. This stiff material created a risk of fraying the stitching during movement. Comparative load tests demonstrated that the manufacturer's box stitch failed at approximately 14 kN, whereas a zigzag stitch remained intact up to 36 kN.
Furthermore, the aluminum spreader bar fittings were found to be prone to cracking due to undersized fillet radii. The investigation also identified a malfunction in the reserve parachute deployment mechanism, where the core cord would bunch up, preventing activation.
Findings
- The primary cause of the accident was the pilot flying into turbulent air on the windward side of the ridge.
- The structural failure of the aircraft was caused by aerodynamic overstressing.
- The weather conditions on the day of the accident were described by other competitors as critical.
- The low altitude at the time of the breakup likely rendered the accident unsurvivable, even if the reserve parachute had been functional.