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: HB-1638 lined up north of grass RWY 25, offset 75 m from the hard-runway centerline per the Sion procedure; the Robin DR400 tow plane positioned ahead and angled slightly toward the grass runway centerline, requiring a curving rollout. Wing tanks held 120 L of water ballast.
2. Stacked operational risks: Strong gusty headwind (245°/15 kt, gusts 25 kt); curving rollout required by the offset procedure; 25.5 m wingspan with high yaw moment if a wingtip touches; tow via the CG (Schwerpunkt) hook gives less directional restoring moment than a nose hook; water ballast can slosh during rollout.
3. Pilot's prior Sion launch trouble: SUST noted the pilot had previously had difficulty maintaining direction behind the tow plane during a 2015 aerotow at Sion and had released the cable during the rollout. The pilot believed the offset-procedure geometry contributed to the risk.
4. Right wingtip drops at release: The wingman held the wing horizontal for over 5 s before the takeoff roll. After release, the right wingtip dropped sharply to the grass — likely caused by water-ballast imbalance or by the pilot intuitively holding the stick toward the curving rollout direction in the strong headwind.
5. S-shaped PIO behind tow plane: The wingtip ground touch yawed the aircraft right. The pilot applied left rudder and aileron; the right wing rose after ~3 s but the glider rolled left past the tow-plane axis. Subsequent strong right-then-left rudder corrections built into an S-shaped oscillation behind the tow plane.
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 the left wingtip and came to rest on its back on the hard RWY 25. The tow rope broke from excess tension just before impact. Two runway lights were damaged; the glider was destroyed; the pilot was seriously injured (per SUST the accident was barely survivable). Tow plane and tow pilot unharmed.