What happened
On August 5, 1977, at approximately 10:40 AM, a Piper PA 18-150, registration HB-OMK, departed Bad Ragaz airfield to perform a glider tow operation. The aircraft was towing a Pilatus B 4 HB-1257 glider. The flight proceeded normally until the aircraft reached the Vilan region near the Sadreinegg ridge.
During the maneuver, the tow pilot executed a sharp right turn toward the slope, followed immediately by an abrupt transition into a sharp left turn. During these maneuvers, the glider pilot lost visual contact with the tow plane. The glider ended up in an elevated position relative to the tow aircraft, causing the tow rope to slacken. As the rope suddenly tensioned, the force caused the tail of the Piper PA 18-150 to lift abruptly, leading to a sudden decrease in the angle of attack and a subsequent nose-down pitch. The aircraft entered a steep dive and struck a steep slope at 10:55 AM. The pilot of the tow aircraft was killed, and the aircraft was destroyed. The glider pilot was able to return to the airfield safely.
The investigation
The investigation examined the flight path, the mechanical condition of the aircraft and tow rope, and the actions of both pilots. Investigators noted that the tow pilot had a history of flying at altitudes and distances from the slope that were below legal minimums, which had been previously noted by glider pilots.
Technical analysis of the 43-meter nylon tow rope revealed that the stitching on the glider side had failed. It was discovered that the thread used for the stitching was likely cotton rather than nylon, creating mismatched elasticity. Furthermore, the breaking strength of the rope's attachment points was found to be significantly higher than the required specifications, meaning the safety release points were over-dimensioned.
Findings
- The primary cause of the accident was the uncontrolled flight attitude of the tow aircraft following an abrupt change in direction in close proximity to the terrain.
- The glider pilot's failure to release the tow rope immediately upon losing visual contact with the tow plane contributed to the accident.
- The sudden tensioning of the slack rope exerted extreme force, which triggered the mechanical failure of the rope stitching and the subsequent pitch-down of the tow aircraft.
- The tow pilot was flying at an altitude and distance from the slope that were below the established safety minimums.
- The tow rope's safety release points were not functioning according to the required breaking strength specifications.