What happened
On July 14, 1978, at approximately 18:29 CEST, a Piper PA 18 (registration HB-OVZ) was performing a glider tow operation at Grenchen Airport. The pilot had completed 38 successful towing maneuvers earlier that day, involving a K 13 glider. Following the release of the tow rope, the aircraft began its turn from the downwind leg toward the final approach for runway 07.
During the turn, the aircraft's airspeed dropped below the required minimum. The aircraft entered an uncontrolled state, characterized by witnesses as a developing spin or spiral. The aircraft's right wing struck the ground first, followed by a nose-first impact that left the plane resting in a vertical position. The pilot sustained serious injuries during the impact, while the aircraft was destroyed.
The investigation
An investigation by the Swiss Transportation Safety Investigation Board (SUST) examined the aircraft, the pilot's records, and various eyewitness accounts. The investigation confirmed that the pilot held a valid private pilot license and that no underlying health issues were identified as contributing to the accident. The aircraft was properly maintained, with its most recent 100-hour inspection completed in June 1978, and its weight and balance were within permissible limits.
Witnesses at the airfield observed the aircraft performing a turn at an altitude of approximately 50 to 70 meters. Some observers noted the aircraft began to bank steeply and enter a right-hand spiral. The investigation also looked into the pilot's workload, noting that this was the 39th towing mission of the day.
Findings
- The primary cause of the accident was the undershooting of the minimum flying speed during the final approach turn.
- The investigation could not definitively determine why the airspeed dropped so significantly.
- The pilot's high workload, having performed 39 towing operations in a single day, may have led to fatigue, potentially impairing situational awareness or flight control precision.