What happened
On July 20, 2002, a Schempp-H. Discus-2T glider, registration PH-1248, was performing a local flight at the Terlet Glider Center. During the approach phase at an altitude of approximately 350 meters, the pilot deployed the airbrakes while traveling at 110 km/ h to reach a target altitude of 250 meters. Upon reaching the desired height, the pilot attempted to retract the airbrakes, but discovered the control lever was completely jammed in both directions.
To avoid an impact with trees due to the steep descent angle, the pilot executed an unplanned approach toward the eastern part of the T-strip. During this maneuver, the aircraft crossed an active winch runway at an altitude of roughly 20 meters. Because the onboard radio was not tuned to the correct frequency, the pilot was unable to communicate the deviation to the airfield operations. The aircraft touched down and slid for approximately 150 meters. After striking an old path, the aircraft bounced slightly, and the landing gear collapsed just before the aircraft came to a halt. There were no injuries to the pilot, and the aircraft sustained light damage.
The investigation
The investigation focused on the mechanical failure of the airbrake control system. The manufacturer retrieved the aircraft to examine the components. The analysis revealed that when airbrakes are retracted, aerodynamic forces bend the upper plate of the brake toward the rear. Under normal design specifications, the upper plate should stop against the lower plate, causing the lower plate to flex forward. However, a manufacturing defect was identified in one of the airbrakes on PH-1248, where the overlap between the two plates was only 1.5 mm instead of the required 3.0 mm.
Findings
- A manufacturing defect caused insufficient overlap between the airbrake plates.
- Abrupt movement of the control lever allowed the upper plate to slide over the lower plate, resulting in a mechanical jam.
- The pilot's inability to communicate the emergency approach was due to the radio being set to the incorrect frequency and high cockpit workload.
- The manufacturer's subsequent inspection of other Discus-2 aircraft determined this was an isolated incident.