What happened
On August 17, 2005, a EUROCOPTER AS 350 B3 helicopter, registration LV-ALN, was engaged in aerial firefighting operations near Lobios, Ourense. While approaching the Lindoso reservoir to refill its water bucket, the pilot observed the engine chip indicator illuminate. Immediately following this warning, the pilot experienced a loss of lift and a sudden low rotor RPM warning.
In an attempt to manage the emergency, the pilot initiated an emergency landing in a nearby field. During the descent, the aircraft's nose turned left, and the tail and right skids struck the ground first. A main rotor blade struck the tail cone, causing the helicopter to overturn on its left side. The pilot, who sustained minor injuries, was able to exit the aircraft through the top. The helicopter suffered major damage to its fuselage, landing gear, and dynamic components.
The investigation
The CIAIAC investigation focused on the engine's mechanical state and the maintenance history of the Turbomeca Arriel 2B engine. Investigators discovered that the engine's M01 module had been showing signs of degradation days before the accident. Specifically, on August 7 and August 15, 2005, the engine chip warning light had illuminated, and metallic particles were found in the magnetic plug.
Detailed disassembly of the engine revealed that the driven bevel gear in Module M01 had fractured. This failure was traced back to the heavy degradation of the drive gear upper thrust bearing, which caused excessive axial movement and a subsequent secondary bending moment that led to the gear's fatigue and eventual breakage. This mechanical failure caused a decoupling between the gas generator and the accessory box, which interrupted the fuel pump and led to the in-flight engine stoppage.
Findings
- The primary cause of the accident was the in-flight failure of the engine due to the breakage of a gear in the module connecting the drive shaft to the accessory box.
- The operator failed to follow the manufacturer's maintenance manual procedures after the appearance of metallic particles and chip warnings.
- Maintenance records for the aircraft and engine did not accurately reflect the recent engine chip warnings or the presence of particles in the magnetic plugs.
- The degradation of the drive gear upper thrust bearing was the root cause of the gear fracture, though the exact origin of the bearing's degradation (assembly shock, contamination, or early fatigue) could not be determined.
- The maintenance and operational oversight by the leasing company was insufficient to ensure the aircraft's airworthiness following these anomalies.