Introduction
On 6 August 1988, a SZD-51-1 "Junior" glider, registration HB-1928, was involved in an accident during a winch launch attempt at the mountain landing area of Croix-de-Coeur, in the Swiss canton of Valais. The accident occurred at approximately 1600 local time (UTC+2). The glider was seriously damaged, and the pilot sustained serious injuries.
Accident Sequence
The launch was part of a test to assess the feasibility of winch-launching gliders from the Croix-de-Coeur site. The winch system, of French design and originally intended for slope gliders, was installed earlier that day. The pilot, a Swiss citizen born in 1938, held a valid glider pilot license and had logged 605 total flight hours, with only 18 minutes on the accident type, all accrued earlier that day while flying the glider from Sion to Croix-de-Coeur.
After the winch operator expressed reservations due to a tailwind estimated at 20 km/h, the project promoters decided to proceed. The glider was placed on the launch line with two wingtip helpers. At about 1630, the pilot gave the start signal via radio. The glider, towed from its central hook, accelerated normally while hopping slightly on its main wheel and drifting left. Upon passing a ridge marking a change in slope gradient, the glider abruptly yawed about 20° to the right. Believing takeoff was imminent, the pilot did not release the cable. However, the glider did not reach takeoff speed due to increasingly rough terrain. The pilot, subjected to severe vibrations, could not activate the release and lost control. The glider struck terrain irregularities, causing the cable to break, and came to rest to the right of the runway in a grassy area.
Established Facts
- The pilot held a valid glider pilot license with limited experience on the accident type (18 minutes).
- The glider had valid certificates of airworthiness and registration, with weight and center of gravity within limits.
- The winch installation comprised a Jeep Cherokee Chief 17, a winch drum, a dynamometer with electronic tension display and audible alert, a pulley mounted on a tripod 13 m to the right of the runway axis at 165 m from the launch point, and an emergency cable cutter.
- The Croix-de-Coeur landing area is at 2160 m altitude, with a T-shaped maneuvering area oriented SW uphill. The natural runway was about 200 m long, with a gentle slope on the first half and a steeper slope on the second. It was partially deteriorated due to recent storms, with only the first 100 m considered usable for the test.
- The main wheel track showed a nearly straight trajectory slightly left of the axis, then a sharp right turn of 20–25° after 95 m.
- Weather: anticyclone centered over SE Germany; clear skies, visibility >20 km, temperature 17°C. Wind from SSW at 20 km/h, as estimated by the winch operator and uncontested by the pilot.
- In prior tests at Sion with a Ka 8 glider and the same pilot, the glider had taken off after about 80 m with a headwind of approximately 10 km/h.
Analysis
The winch system, though designed for slope gliders, had proven capable of launching heavier gliders in the Sion tests. At Croix-de-Coeur, the combination of higher density altitude, tailwind, and degraded runway prevented the glider from attaining takeoff speed within the usable runway length. The sudden yaw at the critical point was attributed to the pulley location to the right of the runway axis. During early acceleration, the pilot could maintain direction using the tailwheel and rudder, but at the ridge, the traction line dictated by the pulley forced a heading change, exacerbated by the cable attachment point being left and slightly ahead of the main wheel. The glider's flight manual explicitly states that during winch launches using the central hook, the tow cable must not pass to the right of the roll axis.
Cause
The accident was due to the pilot losing yaw control during the ground acceleration phase of a winch launch attempt, resulting from the combined effect of a high tailwind component, a partially deteriorated runway, and inadequate positioning of the winch cable pulley relative to the glider's center-of-gravity hook.