What happened
On April 18, 1997, a Bell 206 L-3 helicopter, registration HB-XJL, was performing a private VFR flight to the Trient Glacier in Switzerland. The flight departed from Le Châble with three occupants on board.
During the approach to the landing site near the Aiguilles Dorées, the pilot attempted to land at an altitude of approximately 300 feet while maintaining a speed of 60 knots. During this reconnaissance phase, the aircraft encountered a sudden down-drapt and a gust of wind from the southeast, which was contrary to the previously forecast northwest winds.
In an attempt to arrest the resulting descent, the pilot increased the collective, which triggered a clockwise spin of the fuselage. To mitigate the torque, the pilot subsequently lowered the collective, which further increased the rate of descent. The aircraft entered a series of spins and eventually struck the glacier surface, coming to rest on its left side.
The investigation
The investigation established that the aircraft's weight was approximately 3,650 lbs, which was within the prescribed limits for the aircraft's performance capabilities. While the pilot had obtained a Swiss extension for mountain landings, the investigation noted that the pilot's flight instructor had previously reported inadequate mountain landing behavior to the authorities one month prior to the accident.
Investigators examined the meteorological conditions, noting that while general Swiss weather reports indicated a 10-knot northwest wind at high altitude, local conditions at the glacier were influenced by surrounding ridges. This likely created turbulence and shifting wind directions. The investigation also found that the pilot did not perform a proper reconnaissance of the specific landing site before deciding to land, and the chosen landing area was outside the authorized 400-meter radius of the official Trient landing site.
Findings
- The primary cause of the accident was inadequate flight preparation and improper mountain landing tactics.
- The pilot failed to conduct a thorough reconnaissance of the landing area to establish a safe approach strategy regarding local wind and obstacles.
- The aircraft encountered unexpected wind gusts and turbulence caused by local topography.
- The decision to execute a go-around at low altitude and low speed, while outside of ground effect, left the aircraft vulnerable to sudden wind shifts.
- The loss of directional control led to a descending vortex situation from which recovery was impossible due to insufficient altitude.