What happened
On March 22, 1972, a pilot was operating a Pilatus B-4 glider, registration HB-1050, at the Buttwil aerodrome. The pilot had received permission from Pilatus Flugzeugwerke AG to fly the demonstration aircraft. Following a pre-flight inspection and the manual attachment of the tow rope, the glider was prepared for a towed takeoff using a Piper aircraft, registration HB-OIR.
Shortly after liftoff, at an altitude of approximately 15 meters, the glider experienced a sudden roll and the tow rope became disconnected. Following the release, the glider climbed steeply to an altitude of roughly 50 meters before banking left and descending at a steep angle toward a wooded area. The aircraft struck the ground in marshy terrain approximately 90 meters west of the runway end. The pilot sustained severe injuries, and the B-4 glider was destroyed.
The investigation
Investigators examined the aircraft, the tow rope, and the circumstances of the takeoff. The investigation confirmed that the HB-1050 was airworthy and that its weight and center of gravity were within limits. No mechanical defects were found in the glider's structure. The investigation also noted that the pilot had not received a formal type rating for the B-4 as required by flight training guidelines, and it was his first time operating this specific model at this airfield.
Findings
- The glider experienced significant oscillations around its longitudinal axis during the initial climb.
- These oscillations likely caused a large amount of rope sag.
- The automatic release mechanism of the tow hook likely triggered due to this rope sag, rather than manual intervention by the pilot.
- The pilot's sudden pull-up maneuver following the unexpected release caused the aircraft to drop below its minimum flying speed.
- The pilot failed to execute an emergency landing in the nearby Reussevene, which was a viable option given the glider's glide ratio.
Safety action
No specific safety recommendations were recorded in the final report.