ASK21 stalls in low-altitude turnback after tug-requested release

An ASK 21 crashed in a low-altitude turnback at Sarreguemines-Neunkirch during a training aerotow; one occupant was seriously injured and the glider destroyed. It was the club's first biplace tow with a new DR400-180 (the usual Rallye unavailable), without pre-planned trajectories for an early release. The instructor on the rear seat had forward view blocked by a tall student; the tug reached only 90 km/h at mid-runway (vs 115 expected) and called for release. The instructor released at ~10 m AGL and turned left for the parallel airplane runway. The glider stalled at ~80 km/h. BEA cited stall in the low-altitude half-turn.

1. First biplace tow with DR400-180: Schleicher ASK21 (F-CHIN) on a training day at the start of the season at Sarreguemines-Neunkirch (Moselle). Two-seat trainer, max takeoff mass 600 kg, day's mass ~582 kg. Crew: instructor (70 yo, PPL avion 1971, SPL planeur 1969, tow rating since 1971, glider instructor since 2015, ~1,810 h glider) on rear seat; pilot in training (51 yo, SPL since Nov 2020, ~51 h glider) on front seat — student stated he never touched the controls during the accident flight. Weather: wind 210°/10 kt, CAVOK, 16 °C.
2. Rallye unavailable, new DR400-180 used: The club's usual MS893 Rallye towplane developed a battery problem after the first tow of the day. The club switched to a DR400-180 (F-HIGM) it had acquired ~7 months prior. The DR400 had been used by a German club for ~2,500 h of tow work; since acquisition the Sarreguemines club had used it only for nav and local flights, never tow. The accident flight was the club's first biplace tow with this DR400 from this airfield. The tow pilot (68, glider+airplane+instructor licences, ~504 h airplane / 8,200 h glider) had not been on duty that day; he was the only club member type-rated on DR400 and stepped in for the tows.
3. No release trajectories pre-planned: The club had not formalised any specific procedures or trajectories in case of cable release during DR400 takeoff. Pre-flight: brief verbal briefing that the instructor would release on tow pilot's request if speed issue arose; no agreed glider/tug stop trajectories. Tow pilot had not done a mass/balance calculation nor takeoff distance calculation; the external mirror had been removed from the DR400 to reduce drag for nav flights and not reinstalled.
4. Instructor rear, forward view blocked: Instructor took the controls for the takeoff from the rear seat, with intent to hand back to the student in the initial climb. The student in the front seat was tall, which obstructed the instructor's forward view — he could not clearly see the tow plane or remaining runway.
5. Tug speed 90 vs expected 115 km/h: The glider lifted off at 90 km/h before reaching the restaurant (mid-runway visual reference). At the restaurant, the tow pilot saw airspeed only 90 km/h instead of the 115 km/h rotation reference used with the Rallye. The tow pilot radioed asking whether the glider had lifted off, then without waiting for a reply asked the instructor to release the cable.
6. Release, left turn for runway 05R: Instructor released the cable. Concerned about hitting the tug if it had aborted, and about overrunning into a ditch ~290 m beyond the runway end, he instinctively initiated a left turn at ~10 m AGL to land opposite direction on the parallel airplane runway 05R (the glider grass strip 23R was muddy/uneven on its right side). The glider had ~240 m of runway ahead and ~80 km/h ground speed at the start of the turn.
7. Glider stalls in low-altitude turnback: The instructor felt the glider stall during the turn as it transitioned through the crosswind component (estimated 10-15 km/h). ASK21 stall speed per the flight manual is 74 km/h; the glider was at ~80 km/h with crosswind component reducing effective airspeed during the bank.
8. Ground impact, tailboom broken: Left wing then nose contacted the ground; the tailboom broke completely from the main fuselage. Glider came to rest on the airplane runway, nose pointing northwest. One occupant seriously injured. BEA contributing factors: (i) no pre-planned trajectories in case of cable release; (ii) insufficient preparation for the club's first biplace tow with the DR400-180 from this airfield; (iii) takeoff performed with glider command from the rear seat reducing forward visibility; (iv) half-turn at low height and low speed.