What happened
On October 12, 2013, an AS-350B2 helicopter, registration PT-YSS, was conducting a wildfire monitoring mission in the state of Bahia, Brazil. The aircraft was operating in the Cotegipe region with a pilot and four passengers on board. Approximately 30 minutes into the flight, the pilot observed a metal chip warning light, followed by loud noises and a complete engine failure.
In an attempt to perform an emergency autorotation landing, the pilot encountered an obstacle (a termite mound) that forced a change in the approach path. This maneuver caused the aircraft to touch down 29 meters away from the intended site. During the final moments of the landing, the main rotor blades struck the tail boom, resulting in the severance of the tail rotor drive shaft. The aircraft sustained substantial damage, including impacts to the main rotor assembly, the tail boom, and the tail rotor drive shaft. All five occupants survived the incident without injuries.
The investigation
CENIPA's investigation focused on the cause of the engine failure and the subsequent structural damage. Investigators examined the engine components and found that seven of the thirteen axial compressor blades exhibited wear exceeding manufacturer limits. Additionally, a significant accumulation of laterite (soil/clay) was found inside the hollow shaft of the gas generator module 3. The mass of the detached laterite alone was 8.42 grams, which exceeded the 8-gram limit established by the manufacturer.
The investigation established that the accumulation and subsequent detachment of this material caused an imbalance in the rotating assembly of the module 3. This imbalance generated vibrations that damaged the gas generator bearing, ultimately leading to the unexpected engine shutdown. The investigation also noted that the aircraft had been operating in a dusty environment, which necessitated specific maintenance considerations.
Findings
- Inadequate maintenance monitoring: The maintenance program failed to properly monitor the wear of the axial compressor blade leading edges and the accumulation of laterite within the gas generator shaft.
- Deficient technical supervision: There was a failure to strictly adhere to Service Bulletin A292 72 0230, version C, regarding engine maintenance for Arriel 1 series engines.
- Organizational weaknesses: The operator failed to provide the pilot with established Standard Operating Procedures (SOPs) specific to the type of operation being performed.
- Operational environment: The maintenance program did not adequately account for the high-dust environment in which the aircraft was operating.