What happened
On July 22, 1976, a flight instructor was performing a glider tow operation to return two aircraft from a flight camp in Saanen to La Chaux-de-Fonds. The flight involved a Piper PA-18-150 acting as the tug and an SZD-30 "Pirat" glider, registration HB-120 and 09.
While flying near Le Crét, the instructor initiated a 270-degree right turn to assess changing weather conditions near Le Locle. During this maneuver, the aircraft encountered a lowering cloud ceiling and rain. As the tug aircraft descended to stay below the clouds, the glider pilot, flying at a higher altitude, lost visual contact with the towplane and the towline. In an attempt to correct the flight path, the glider pilot applied a significant correction that caused the aircraft to surge, resulting in a sudden tension spike. This caused the towline to snap, leading to a loss of control in the glider. The SZD-30 "Pirat" subsequently struck trees and crashed into the ground. The pilot of the glider survived the impact without injury, as the wings were torn off upon hitting the trees, absorbing much of the impact energy.
The investigation
SUST examined the flight conditions, the mechanical state of the equipment, and the pilots' preparations. The investigation established that the weather was highly unstable, with visibility dropping significantly due to rain and fog.
Technicians tested the towline and found it was in poor condition. While a new line of this type should withstand between 2600 and 3200 kp, the tested line failed at only 1050 kp. Furthermore, a knot in the line acted as a major weak point, failing at 770 kp. This indicated the line's strength had degraded to roughly 22% to 27% of its original capacity, failing to meet the minimum required standards. Conversely, the investigation found that the release fuses on the aircraft were oversized, exceeding the maximum allowable limits set by the authorities.
Findings
- The primary cause of the accident was the continuation of a VFR flight in unfavorable meteorological conditions.
- The towline was heavily worn and significantly below the required safety strength.
- Flight preparation was insufficient, as the pilots did not adequately account for the rapidly changing weather.
- The glider pilot lost visual reference to the towplane and the rope, leading to an aggressive corrective maneuver that broke the weakened line.