Défaillance structurelle du S-18 lors d'un lancement au treuil à l'aérodrome de Bad Ragaz entraîne un décès

Le 1er janvier 1964, un AG Wynau S 18 M a subi une défaillance structurelle en vol lors d'un lancement au treuil à l'aérodrome de Bad Ragaz en Suisse. Le planeur s'est désintégré avant d'atteindre le sommet du lancement, entraînant un crash mortel. Le pilote, un élève avec une expérience limitée de vol en solo, a été tué à l'impact. L'enquête a révélé que la défaillance structurelle était due à des faiblesses à long terme dans la construction de l'appareil qui n'ont pas été correctement corrigées lors de la maintenance. Les conditions météorologiques étaient claires avec des vents variables au moment de l'incident.

1. Winch launch climb: During a training winch launch in calm weather, the S-18 glider climbed normally toward the apex of the launch with the student pilot at the controls.
2. Aged weakened structure: The glider’s rear fuselage structure around frame 8a had long‑term weakening from design discontinuities, moisture, multiple past damage events, and some unsatisfactory repairs that were not fully recognized or corrected in maintenance.
3. Over-strong weak link: The winch cable was fitted with a supposed 540 kg weak link that in fact had a measured breaking load of about 800 kg, allowing higher tow loads to be transmitted to the airframe than intended.
4. Structural failure in climb: Shortly before reaching the launch apex at about 180 m above ground, the rear fuselage between wing and tail deformed and the plywood skin began to crack and shed pieces as the weakened structure failed under winch tow loads.
5. Fuselage breaks apart: The fuselage burst apart with the tail section hinging downward, leading to loss of structural integrity and control as the glider rolled right and pitched into a steep dive.
6. Winch cable released: The winch operator cut the tow cable after seeing the breakup, but the glider was already uncontrollable and continued descending in a steep, rolling attitude.
7. Crash - fatal: The glider impacted the ground in a steep inverted attitude near the airfield, was completely destroyed, and the student pilot was killed on impact.