What happened
On February 20, 2011, an Agusta A109E, registration PR-SSK, was preparing for takeoff from a helipad in Angra dos Reis, Brazil. While the pilot was hovering in the parking area to reposition the aircraft, the pilot applied left pedal input. During this maneuver, the pilot noticed the pedal sinking and heard an abnormal noise. Simultaneously, the tail rotor lost efficiency due to components detaching from the drive shaft, causing the helicopter to enter a rapid, violent 180-degree turn to the right.
In response to the loss of directional control, the pilot immediately lowered the collective. The aircraft completed a 180-degree rotation before the landing gear made initial contact with the ground, followed by an additional 40-degree turn before coming to a complete stop. There were no injuries to the pilot or the two passengers on board, though the aircraft sustained minor damage to the tail rotor drive system.
The investigation
CENIPA investigators examined the tail rotor drive system, which consists of a drive shaft divided into three sections (Forward, Middle, and Long) and a 9-degree gearbox. The investigation focused on the Long Section of the shaft, where an adapter is fixed using adhesive to allow for rotational and longitudinal movement.
Investigators discovered that a previous repair to the aircraft's tail boom—conducted to address cracks found during an 800-hour inspection—had been performed using methods that deviated from the manufacturer's authorized instructions. Specifically, the repair involved replacing part of the sheet metal but did not follow the approved structural repair manual (SRM) procedures. Post-occurrence analysis revealed a 1mm misalignment in support nº 4 and significant discrepancies in the torque applied to support screws, which were approximately twice the required value.
Findings
- The primary cause of the tail rotor failure was the degradation of the adhesive used in the Long Section shaft adapter, which led to the detachment of components and the shearing of the safety pin.
- This adhesive failure was triggered by unforeseen stresses caused by the misalignment of the tail rotor drive axis.
- The misalignment was a direct result of improper tail boom repairs that did not adhere to manufacturer-authorized procedures.
- Inadequate maintenance management oversight contributed to the incident, as the alignment of supports following the tail boom repair was not properly recorded in the aircraft's service orders.
- The investigation also noted that the maintenance organization failed to account for potential hidden failures in the drive shaft during the tail boom repair process.