Flight History
The helicopter departed Sion at 0705 hours local time on 26 April 1987, heading to Turtmann with the pilot, an auxiliary flight assistant, and another passenger. After landing at the military airfield, it took off at 0735 with four passengers, including the assistant, for a mission to transport skiers to the Ebnefluh mountain landing site. Due to unfavorable weather conditions, the pilot decided to divert to the Petersgrat landing site.
Meteorological Conditions
On the day of the accident, weather was characterized by changing cloud cover, partially or completely obscuring ridges and affecting visibility. Winds were light to moderate from the west. Temperature at the landing site was -3°C.
Accident Sequence
During the approach to Petersgrat, the pilot observed that the intended landing site was clear of clouds, but the wall behind it was constantly obscured by moving fog banks. Concerned, the pilot aborted the landing and chose a lower site with a gentle slope. Approaching from the southeast perpendicular to the slope, visibility was good but light was diffuse, making height and distance estimation difficult. The pilot asked the flight assistant to indicate height. As the left skid touched the snow, rotor downwash raised a whirlwind of fresh snow, causing the pilot to lose all visual references. The helicopter then leaned on its skid and overturned. The assistant, who had unbuckled, was ejected between the cabin and the rotor blades, which struck the ground. The pilot and other passengers remained harnessed. No one was injured.
Investigation Findings
The pilot held a valid professional helicopter license with Canadian validation. The helicopter had valid airworthiness and registration certificates; no pre-existing technical anomalies were found. Mass and center of gravity were within limits. The pilot had previously landed at Petersgrat but not at the accident site. The official landing zone at Petersgrat is defined by a 400-meter radius; the accident occurred outside that zone. Traces indicated forward speed was nearly zero at impact. Fuel remaining was 150 liters.
The investigation noted that the pilot prudently abandoned the Ebnefluh landing due to weather and made a judicious decision to abort the Petersgrat approach when reference conditions were inadequate. However, the approach tactic—perpendicular to the slope—deprived the pilot of visual cues over a featureless white surface. A calm wind approach from the east along the slope would have provided necessary references. During the hover, rotor downwash engulfed the helicopter in powder snow, eliminating ground reference. The investigation highlighted that hovering at a few meters to allow snow to dissipate could have prevented loss of control. The pilot's reliance on an assistant for height estimation was considered illusory and dangerous.
Cause
The accident was due to a loss of control of the helicopter following the absence of external visual references and an approach tactic not adapted to the circumstances.