ASK 23 aerotow PIO from nose-heavy trim — late abort, hard runway impacts

An ASK 23 B was heavily damaged on aerotow from Zweisimmen RWY 35 (Switzerland) behind a Robin DR 400; the pilot was uninjured. Elevator trim was set nose-heavy. The glider rotated abruptly and entered a pilot-induced oscillation with two touchdowns. After the tug lifted off, the glider sank, touched down again, and entered a second oscillation with progressively heavier impacts. Only after the second oscillation had developed did the pilot release the rope. Limited aerotow recency on a predominantly winch background and the nose-heavy trim setting were identified as primary factors.

1. Aerotow from Zweisimmen RW 35: ASK 23 B aerotowed at 15:52 behind a Robin DR 400 (55 m rope, nose hook). Wind W-NW 8 kt.
2. Limited aerotow recency: 62 yo pilot with 972 h total; only 11 aerotow starts in last 24 months, last on type July 2019. Background predominantly winch.
3. Nose-heavy elevator trim: Elevator trim set forward (kopflastig) — a winch-launch convention used in the operator's school. The flight manual has no specific aerotow recommendation.
4. Abrupt liftoff and first PIO: The glider rotated sharply and lifted off; immediately entered pitch oscillation with two touchdowns.
5. Second PIO with heavy impacts: Tow plane was now climbing; glider sank, touched down again and entered a second PIO. Impacts grew progressively harder — wing flex visible, impact marks on the runway.
6. Tow release (too late): Pilot released the rope to abort — SUST: decision was safety-conscious but too late, as PIOs escalate rapidly.
7. Heavy damage, pilot uninjured: After release the glider climbed steeply (elevator forward range limited by fuselage damage), reached ~20 m AGL at runway end, then nose dropped and the glider landed in a meadow past the runway. Aircraft heavily damaged; pilot uninjured. Tow plane completed a circuit normally.