Circumstances
On 18 April 1997, at approximately 1050 local time, a Bell 206 L-3 Long Ranger helicopter, registered HB-XJL, was involved in an accident while landing on the Glacier du Trient in Valais, Switzerland. The helicopter was operated privately with one pilot and two passengers on board. The flight had departed from Le Châble around 1030 hours.
According to the pilot, the weather forecast indicated a constant wind of 10 knots from the northwest, but upon arrival at the glacier, there was no wind. The pilot made an initial approach toward the northwest of the Plateau du Trient, near Les Aiguilles du Tour, and executed a go-around. He then decided to land toward the southeast of the plateau, close to Les Aiguilles Dorées. While conducting a reconnaissance at about 60 knots and approximately 300 feet above the ground, the helicopter encountered a sudden downdraught from the southeast, nearly opposite the forecast direction. The pilot increased collective pitch to arrest the descent, causing the fuselage to spin clockwise despite full left pedal input. Reducing collective to lower torque increased the rate of descent, and after several spins, the aircraft struck the ground.
The front left passenger reported that the helicopter began to be affected by gusts near the Orny hut, and violent intermittent wind was present near the Trient hut. The pilot attempted to land without a prior overflight, then aborted and apologized for the hasty approach. Shortly after, at around 20 meters above the ground, the pilot lost control, the helicopter made one or two 360-degree rotations, and the left skid contacted the ground first. After two or three rolls, the helicopter came to rest on its left side. The occupants evacuated unaided; the pilot sustained minor cuts to the head, while the passengers were uninjured.
A pilot of another helicopter reported excellent visibility and a cloudless sky over the Trient plateau that morning but noted extremely violent wind from the northwest. The guardian of the Trient hut, who witnessed the accident, also mentioned strong gusty wind.
Investigation
The investigation established that the pilot held a validation of a foreign (FAA) license for airplanes and helicopters with a mountain landing extension obtained in Switzerland. In the two months preceding the accident, the pilot had performed 20 glacier landings, 12 of them at Trient using HB-XJL. No evidence indicated any physical impairment during the flight. The helicopter was approved for VFR day and night non-commercial operations and VFR day only for commercial operations.
Mass and balance calculations for the intended two rotations were performed; the accident occurred during the first rotation. The estimated mass at the time of the accident was 3650 lbs, within prescribed limits. The last 50-hour inspection was conducted on 24 March 1997 at 645 flight hours; at the accident, total flight time was approximately 662 hours. The helicopter was equipped with numerous options, including a power augmentation kit, allowing hover out of ground effect at about 4000 lbs.
No reconnaissance overflight of the initial landing site occurred. After a go-around, the pilot decided to land southeast of the plateau, near Les Aiguilles Dorées. Both the originally intended and the actual landing sites were outside the authorized 400-meter radius from the official Trient coordinates. One month before the accident, the instructor responsible for the pilot's mountain training had warned the Swiss Federal Office for Civil Aviation about the pilot's inadequate behavior during glacier landings, which he had observed on several occasions.
Meteorological conditions at the time, according to the Swiss Meteorological Institute, indicated a high-pressure influence. At the accident location, the sky was cloudless, visibility was 30–35 km, wind was from about 130° at 10–15 knots, temperature was -9°C, and dewpoint was -20°C. The wind values were estimated from surrounding stations and radiosondes; the crests south of the impact point (Aiguilles Dorées) likely influenced the actual wind, possibly causing light turbulence on their north side. A light mountain wind probably prevailed over the glacier surface.
Cause
The accident was due to inadequate flight preparation followed by inadequate flight tactics for mountain landings.