Arcus terrain collision in slope soaring after lee-side downdraft

A Schempp-Hirth Arcus on a mountain-flying instruction flight struck a mountainside on the Puigmal; the instructor was injured and the glider destroyed. After release at ~2,300 m and slope soaring in the Llo valley in weak lift (~1 m/s) and SW wind, the instructor took the controls to reach the cirque at the valley head. Crossing a rocky spur at low height with planned -2 m/s sink, the glider met a marked lee-side downdraft. The fuselage hit the ground, the left wing detached, and the glider slid ~150 m. Investigators cited the lee-side downdraft over the rocky spur, with recent flights in the same sector contributing.

1. Mountain instruction from La Llagonne: The instructor and pilot took off by aerotow at 12:29, releasing at 685 m height near the entrance of the Llo valley, in calm anticyclonic weather with a light southwesterly wind.
2. Weak lift entering Llo valley: Conditions in the valley matched the briefing: weak lift around 1 m/s with some sink. The instructor aimed for a cirque where raptors had been climbing well; the glider was already low.
3. Recent flights in same sector: The instructor had flown the same sector with other students recently and regularly ran mountain-flying courses for the club, a familiarity that may have bred overconfidence.
4. Decision to cross redan at low height: A rocky spur lay between the glider and the cirque; judging he could clear it, the instructor overflew it directly instead of turning back, and the student assumed he would divert.
5. Marked lee-side downdraft: Just before the spur, the glider entered the descending lee-side airflow; the actual sink exceeded the assumed rate, causing a rapid, hard-to-anticipate loss of altitude.
6. No margin to escape toward valley: With the unexpected sink and low height, the instructor had no margin to turn toward the valley; he lost control of the trajectory and could not avoid the collision.
7. Terrain impact, aircraft destroyed: The fuselage struck the slope, the left wing detached, and the wreckage slid down the steep slope. The instructor was injured and the aircraft destroyed; no malfunction was found.