General Information
The accident involved a Piper PA-18-150, registration HB-OZE, owned and operated by Segelfluggruppe Säntis of St. Gallen, Switzerland. The aircraft was powered by a Lycoming O-320-A2B engine. The pilot, a Swiss citizen born in 1953, was conducting a glider towing flight. The glider was an ASW 15, registration HB-1029, piloted by Pilot Y.
Accident Sequence
On 30 April 1978, at 16:26 GMT, the towplane departed from Altenrhein airfield on grass runway 10. The flight was the 32nd of the day for the pilot, who had accumulated 3 hours 54 minutes of flight time. The glider pilot was continuing his type familiarization on the ASW 15, which used a center-of-gravity tow hook for normal towing. The towrope, made of Perlon, was approximately 50 meters long and was equipped with weak links: 850 ± 50 kp at the towplane end and 600 ± 30 kp at the glider end. However, the weak link values exceeded those specified in the Swiss Federal Air Office directives and the flight manual.
The initial takeoff and climb were normal. During a left turn to crosswind or downwind leg at an altitude of about 150 meters, a significant height difference developed between the two aircraft. The glider, flying higher, raised the tail of the towplane, causing it to pitch into a dive. The towplane pilot was unable to recover. The glider pilot attempted to release the towrope, but the rope broke before he could do so, at the glider-side weak link. The towplane impacted the ground in a left-banked, nose-down attitude, with a longitudinal inclination between 30 and 50 degrees. The impact was partially cushioned by dense young forest, but a fire immediately broke out, resulting in the pilot's death. The glider pilot returned to Altenrhein and landed safely.
Investigation Findings
The investigation determined that the accident occurred because the glider flew higher than the towplane. A contributing factor was the use of the center-of-gravity tow hook, combined with the glider pilot's low experience on the ASW 15 type. The towrope's weak links were found to have breaking loads higher than recommended, which may have affected the sequence of events.