DG-1000S airbrakes jam on final from linkage failure, ends in river

The instructor and student of a DG-1000S were lightly injured on a training flight near Kägiswil and the glider was destroyed. On final to RWY 03 the student tried to retract the airbrakes, which were about two-thirds extended; the lever moved freely. A misassembled rod-end fitting in the airbrake linkage had failed under stress. At about 50 m AGL with reduced glide, the instructor took control and rejected a tree-blocked field landing for the runway. The glider struck a house roof and a tree 270 m short of the threshold, overturned and came to rest inverted in the Sarner Aa. A misassembled airbrake rod-end fitting caused the linkage failure.

1. Aerotow training from RWY 03: At 12:14 LT on 3 August 2013 an instructor and student launched a DG-1000S (HB-3410) on aerotow from RWY 03 of Flugfeld Kägiswil (LSPG) for a planned training flight including a spin / hazard introduction exercise. They released at about 2,000 m AMSL after an uneventful tow.
2. Rod-end fitting mis-assembled: A rod-end fitting in the airbrake control linkage — an M6 connection between control rod 10St71 and its rod end — had been incorrectly assembled before the flight. Under flight loads the joint subsequently failed. Such a hidden assembly fault inside the linkage would not be detectable by routine daily or pre-takeoff functional checks.
3. Retract fails at ~50 m; lever loose: After using the airbrakes earlier in the circuit, the student extended them again at 12:43:17 LT to adjust the glide path. At 12:43:57 LT on final he tried to retract them — they were about two-thirds extended; the lever moved freely with no effect on the airbrakes. The instructor heard a rattle (Scheppern) behind him as the linkage failed. The aircraft was then at about 50 m above the airfield.
4. Instructor takes over; rejects field: The instructor took control of the DG-1000S. He rejected an off-field landing in a field in front of the Sika Sarnafil industrial building because of trees blocking the approach and tried instead to reach the runway.
5. Hits roof + tree 270 m short: The last FLARM point at 12:44:13 LT shows the glider at 484 m AMSL and about 90 km/h ground speed. About 270 m before the threshold of RWY 03 it struck the roofs of houses and a tree.
6. Inverted in river; both lightly hurt: The glider overturned and came to rest inverted in the Sarner Aa river. Both occupants exited the wreck under their own power and waded to the bank; the student was briefly checked at hospital for mild nausea and quickly released. The glider was destroyed. Light damage to house roofs.