What happened
On September 26, 2011, a Bell 206L-4 helicopter, registration PR-HLB, departed from the Itaituba aerodrome in Pará, Brazil, bound for Uruá. The aircraft was operated by IBAMA and carried three crew members. Approximately three minutes after takeoff, the crew heard a loud noise originating from the tail cone area.
Following the noise, the pilot experienced a loss of tail rotor effectiveness. In an attempt to return to the aerodrome for a running landing, the aircraft lost airspeed and began an uncontrolled vertical axis rotation. The helicopter subsequently collided with buildings, including a church and a residence, before impacting the ground. While the aircraft sustained substantial damage, all three crew members survived the accident without injuries.
The investigation
CENIPA investigators examined the wreckage and recovered a sheared portion of the tail rotor drive shaft assembly approximately 300 meters from the main debris field. The investigation focused on the mechanical integrity of the drive shaft and the maintenance history of the aircraft.
Investigators found that a bolt in the drive shaft assembly had exited its position, which subsequently caused the second bolt to fail due to overload. This failure broke the connection between the first and second shafts via the Thomas Coupling system. The investigation also reviewed the maintenance logs, noting that the aircraft had undergone several inspections at different secondary bases. The investigation revealed that a task involving the disassembly of the tail rotor drive shaft had been performed outside of the manufacturer's prescribed periodicity.
Findings
- Mechanical failure of the tail rotor drive shaft caused the loss of directional control.
- The disassembly of the drive shaft was performed outside of the manufacturer's recommended intervals.
- Evidence suggested the bolt securing the assembly may have failed due to an ineffective locking system, potentially caused by incorrect torque application or the reuse of coupling nuts.
- Inadequate management oversight by the operator (IBAMA) allowed for a maintenance model that lacked sufficient supervision of outsourced maintenance services.
- Deficiencies in maintenance supervision by the operator (Helisul Táxi Aéreo Ltda.) meant that unscheduled tasks were performed during inspections without proper oversight.
- Possible improper application of flight controls during the emergency may have contributed to the loss of control.