What happened
On November 27, 2011, a Eurocopter BO-105S, registration PR-MEW, was performing seismic survey operations near Carauari, Amazonas. After a series of shuttle flights between various clearings, the aircraft was cruising at 1,000 feet and transitioning to a cruise speed of approximately 100 knots when the pilot experienced a violent impact and heavy vibrations.
The pilot felt a strong strike against the tail rotor, accompanied by changes in heading, altitude, and stiffening of the flight controls. The pilot immediately decided to return to the Carauari airfield, where the aircraft landed safely. Upon inspection, it was discovered that the upper left engine cowling had detached in flight, striking the horizontal stabilizer and the tail rotor blades.
The investigation
The investigation by CENIPA focused on why the cowling became unsecured. It was noted that prior to the first flight of the day, an external audit had required the opening of the engine cowlings for visual inspection. A mechanic had subsequently closed the cowlings and released the aircraft for service.
Investigators found that while there was no physical evidence of improper closure, two DZUS fasteners remained unsecured on the right side of the cowling, which would have failed to properly lock the left side. The investigation also noted that during several intermediate stops, the pilot performed refueling and passenger transfers without shutting down the engines. This practice prevented the opportunity to conduct a thorough inspection between flight legs.
Furthermore, the investigation examined the operational environment. The operator's infrastructure in Carauari was found to be inadequate, with maintenance supplies and crew housing sharing a single residential building. This lack of proper facilities and the resulting potential for crew fatigue and interpersonal conflict were identified as factors that could have led to oversight during maintenance or pre-flight inspections.
Findings
- The detachment of the left engine cowling caused structural damage to the fuselage and horizontal stabilizer, as well as mass loss from the tail rotor blades.
- Incomplete closure of the engine cowling fasteners was the primary cause of the component detachment.
- The decision to perform refueling and passenger boarding without shutting down the engines precluded a secondary inspection of the cowling.
- Inadequate maintenance and crew infrastructure may have contributed to a lack of attention to detail during maintenance and pre-flight procedures.