What happened
On the afternoon of August 26, 1973, a student pilot initiated a flight from the Olten airfield in a Ka 8B glider, HB-668. The glider was being towed by a Robin DR 400/180R, registration HB-EUY. The flight was intended to be the student's third solo flight as part of his licensing requirements.
During the climb, the tow pilot signaled for the glider to release from the rope using aileron inputs. However, the student pilot failed to release. As the tow aircraft entered a left-hand turn and encountered patches of fog, the tow pilot reduced power to maintain level flight. This maneuver caused the tow rope to slacken significantly. The glider subsequently drifted below the level of the tow aircraft, causing the rope to loop over the glider's wings and tail.
The resulting entanglement caused the glider to become uncontrollable. Witnesses observed the aircraft performing erratic maneuvers, including a nose-down pitch and a spiral descent. The glider struck the southern slope of the Born mountain at approximately 500 meters altitude. The pilot attempted to bail out, but the altitude was too low for the parachute to deploy fully before impact.
The investigation
An investigation conducted by the Swiss authorities, in collaboration with the Solothurn Cantonal Police, examined the wreckage and flight data. Analysis of the barograph confirmed that the tow lasted approximately 2.4 minutes and that the aircraft entered a rapid descent following the rope breakage.
Physical examination of the Ka 8B revealed that the left aileron had been damaged or blocked, likely by the rope, and parts of the right horizontal stabilizer had been torn away. The investigation found no evidence of mechanical failure in the aircraft's controls or the release mechanism. The tow rope's weak link had also broken during the incident.
Findings
- The primary cause of the accident was the entanglement of the glider in the tow rope, which rendered the aircraft uncontrollable.
- The student pilot's limited flight experience contributed to the inability to react correctly to the tow pilot's maneuvers.
- The tow pilot's maneuvers, specifically the reduction in power and the turn, created the slack necessary for the rope to loop over the glider.
- Adverse weather conditions, including poor visibility due to fog and moderate turbulence, played a role in the sequence of events.
- The pilot's decision to exit the aircraft occurred at an insufficient altitude to allow for the successful deployment of the parachute.