What happened
On April 9, 1980, at approximately 15:57 local time, a student pilot began a solo training flight at Grenchen Airport. The flight was being conducted via aero-tow, with a Piper Super Cub PA-18 (registration HB-PDL) pulling an ASK-13 glider (registration HB-926).
Shortly after takeoff, at an altitude of roughly 80 meters, the tow cable unexpectedly detached from the towing aircraft. Following the release, the glider entered a shallow left turn that progressively steepened into a dive. The aircraft struck the ground at approximately 15:59 with a significant nose-down pitch. The student pilot sustained fatal injuries and passed away later that day. The glider was destroyed in the impact.
The investigation
Investigators examined the mechanical state of both aircraft and the sequence of events leading to the loss of control. The investigation confirmed that the ASK-13 was within its weight limits, though its center of gravity was positioned toward the rear of the allowable range. The towing aircraft, HB-PDelung, was also found to be in a valid state of airworthiness.
Technical examination of the release mechanism on the HB-PDL revealed that the release handle was heavily soiled with dirt. While the release mechanism itself functioned correctly when tested, investigators noted that the handle had not fully locked past the dead point during the initial connection. This likely allowed vibrations during flight to cause the cable to slip from the hook.
Findings
- The primary cause of the accident was the unintended release of the tow cable from the towing aircraft.
- The heavy accumulation of dirt on the release mechanism of the HB-PDL may have prevented a secure, full engagement of the hook.
- The student pilot's sudden reaction to the unexpected situation led to an increasingly steep left turn and subsequent loss of control.
- The glider's center of gravity was located in the rear portion of the permitted range.