Accident Overview
On 12 August 1999, a Eurocopter AS350B2 (registration HB-XVM) operated by Heli Rezia SA departed Ambri aerodrome for timber transport operations in the Disentis region. On board were a pilot and two flight assistants. Shortly after 10:00 local time, while cruising near Lake Ritom, the crew heard a loud detonation followed by vibrations and a loss of engine power.
Emergency Landing
The pilot conducted an autorotation and landed the helicopter in an alpine meadow. During touchdown, the helicopter rotated about 90 degrees to the right, damaging the landing gear. The rotor stopped immediately after landing, and the pilot noted the engine was already stopped. The emergency locator transmitter had activated but was switched off by the pilot as no injuries occurred and telephone contact with a nearby municipality was possible.
Crew and Aircraft
The pilot held a professional helicopter pilot license with endorsements for the AS350 and other types, with 4570 total flight hours. The two flight assistants were not injured; all crew members wore helmets, and the assistants were not restrained. The helicopter had accumulated 3553 hours at the time of the accident and was equipped with a Turbomeca Arriel 1D1 turboshaft engine. The engine had accumulated 2260 hours and 8349 gas generator cycles at the time of the accident, with a life limit of 14000 cycles. Maintenance records showed compliance with airworthiness directives. A police alcohol test on the pilot was negative.
Weather Conditions
Weather conditions at the time included scattered clouds with bases around 2200 m and 4000 m, visibility 25 km, southeast wind 5-10 knots, temperature 8°C, dew point 6°C, and QNH 1016 hPa.
Investigation Findings
Examination of the engine revealed extensive damage. Laboratory analysis identified a fracture in one blade of the radial compressor. The fracture occurred in two stages: an initial progressive fatigue crack caused by vibrations from acoustic resonance within the compressor, followed by a final catastrophic fracture due to centrifugal forces. No foreign object debris was found. The main transmission showed no anomalies.
Probable Cause
The accident was caused by an engine failure resulting from a blade fracture in the radial compressor.