What happened
On August 7, 2009, an AS350BA helicopter, registration PP-FJC, was performing a vertical takeoff from the Civil Police helipad in Brasília, Brazil. During the initial phase of a normal takeoff, the crew heard two abnormal noises, followed by a sudden yaw to the right and the activation of the low rotor RPM warning alarm.
As the aircraft lost altitude, the tail rotor struck a spotlight, and the tail boom collided with a wire fence. The helicopter continued to lose height while rotating, eventually striking the ground approximately 18/0 degrees from its original heading. Despite the substantial damage to the engine, airframe, stabilizers, and tail rotor, the two pilots and one passenger escaped the accident without injuries.
The investigation
CENIPA investigators examined the engine and transmission components to determine the cause of the power loss. The investigation focused on the connection between the Turbomeca engine (GTM) and the main gearbox (CTP).
Technical analysis revealed that the connection between the engine and the main gearbox had failed. Specifically, the three bolts securing the flexible coupling to the transmission shaft had fractured. While the engine was still operating normally and transmitting power to the tail rotor, the mechanical link to the main rotor was severed.
Testing by the Department of Science and Technology confirmed that the bolts failed due to overload at the safety wire holes. Investigators found that the castellated nuts had unscrewed to a point where the bolt's cross-section was significantly reduced, creating a stress concentration that led to the fracture. Crucially, no traces of safety wire were found at the connection, indicating that the nuts had been installed without proper locking.
Findings
- Inadequate maintenance was the primary contributing factor, as technicians failed to install safety wires on three bolts connecting the flexible coupling to the transmission shaft.
- The absence of safety wiring allowed the nuts to gradually loosen until the bolts reached a high-stress area, resulting in a rupture by overload.
- The resulting collapse of the coupling prevented the transmission of power and rotation from the engine to the main rotor blades.
- The sudden increase in collective pitch during takeoff accelerated the decay of rotor RPM once the power link was lost.