Introduction
On 21 May 2016, at 13:27 local time, a DG-400 glider with registration HB-2139 collided with terrain approximately 4.1 km northwest of Montricher airfield (LSTR) in the municipality of Montricher, canton of Vaud, Switzerland. The aircraft was destroyed and the pilot was fatally injured. Light forest damage occurred.
Flight Sequence
The pilot, a Swiss national born in 1951, held a valid EASA sailplane pilot license. He had accumulated 2,985 total flight hours, including 773 hours on the DG-400 type. On the day of the accident, he participated in the morning briefing at 09:30 and appeared relaxed and in good health to fellow glider pilots.
The pilot departed from Montricher at 12:42 using the self-launching capability. After climbing, he retracted the engine at 2,034 m AMSL approximately 10 minutes after takeoff. He flew north along the Dent de Vaulion, then reversed course and returned to the area northwest of Montricher. At around 13:06, he crossed the Col du Mollendruz at 1,450 m AMSL. Subsequently, over the Combe à Berger, he performed a full circle and then flew toward a forested slope oriented east-southeast, where he executed numerous circles and figure-eights for about 17 minutes at altitudes between approximately 1,170 and 1,250 m AMSL.
A witness in Montricher observed the glider from about 2 km distance, briefly seeing it fly below the ridge line with wings "perpendicular to the horizon." One wing then grazed the trees, and the glider disappeared horizontally into the forest.
Impact and Damage
The left wing separated from the fuselage upon striking tree crowns and remained lodged in a tree about 20 m above ground. The rear fuselage and empennage were torn off during impact with tree trunks. The forward fuselage and right wing struck the forest floor approximately 60 m from the left wing.
Meteorological Conditions
Conditions at the time were sunny with scattered cumulus clouds at about 4,400 ft AMSL and high cirrus. Visibility was 50 km. Surface wind was from 180° at 5 kt, varying between 150° and 210°. Wind measurements at Berolle (740 m AMSL) showed southerly to south-southwesterly winds with gusts of 20–25 km/h. On the Jura slope, wind was from the southeast. The gliding weather forecast for the afternoon indicated moderate to good thermal activity below 1,500–1,800 m AMSL.
Aircraft Information
The DG-400 is a single-seat, high-performance, self-launching glider with a retractable engine, manufactured by DG Flugzeugbau GmbH in 1983. Its wingspan is 17 m. At the time of the accident, mass and center of gravity were within permissible limits. Stall speeds ranged from 71 km/h (flaps +8°) to 84 km/h (flaps -10°) at a wing loading of 40 kg/m².
Investigation Findings
The Swiss Transportation Safety Investigation Board (STSB) determined that the accident occurred when the glider grazed the treetops of a hillside forest, resulting in loss of control and subsequent impact with the ground. Contributing factors included flying close to terrain with a low safety margin and a probable sudden incapacity of the pilot due to health issues.
Probable Cause
The accident is related to the fact that the glider grazed the treetops of a forest on a hillside, causing the glider to no longer be under control and finally crash onto the forest floor. Contributing factors: slope flying near terrain with a low safety margin; probable sudden incapacity due to health problems.