ASK 13 wing strikes tree on short final in gusty wind

An ASK 13 was substantially damaged on a training flight at Iserlohn-Suemmern (Germany); both occupants were seriously injured. After a winch launch the crew began final approach to RWY 23 in very gusty wind from 210-300 degrees at 10-22 kt. Witnesses saw the glider lower than usual; about 100-200 m before the threshold it deviated sharply right and the instructor reported the controls felt soft. The left wing struck a tree at 15 m, the glider rotated 270 degrees counterclockwise, and it impacted 40 m short of the threshold. The instructor held 1,969 h total but only 11 h on type, with 2.5 h since the start of the year.

1. Winch launch RWY 23 12:57 LT: At 12:57 LT on 9 May 2015 an instructor and student launched an ASK 13 on a winch tow from RWY 23 of Iserlohn-Sümmern (488 ft AMSL, 800 m grass strip) for a standard circuit. FLARM data show a right turn after release, then alternating right and left turns over about five minutes.
2. Gusty wind 210-300° / 10-22 kt: The airfield reported wind very gusty from 210° to 300° at 10-22 kt — a 90° directional spread and 12 kt gust range. Visibility was over 10 km with broken cumulus base at 2,500 ft. Landing was on RWY 23 (heading 230°), inside the wind variability arc.
3. Instructor: 11 h on type, 1,969 h: The instructor held an unlimited glider licence with about 1,969 hours total. He had only 11 hours on the ASK 13 type — 2.5 hours over 18 starts and landings since the start of 2015, including 7 winch launches earlier on the day of the accident. The student was at the beginning of training and had completed one prior flight with the instructor on the type that day.
4. Final approach 23, lower than usual: After about five minutes in the circuit the crew commenced the final approach to RWY 23. Witnesses noted that the glider was lower than usual on final.
5. Sharp right deviation on short final: About 100 to 200 m before the runway threshold the glider made a marked change of direction to the right.
6. Correction; controls feel "soft": The instructor reported that he tried to correct the unintended right turn and noticed that the controls of the glider felt "soft" — a feel consistent with airspeed near the stall.
7. Wing strikes tree, 270° rotation: At about 15 m above the ground the left wing struck a tree at the edge of the airfield. The glider rotated approximately 270° counterclockwise and impacted the ground about 25 m beyond the tree, roughly 40 m short of the RWY 23 threshold, where it then came to rest. The cabin area was destroyed and both wings broke at the brake-flap box; tail boom and tail surfaces sustained only minor damage. No technical defects were found. Both occupants were seriously injured.