Nimbus-3 wingtip touch escalates to PIO and overturn on aerotow start

The pilot of a Nimbus-3 was seriously injured on aerotow takeoff at Sion and the aircraft was destroyed. The glider carried 120 litres of water ballast in gusty wind (245 deg, 28 km/h gusting 46 km/h) and was offset 75 m from the hard-runway centreline per Sion procedure, requiring a curving rollout. After the wingman released, the right wingtip touched the grass and the aircraft yawed right. Towed via the CG hook (less directional restoring moment than a nose hook), pilot corrections built into an S-shaped PIO. A second left-wingtip touch pitched the glider up about 45 degrees; it rolled inverted over the left wingtip onto the hard runway.

1. Aerotow lineup, Sion offset: The glider lined up north of the grass runway, offset 75 m from the hard-runway centerline; the tow plane was angled toward the grass runway centerline, requiring a curving rollout.
2. Stacked operational risks: Strong gusty headwind; curving rollout by the offset procedure; 25.5 m wingspan with high yaw moment if a wingtip touches; tow via the CG hook gave less directional restoring moment.
3. Pilot's prior Sion launch trouble: The pilot had previously had difficulty maintaining direction behind the tow plane during an earlier aerotow at the same site and had released the cable during the rollout.
4. Right wingtip drops at release: The wingman held the wing horizontal before the roll; the right wingtip then dropped sharply to grass after release, likely from ballast imbalance or stick position toward the curve.
5. S-shaped PIO behind tow plane: The wingtip touch yawed the glider right; left rudder and aileron raised the right wing, but the glider rolled left past the tow axis, and corrections grew into an S-shaped oscillation.
6. Left wingtip touch, pitch-up ~45°: After a second ground touch by the left wingtip, an abrupt left yaw and sufficient forward speed let the right wing generate enough lift to pitch the glider up at about 45° on one side.
7. Inverted overturn on hard runway: The glider flipped over its left wingtip onto its back on the runway; the tow rope broke from excess tension just before impact, and the pilot was seriously injured.