What happened
On August 3, 2013, a DG-1000S glider, registration HB-3410, was conducting a training flight near the Kägiswil airfield in Sarnen, Switzerland. The flight, which included a flight instructor and a student, was intended to involve spin training maneuvers. During the final approach to runway 03, the student pilot attempted to adjust the glide angle by retracting the airbrakes. However, the control mechanism failed to respond, and the airbrakes remained partially extended.
As the aircraft continued its descent, the instructor took control of the glider. While attempting to reach the airfield, the aircraft struck the roof of a house and a tree approximately 270 meters before the runway threshold. The impact caused the left wing to separate from the fuselage, and the glider overturned before coming to rest in the Sarner Aa river. Both occupants sustained two light injuries and were able to exit the wreckage and reach the riverbank on their own.
The investigation
Investigators from the Swiss Transportation Safety Investigation Board (SUST) examined the wreckage and the flight data recorded by the FLARM collision warning system. The investigation focused on the failure of the airbrake control system. Upon inspection of the wreckage, investigators discovered that the M6 threaded connection at the rod end of the airbrake control linkage had been torn out.
Findings
Technical analysis revealed that the failure was caused by the incorrect installation of a rod end. Specifically, the rod end used featured a right-hand thread, whereas the pushrod was designed for a left-hand thread. To facilitate the installation of this incompatible part, the internal threads of the pushrod had been modified, which significantly compromised the structural integrity of the connection. Under the operational stresses of flight, this weakened thread could no longer withstand the load, leading to the mechanical failure that prevented the airbrakes from being retracted.
Safety action
Following the accident, the aircraft manufacturer, DG Flugzeugbau GmbH, issued Service Information No. 83-13. This directive instructed operators of all DG-1000 variants to inspect the clearance of the threads between the control rods. The manufacturer mandated that if any right-hand thread was discovered, or if the free play exceeded 2 mm, the affected pushrod and rod end must be replaced to prevent similar failures.