What happened
On May 3, 2020, a DG-1000S glider, registration HB-3444, was engaged in a recreational flight near Grenchen, Switzerland. After being towed from Grenchen Airport, the pilot spent approximately 90 minutes soaring along the southern Jura ridge. During the flight, the pilot performed several maneuvers, including high-performance turns and accelerating through descents.
At approximately 14:35, the pilot initiated a 180-degree right-hand turn close to the terrain near the Ängloch ridge. While flying in a descent at a ground speed exceeding 170 km/ . During this phase, the left wing of the aircraft struck a suspended power line stretching between the Bettlachberg and the Wandflue. The impact severed the outer portion of the left wing, causing the aircraft to enter an uncontrolled spin and crash into the steep, forested terrain. The pilot sustained one serious injury, and the aircraft was destroyed.
The investigation
Investigators from SUST analyzed GPS flight data from the aircraft's LX8000 flight computer, eyewitness accounts, and video footage. The investigation focused on why the pilot failed to detect the wires and why the onboard collision warning system did not alert the crew.
Technical analysis of the Flarm collision avoidance system revealed that the specific power line involved in the accident was not present in the "ALPS20200130" obstacle database installed in the aircraft. While the system was capable of alerting the pilot if the obstacle were known, the database used for the large Alps region prioritized storage efficiency by omitting certain low-visibility objects like power lines. Simulations conducted by investigators proved that had the power line been included in the database, the system would have provided a warning approximately nine seconds before the collision.
Findings
- The pilot performed a risky and unnecessary flight maneuver by turning close to the terrain and accelerating at low altitude.
- The pilot's attention was diverted by mental preparations for landing and communicating with air traffic services, leading him to overlook the known power lines.
- The onboard collision warning system failed to trigger because the power line was missing from the active obstacle database.
- The aircraft's emergency locator transmitter (ELT) had not been activated prior to the flight.