Accident Summary
On 29 January 1999, at approximately 1705 local time, a McDonnell Douglas MD520N "NOTAR" helicopter, registration HB-XGQ, collided with steep, snow-covered terrain beside a ski slope near the mountain station of Hoch-Ybrig, Canton Schwyz, Switzerland. The helicopter was destroyed, and all four occupants—the pilot and three ski patrollers—sustained fatal injuries. The accident occurred during an avalanche control operation, with 68 kg of explosives on board, none of which detonated.
Flight History
The pilot, a Swiss citizen born in 1947, held a commercial helicopter pilot license with mountain and night flight endorsements and had accumulated 2914:34 flight hours, with 2484:30 hours on the MD520N. He began his workday at about 0800 and had complied with rest requirements. The flight originated from Schindellegi at 1630, landing at the "alten Sternen" landing site near the Hoch-Ybrig mountain station. There, he picked up three ski patrollers (born 1951, 1967, and 1975) who were carrying 17 explosive cartridges (68 kg). The helicopter departed at 1650 for avalanche control operations. Witnesses observed the helicopter flying over the mountain station toward the "grossen Sternen" avalanche slope and then toward the Forstberg, where it disappeared from sight. No explosions were heard. The final segment of the flight and the impact were not observed. Impact marks indicated a collision with a southwest heading in a steep couloir about 50 m east of the Weglosen ski run.
Wreckage and Investigation
The wreckage was located the following day at 0250 by a search helicopter from the Federal Office of Civil Aviation. The investigation began at 0830 in collaboration with the Cantonal Police of Schwyz, the examining magistrate, the Institute of Legal Medicine Zurich, and the Scientific Service of the Zurich City Police. The helicopter lay with about 30° sideslip in approximately 2 m of snow, with the cockpit bent downward and the tail boom largely intact. All five rotor blades were severely deformed; one was found 20 m downhill, and two were embedded in rocky ground. Technical examination, including fraktographic analysis of the fractured freewheel coupling by the Swiss Federal Laboratories for Materials Testing and Research (EMPA) and engine inspection by Rolls-Royce Allison at MTU Munich, found no pre-impact mechanical failures. The engine exhibited evidence of operation at the time of impact. The helicopter was equipped for VFR day operations with no flight recorder or artificial horizon. The emergency locator transmitter antenna was buried in snow, producing a weak signal.
Medical Findings
Autopsy of the pilot and inspection of the three other occupants revealed severe traumatic injuries consistent with sudden, blunt force trauma from the front, with no signs of protective actions, suggesting a sudden and unexpected event. Toxicology screening found no alcohol, medications, or drugs in the pilot. No medical cause for the accident was identified.
Probable Cause
The investigation did not assign a specific probable cause. Technical examination indicated that no mechanical defect likely contributed, and the impact signature suggested a controlled horizontal flight path into terrain.