Arcus M stall in mountain hang turn — terrain collision

After a stall during mountain hang turns about 1 km southeast of Chistehorn at 2,126 m MSL, both pilots of an Arcus M were fatally injured and the aircraft destroyed. After 2.5 hours on day one of a planned cross-country tour from Birrfeld, the glider had been flying figure-8 hang turns along the Rhone ridge for about 12 minutes. A right turn was not completed in sinking air; the critical angle of attack was exceeded and the glider stalled into a steep spiral, striking the rising terrain. A possibly aft CG and unclear two-pilot role-sharing were noted as contributing factors.

1. Mountain ridge soaring: On day one of a planned two-day Wandersegelflug (Birrfeld → Grenoble → Langenthal), the Arcus M was ridge-soaring at the Chistehorn for about 12 minutes, flying figure-8 hang turns with the half-circles turning away from the slope (recommended technique).
2. Limited mountain hang-flying time: Pilot A (front, age 46): 1377h total but only 6:10h on the Arcus M across 3 flights. Pilot B (back, age 72): 441h total, of which 46:16h on the Arcus M — but all on flat-terrain cross-country flights in Namibia. Both pilots' mountain hang-flying experience on type was limited.
3. Possibly aft CG: SUST cites a possibly aft CG as a contributing factor. The rudder-fin tank had 4 L of water (3-L holes taped), placing CG in the mid-to-aft allowable range per AFM. SUST notes that an aft CG accentuates departure behaviour, especially in turning flight.
4. Two-pilot role-sharing unclear: SUST notes that work-sharing between pilots of different backgrounds can support safety only if pre-flight rules for decision-making and intervention are clearly defined and accepted by both. Whether the two pilots had agreed on roles remains open.
5. Critical AoA exceeded in turn: At 15:52:58 a right turn was initiated in sinking air; IAS dropped 25 km/h in 15 seconds while the glider gained ~10 m height. SUST analysis indicates the critical angle of attack was exceeded as the aircraft was pulled up in sinking air during the turn.
6. Stall, steep spiral into terrain: At about 15:53:07 the airflow separated. The aircraft continued turning toward the steeply rising slope and descended at ~23 m/s from ~200 m AGL. Pilot B's hand was found on the flap lever — possibly an attempt to recover from incipient spin by switching to negative camber per the AFM emergency procedure — but altitude was insufficient.
7. Terrain collision at 2126 m/MSL: The Arcus M collided with the ground in a near-vertical steep spiral at 2126 m/MSL, approximately 1 km southeast of the Chistehorn. Both pilots were fatally injured (polytrauma, instant); the aircraft was destroyed. ELT activated. SUST renews safety recommendation #409 (mandatory stall warning systems on sailplanes) and adds #557 (community sensitization on LOC-I in hang flying).