Pilatus B4 Schleppkupplungszwischenfall auf dem Flugplatz Buttwil führt zu Bruchlandung
Am 22. März 1972 kam es bei einem Pilatus B4 kurz nach dem Start auf dem Flugplatz Buttwil in der Schweiz zu einem unerwarteten Ausklinken der Schleppkupplung. Das Segelflugzeug, gesteuert von einem lizenzierten Segelflugpiloten mit begrenzter aktueller Flugerfahrung, stieg steil auf etwa 50 Meter, bevor es in einem steilen Winkel abstieg und abstürzte. Der Pilot erlitt schwere Verletzungen, und das Segelflugzeug wurde zerstört. Die Untersuchung stellte fest, dass das Schleppseil wahrscheinlich aufgrund von Durchhang automatisch ausklinkte, was zu dem Zwischenfall führte. Die Wetterbedingungen waren zum Zeitpunkt des Unfalls günstig.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.