Incident Overview
On 12 August 1967, a Rhönlerche II glider (registration HB-631) departed Saanen, Switzerland, under tow by a de Havilland Chipmunk (HB-TUC). The intended destination was Spreitenbach. Approximately 2.5 km south-southeast of Boltigen, at an altitude of about 1600 m, the glider separated from the tow rope while in level flight. A witness on the ground observed the glider's right wingtip flutter audibly and visibly at the moment of separation.
Sequence of Events
Immediately after separation, the glider entered a steep dive, gaining speed and moving ahead of the tow plane. The flutter at the wingtips intensified. About half a minute after separation, the glider's canopy opened with a loud noise, the plexiglass shattered and fell away, and the steel-tube frame pivoted backward. Shortly thereafter, two pieces of the right aileron detached, one trailing a long streamer of fabric. The glider then turned right towards the slope and, in a nose-down attitude of approximately 40-45°, struck the ground at an elevation of about 1340 m on a pasture called "Fäng" near Boltigen. The aircraft was destroyed. The pilot, born 1915, was ejected from the open cockpit; he died in the accident. He had apparently prepared to bail out but did not leave the aircraft.
Aircraft and Pilot Information
The glider was a Rhönlerche II, a two-seat, strut-braced high-wing mixed-construction design (wooden wings, steel-tube fuselage) manufactured by Schleicher in 1959 (serial number 562). Its empty weight was 217 kg, maximum takeoff weight 400 kg, and weight at the time of the accident approximately 307 kg. The center of gravity was within limits. The ailerons were cable-actuated and had no mass balance. The pilot held a glider pilot license issued 5 June 1959, with a passenger endorsement, and had 182.5 flight hours in 309 flights. The tow pilot held a private pilot license and a glider pilot license, with 102 hours of powered flight and about 70 hours of gliding.
Weather Conditions
At the time, the synoptic situation featured a trough over western Europe with a cold front approaching. Over the Simmen valley, cloud cover was 6-7/8 oktas, with cumulus bases at 2000-2500 m. Winds were southerly at 10 kt at 800 m, southwesterly at 15 kt at 1600 m, and southwesterly at 25 kt at 2500 m. Light thermal turbulence was present over the valley, with possible moderate to strong shear turbulence near the Jaunpass. No precipitation, visibility 20 km.
Investigation and Cause
The investigation was conducted by the Swiss Federal Aircraft Accident Investigation Commission. The probable cause, as stated in the final report, was that the glider pilot lost control of the aircraft due to wing/aileron flutter, which ultimately led to the fracture of the right aileron, resulting in collision with terrain.