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. Investigators cited stall in the low-altitude half-turn.

1. First biplace tow with DR400-180: A Schleicher ASK21 trainer flew a season-opening training day at Sarreguemines-Neunkirch; the instructor sat rear, a student who never touched the controls sat front.
2. Rallye unavailable, new DR400-180 used: The club's Rallye tug developed a battery fault, so it substituted a DR400-180 bought ~7 months earlier and never used for towing there; this was the club's first glider tow with it.
3. No release trajectories pre-planned: The club had no formal procedure for a cable release on DR400 takeoff, only a verbal plan for the instructor to release on request; no stop trajectories or takeoff calculations were made.
4. Instructor rear, forward view blocked: The instructor took off from the rear seat, meaning to hand control to the student in the climb; the tall student in front blocked his forward view of the tug and runway.
5. Tug speed 90 vs expected 115 km/h: The glider lifted off at 90 km/h, well below the 115 km/h reference used with the Rallye; the tow pilot radioed, then without a reply asked the instructor to release.
6. Release, left turn for runway 05R: The instructor released and, fearing a collision with the tug or a ditch beyond the runway, turned left at ~10 m AGL to land on the parallel runway, ~240 m ahead at ~80 km/h.
7. Glider stalls in low-altitude turnback: The instructor felt the glider stall in the turn crossing the crosswind (~10-15 km/h); ASK21 stall speed is 74 km/h, and the crosswind cut the effective ~80 km/h further.
8. Ground impact, tailboom broken: The left wing then nose struck the ground and the tailboom broke off; one occupant was seriously injured. Factors: no release procedure, poor DR400 preparation, blocked view, low slow turn.