Escape turn from sink at low speed stalls LS 8-18 into ridge

The pilot of an LS 8-18 was seriously injured during a soaring flight near Ulrichen in the Walliser Alps and the glider was destroyed. After an early release at about 2,200 m MSL to avoid traffic, the pilot flew ridge figure-8s on an unfamiliar slope at about 112 km/h. Lift stopped unexpectedly and the aircraft entered sink. The pilot initiated a 180 degree reversal turn but airspeed fell below the minimum flying speed; with the aft CG — a tail battery had been left installed — aggravating wing drop, the LS 8-18 lost control near the ground and struck the mountainside. Aft CG and a low-speed reversal in sink drove the stall close to terrain.

1. Aerotow, early release at Geschinergale: At 13:11 LT on 25 July 2013 the pilot launched the LS 8-18 (HB-3287) on aerotow behind a Super-Dimona from RWY 05 of Münster (LSPU) for a conservative local flight in the Walliser Alps. About 3 km before the planned release point at the Galmihornhütte, around 2,200 m MSL, he released early near the Geschinergale, 1.5 km abeam the unused Ulrichen RWY 05, to avoid the expected congestion of gliders at the planned area.
2. Aft CG: tail battery installed: The pilot's preferred loading for the LS 8-18 used no nose trim weights and no battery in the tail. On this flight a tail battery was installed (a fact the pilot noted during preflight). Cockpit load was 85.8 kg against a 85 kg minimum, putting the in-flight centre of gravity at 397 mm aft of the reference plane — at the upper end of the 280–400 mm permitted range. An aft CG increases the risk of wing drop at low speed and a developing spin after a stall.
3. Early release in unfamiliar area: Releasing early at the Geschinergale offered the tactical advantage of avoiding a crowded thermalling area. The disadvantage: the pilot had little altitude and time to assess local conditions in the Ulrichengale area, which he did not know from earlier flights as a thermal entry point.
4. Lift stops, sink at Ulrichengale: After short search at the Geschinergale, the pilot moved over the crest to the Ulrichengale and started ridge figure-8s along its south-east corner at a ground speed averaging 112 km/h (about 10–15 % above best L/D). Intending to extend the second figure-8 into the Obertal side valley, he unexpectedly lost lift and the aircraft entered a stronger sink area. The last FLARM point at 13:19:46 LT shows the LS 8-18 at about 2,200 m MSL near the end of the first figure-8.
5. Low-speed 180° reversal turn: Surprised by the sink, with little height above terrain and a rising slope ahead, the pilot was unable to gain speed. He immediately initiated a 180° reversal turn (Fluchtkurve) toward better ground and noted a reduced speed margin during the manoeuvre.
6. Wing drops near ridge: Past about 180° of the turn the right wing dropped toward the slope. Airspeed had very probably fallen below the minimum flying speed; with the aft CG aggravating wing drop, the LS 8-18 entered an unrecovered stall close to the ground. The 100 m of altitude needed to exit a developed spin per the flight manual was not available.
7. Mountain impact, serious injury: The LS 8-18 struck a steep east-facing slope of more than 30° about 2 km north of Ulrichen at 2,170 m MSL, nose first. The cockpit absorbed energy in soft ground, the wreck slid about 19.5 m below the impact point. The pilot was seriously injured but conscious and used his mobile phone to alert a colleague; REGA and Air Zermatt were on scene at 14:10. The LS 8-18 was destroyed.