What happened
On November 11, 2015, a Robinson R66 helicopter, registration PR-EDL, was performing a transport flight from Recife to a power plant in Sirinhaém, Brazil. Approximately five minutes before reaching the destination, the pilot experienced a loud noise and intense vibrations that compromised aircraft control. The pilot immediately initiated an autorotation procedure to perform an emergency landing in an unprepared field at the Trapiche Power Plant.
During the landing sequence, the pilot applied aft cyclic control to prevent the aircraft from striking an obstacle. This maneuver caused the main rotor blades to strike the tail cone, resulting in the sectioning of the tail cone and substantial damage to the vertical and horizontal stabilizers, the tail rotor drive shaft assembly, and the tail rotor gearbox. Despite the significant damage to the airframe, the pilot and passenger were unharmed.
The investigation
CENIPA investigators examined the wreckage and identified a rupture in the shaft weldment. Laboratory analysis conducted by the Aeronautics’ Science and Technology Department (DCTA) revealed that the power axis fracture occurred due to a torsional fatigue mechanism. The analysis found evidence of corrosion near the fracture site, suggesting that corrosion pits likely facilitated the initiation of the fatigue process.
Further investigation into the manufacturing process revealed that the shaft weldment was part of a specific production batch that lacked adequate anti-corrosive protection inside the component. Additionally, investigators noted that the 100-hour inspection procedures lacked sufficient detail regarding the identification of corrosion points on the weldment shaft, particularly in the difficult-to-access areas near the firewall.
Findings
- Manufacturing defect: The shaft weldment was part of a batch produced without the necessary internal anti-corrosion treatment.
- Component failure: The failure was caused by torsional fatigue initiated by corrosion pits on the shaft weldment.
- Control inputs: The application of aft cyclic during touchdown was the decisive factor that caused the main rotor blades to strike the tail cone.
- Maintenance and oversight: While the 100-hour inspection had been performed, the difficulty in accessing the component and a lack of detailed inspection criteria contributed to the failure to identify the corrosion.
Safety action
CENIPA issued several recommendations to the Brazilian National Civil Aviation Agency (ANAC) to improve inspection criteria for the R66 shaft weldment, ensure proper maintenance compliance, and enhance management supervision of technical inspections. The investigation also noted that the manufacturer had subsequently issued Service Bulletins (SB-16 and SB-17) to address the corrosion and replacement of affected shaft weldments.