What happened
On May 21, 2016, a DG-400 glider, registration HB-2139, departed from the Montricher airfield for a private flight. After an initial climb and the retraction of its retractable engine, the pilot proceeded to fly near the Jura mountains. During the flight, the pilot performed several maneuvers, including circles and figure-eights, at low altitudes along a forested slope northwest of Montricher.
At approximately 13:27, the aircraft struck the treetops of a steep, wooded hillside. The impact was severe enough to sever the left wing from the fuselage; the wing remained lodged in the canopy at an altitude of roughly 20 meters. The remaining portion of the fuselage and the right wing continued sliding through the forest for about 60 meters before hitting the ground. The pilot sustained fatal injuries in the accident.
The investigation
Investigators from the Swiss Transportation Safety Investigation Board (SUST) analyzed flight data from both a collision warning device and a navigation unit. The data revealed that for the final 12 minutes of the flight, the aircraft was operating at low altitudes near the terrain.
Technical examinations of the HB-2139 showed no evidence of mechanical failure or pre-existing technical defects. The aircraft's weight and center of gravity were within permissible limits. While the aircraft was equipped with an older-model emergency locator transmitter (ELT), the investigation found no evidence that the device failed to function due to mechanical issues, though it was found displaced in the luggage compartment.
Findings
- The primary cause of the accident was the glider striking the treetops of a forested slope, which led to a loss of control and the subsequent impact with the ground.
- The pilot was flying in close proximity to the terrain with a low margin of safety.
- A sudden inability to act due to potential health issues, such as cardiac arrhythmia or acute unwellness, could not be ruled out, although toxicological results were negative.
- The aircraft was operating under Visual Flight Rules (VFR) in favorable weather conditions, though moderate turbulence was present above 3000 ft AMSL.