What happened
On August 25, 1989, during the "Zürich im Höhenflug" gliding festival at the Allmend Brunau airfield in Zurich, a tragic accident occurred involving the two-seat sailplane HB-571, a Rhönlerche II. The event utilized a dual-winch launch system to tow two gliders sequentially to an altitude of approximately 200–250 meters. The pilot of HB-571 was scheduled for the first launch.
At 10:33 local time, the pilot took off with a passenger in the rear seat. Following the standard procedure, he climbed and turned westward over the Sihl river to begin his circuit. Shortly after, a second glider launched behind him using the secondary tow cable. Before this second cable had fully retracted to the ground, the pilot of HB-571 initiated his landing turn. Instead of completing the prescribed full circuit, he cut across the approach path directly in front of the winch vehicle. During this maneuver, the right wingtip of HB-571 collided with the still-descending tow cable of the second glider. The wire acted like a saw, slicing through the outer section of the right wing. The aircraft lost structural integrity and control, entering an uncontrolled spin before crashing nose-first from an altitude of approximately 30 meters.
The investigation
The Swiss Transportation Safety Investigation Board (STSB) examined the wreckage and the scene to reconstruct the sequence of events. Physical evidence confirmed that the right wing had been severed by the tow wire of the second aircraft. Forensic analysis of the cable showed paint and wood debris consistent with the HB-571's construction, indicating the wire had passed through the wing structure near the parachute deployment point.
The investigation reviewed the pilot's qualifications, noting he held a valid license with passenger flight endorsements and had over 750 hours of gliding experience. He was in good health and had completed required check flights that day. Weather conditions were clear with light winds. The inquiry focused on why the pilot deviated from the standard circuit pattern and failed to notice the descending cable.
Findings
The primary cause of the accident was the pilot's failure to adhere to the prescribed landing circuit, which led to a collision with the tow wire of the following aircraft. By cutting the turn short, the glider entered the danger zone around the winch while the second cable was still airborne. Contributing factors likely included light downdrafts on the northeast side of the Uetliberg mountain, which may have forced the pilot to shorten his circuit unexpectedly. The pilot overlooked the descending wire due to this distraction and spatial disorientation.
Safety action
The investigation highlighted the risks associated with dual-winch operations and tight airfield environments. While no specific new regulations were issued immediately in the report, the incident underscored the necessity for strict adherence to circuit patterns and enhanced situational awareness regarding other aircraft's tow lines during high-density gliding events.