What happened
On August 3, 2015, an Arcus T motor glider, registration HB-2483, was conducting a private VFR flight from Saanen. After encountering much weaker thermal conditions in the Valais region than forecasted, the pilot decided to fly toward Bex, intending to use the retractable engine to maintain altitude.
During the flight, the pilot retracted the engine to reduce drag. However, a mechanical issue caused the spindle drive's main fuse to trip, leaving the engine only capable of being deployed manually. As the aircraft approached the area near Saint-Maurice, the pilot attempted to manually deploy and start the engine via windmilling at a low altitude of approximately 105 meters. Despite the engine running, the aircraft could not maintain altitude while flying in the lee of local terrain.
Realizing the Bex airfield was no longer reachable, the pilot initiated an emergency landing in a field. During a right-hand turn toward a meadow, the right wing of the ArcSB-2483 struck the stalks of a cornfield. This contact caused the aircraft to yaw clockwise, leading to a low-altitude impact that broke the fuselage behind the wings. Both the pilot and the passenger escaped the wreckage without injury.
The investigation
SUST examined the flight data from the aircraft's LX 9000 navigation computer and interviewed the pilot. The investigation focused on the sequence of engine deployment, the impact of meteorological conditions, and the pilot's decision-making process. Investigators also reviewed the technical state of the Solo 2350 D engine and the mechanical failure of the spindle drive fuse.
Findings
- The primary cause of the accident was the aircraft's contact with the ground during a right turn while attempting an emergency landing, which induced a rotation around the vertical axis.
- The pilot failed to adapt flight tactics to the prevailing weather conditions and the specific characteristics of the retractable engine aircraft.
- The pilot did not plan sufficient altitude or distance margins to reach an airfield or a suitable landing site in a timely manner.
- The pilot's decision to deploy the engine at a low altitude (105m AGL) contradicted manufacturer recommendations and standard practice, which suggests deployment only above 500m AGL.
- The pilot experienced significant stress and 'target fascination,' focusing on reaching the airfield despite the diminishing altitude reserves.
- There were no evidence of pre-existing technical defects in the engine or the airframe that contributed to the crash.