Aerotow abort from soft grass; glider stalls past runway end

The pilot of an ASK 21 was seriously injured when the glider struck a ditch embankment past the runway end on an aerotow takeoff; substantial damage, instructor-candidate uninjured. The Varga 2180 tug accelerated poorly on the soft grass strip and the glider lifted off early into a high tow position, loading the tug hook downward so the nose wheel stayed pinned. The tug pilot aborted at ~91 km/h, ~160 m short of the end; the glider overflew the tug, banked left to clear a fence and stalled over a creek. A paved runway of equal length was available; the prior tow had also been sluggish, but its crew was not consulted and no abort plan briefed.

1. Aerotow takeoff on grass RWY 27: An ASK 21 was launched behind a Varga 2180 tug from the soft grass RWY 27 at Hausen am Albis on a training check flight; an equal-length paved runway was available alongside.
2. Soft grass strip; paved alt unused: The grass strip was softer than expected. The previous aerotow on the same runway had also accelerated sluggishly, but the crew was not consulted and no aborted-takeoff briefing was carried out.
3. Tug slow to accelerate: The Varga 2180 tug reached only ~40-45 mph (~65-72 km/h) on the ground roll; ~65 mph was required to lift off. All three wheels stayed on the ground throughout the run.
4. Glider lifts off early in high tow: The pilot rotated early and the ASK 21 lifted off at minimum speed, with the tail briefly contacting the ground. The glider then climbed to ~3-4 m above the tug, generating an upward force on the tug's tow hook that pitched the tug nose-down and prevented the nose wheel from unloading despite full back stick.
5. Tug pilot aborts at ~91 km/h: Concluding the tug could not safely fly, the tug pilot reduced power, called "Startabbruch" on the radio, released the tow rope and steered left toward the paved runway. Ground speed at abort was ~91 km/h, about 160 m short of the runway end. The instructor-candidate in the rear seat then pulled the glider release.
6. Glider stalls past runway end: The pilot, unaware of the tug's position behind, elected to overfly the ditch and land beyond. The ASK 21 passed over the tug on the right, banked left to clear a dog-track fence, then stalled over the creek and the nose dropped.
7. Pilot seriously injured: The glider struck the far bank of the ditch just below the rim, yawed ~150 degrees and came to rest ~10 m beyond. The front-seat pilot was seriously injured; the instructor-candidate in the rear seat was uninjured. The fuselage suffered a torsion break ahead of the empennage.