What happened
On July 22, 2021, during the Swiss Junior and West Swiss Gliding Championships at Bex airfield (LSGB), a Robin DR 400/180 R, registration HB-KAW, was performing glider towing operations. After successfully releasing a single-seat glider near Col de Bretaye, the pilot began a descent back toward Bex. While attempting to retract the 54-meter retractable tow cable using the automatic winch system, the pilot noticed via an analog instrument that the cable was not being reeled in. Although the pilot reported this issue via radio, the cable remained largely extended behind the aircraft.
As the aircraft approached runway 33, the trailing cable struck the wingtip of a glider parked at the edge of the runway. The impact caused the glider to rotate approximately 100 degrees on its vertical axis and resulted in the separation of its left winglet. A second glider positioned nearby sustained minor damage. The collision also caused the tow cable to snap and resulted in minor injuries to a person standing near the aircraft.
The investigation
The investigation examined the flight path and the mechanics of the trailing cable. It was determined that the pilot was unaware of the specific danger posed by the extended cable during the final approach. Because the aircraft was flying at a low altitude—approximately 10 meters above the ground—the trailing end of the 54-meter cable was significantly lower than the aircraft's flight path.
Investigators noted that at Bex, a safe approach that clears obstacles like parked gliders would require an aiming point much further down the runway, potentially hundreds of meters beyond the runway end, which is not feasible for standard landing procedures at this location. The investigation also looked into the functionality of the winch system and the pilot's awareness of the cable's position, noting that the cable was not visible in the cockpit's rearview mirror.
Findings
- The primary cause of the collision was the trailing tow cable remaining extended during the landing approach.
- The pilot was not aware of the immediate hazard the trailing cable posed to ground objects during the final descent.
- The aircraft's low altitude during the approach made it mathematically difficult to clear the parked gliders without the cable striking them.
- The pilot's cockpit instruments indicated the cable was not retracting, but the physical position of the cable was not visible via the cockpit mirror.