What happened
On August 12, 1999, an AS350B2 helicopter, registration HB-XVM, was performing timber transport operations for Heli-Rezia between Ambri and the Disentis region. Shortly after departing Ambri, while flying near the Ritom Lake area, the crew experienced a loud detonation accompanied by intense vibrations throughout the aircraft.
Following the sudden loss of engine power, the pilot executed a successful autorotation, landing the aircraft in an alpine meadow. During the landing maneuver, the helicopter rotated approximately 90 degrees to the right, which resulted in damage to the landing skids. There were no injuries among the three crew members on board.
The investigation
The investigation conducted by the Swiss Transportation Safety Investigation Board (SUST) focused on the engine's condition and the mechanical failure. Laboratory analysis of the engine revealed that a blade in the radial compressor had fractured. The failure occurred in two distinct stages: first, a progressive fatigue crack developed due to vibrations caused by acoustic resonance within the compressor; second, a sudden fracture occurred due to centrifugal forces acting on the weakened blade.
Investigators also examined the maintenance records and the definition of operational cycles. While the engine was within its authorized operating limits, the investigation noted discrepancies in how the operator recorded rotations, landings, and engine starts in the logbook. There was a lack of clarity regarding how "cycles" were calculated for both engine and airframe maintenance.
Findings
- The primary cause of the accident was an engine failure resulting from a centrifugal compressor blade fracture.
- The failure was driven by acoustic resonance within the compressor, which induced fatigue vibrations.
- The engine failure was a known issue within the industry, with the manufacturer, Turbomeca, having previously issued several service letters regarding similar occurrences in Arriel 1 series engines.
- Discrepancies were identified in the maintenance logging, specifically regarding the inconsistent use and definition of the term "cycle" for engine versus airframe maintenance.
Safety action
- The investigation issued a safety recommendation for the Federal Office of Civil Aviation (FOCA) to ensure the term "cycle" is defined and used homogeneously across both engine and airframe maintenance practices.
- A recommendation was also made for FOCA to evaluate the necessity of mandating the installation of advanced flight and engine parameter monitoring systems.