Arcus M stalls out of a ridge turn and strikes terrain

Both pilots of an Arcus M were fatally injured when the glider stalled out of a ridge turn and struck rising terrain near Chistehorn; aircraft destroyed. On the first day of a cross-country tour from Birrfeld, the crew had been flying figure-eight passes along the Rhône ridge for about twelve minutes. A right turn was not completed in sinking air; the glider passed its critical angle of attack, stalled into a steep spiral, and hit the slope. A possibly aft centre of gravity and unclear sharing of control between the two pilots were noted as factors.

1. Mountain ridge soaring: On day one of a planned two-day cross-country flight, the glider had been ridge-soaring at the Chistehorn for about 12 minutes, flying figure-8 turns away from the slope.
2. Limited mountain hang-flying time: Both pilots had limited mountain ridge-soaring experience on this glider type; the pilot in the back seat had accumulated most of his time on type over flat terrain.
3. Possibly aft CG: The rudder-fin tank held 4 L of water ballast, placing the centre of gravity in the mid-to-aft part of the allowable range, which accentuates departure behaviour in turns.
4. Two-pilot role-sharing unclear: It remained unclear whether the two pilots, who came from different flying backgrounds, had agreed in advance on how decisions and control would be shared.
5. Critical AoA exceeded in turn: A right turn was initiated in sinking air; indicated airspeed dropped 25 km/h in 15 seconds while the glider gained about 10 m in height, exceeding the critical angle of attack.
6. Stall, steep spiral into terrain: The airflow separated; the glider continued turning toward the rising slope and descended at about 23 m/s from about 200 m AGL, with too little height left to recover.
7. Terrain collision at 2126 m/MSL: The glider collided with the ground in a near-vertical spiral at 2126 m MSL, about 1 km southeast of the Chistehorn; both pilots were fatally injured and the aircraft was destroyed.