Incident de largage de remorqueur Pilatus B4 à l'aérodrome de Buttwil entraîne un atterrissage forcé

Le 22 mars 1972, un Pilatus B4 a subi un largage inattendu du remorqueur peu après le décollage à l'aérodrome de Buttwil en Suisse. Le planeur, piloté par un pilote de planeur licencié avec une expérience de vol récente limitée, a grimpé abruptement à environ 50 mètres avant de descendre à un angle raide et de s'écraser. Le pilote a subi des blessures graves et le planeur a été détruit. L'enquête a noté que le câble de remorquage s'est probablement largué automatiquement en raison d'un mou, conduisant à l'incident. Les conditions météorologiques étaient favorables au moment de l'accident.

1. Aerotow takeoff roll: The Pilatus B-4 glider began an aerotow launch behind a Piper tug from grass runway 34 at Buttwil, with the initial ground roll and liftoff reported as normal.
2. Pilot recency and type: The pilot had not flown for the previous 90 days, had never flown from Buttwil, and this was his first flight on the Pilatus B-4 without a formal type-specific checkout.
3. No rope-break training: During his glider training the pilot had not practiced rope-break or cable-break exercises, leaving him untrained for an unexpected low-altitude tow release.
4. Automatic tow release: At about 10–15 m above ground, after strong rolling/pendulum motions of the glider, the tow rope automatically released from the B-4, likely due to excessive slack activating the release mechanism.
5. Steep climb after release: Following the unexpected tow release, the pilot pulled the glider up into a steep climb instead of maintaining speed and preparing for an immediate landing.
6. Low-altitude stall: The steep climb at low height led to the glider stalling or ‘abschmieren’ at about 40–50 m above ground and pitching nose-down in a steep descent toward the ground.
7. Crash - serious injury: The glider descended at a steep angle and crashed into soft, slightly swampy terrain about 90 m west of the north end of runway 34, seriously injuring the pilot and destroying the aircraft.