What happened
On July 10, 2017, a Glaser-Dirks DG-202 sailplane, registered as HB-1593, was conducting a local flight near the Obergoms region. The pilot had departed from Münster/VS (LSPU) following a standard daily briefing and had previously completed the annual internal inspection for the flying group earlier that spring.
During the flight, which reached an altitude of 2,300 meters, the pilot decided to terminate the mission early due to approaching bad weather from the west. The pilot opted to land on runway 23, which slopes slightly from northeast to southwest. To account for expected headwinds and light turbulence, the pilot maintained an approach speed of 100 km/h, rather than the standard 90 km/h used in calm conditions.
The aircraft touched down near the middle of the runway, leaving approximately 350 meters of runway remaining for the rollout. As the pilot initiated a right turn to taxi onto the grass strip for parking, the wheel brake failed to function despite the pilot fully pulling the control lever. Consequently, the aircraft continued rolling across a public side road, eventually striking the corner of a hangar, a signal sign, and two barrier posts.
The investigation
The investigation focused on the mechanical operation of the aircraft's braking system. The DG-202 is equipped with a drum brake operated via a Bowden cable. For the brake to be effective, the blue control lever must reach the final third of its travel. This mechanism relies on a metal linkage and a tensioned cable to actuate the brake pads on the wheel.
Investigators examined the aircraft's landing gear and the tension of the control cable. They also reviewed the pilot's pre-flight procedures and the maintenance of the braking mechanism.
Findings
- The investigation determined that the Bowden cable for the brake was loose and not correctly tensioned.
- This lack of tension prevented the brake from being actuated, even when the pilot moved the control lever to its full extent.
- The improperly tensioned cable was identified as the sole cause of the brake failure and the subsequent collision.