What happened
On June 25, 2011, an ATR 72-212A, registration F-OIQU, operated by Air Tahiti, was performing a scheduled passenger flight from Papeete, French Polynesia. During the initial climb phase at approximately 170 knots, the pilot in command noticed the slip indicator ball had moved to the far right limit.
In an attempt to correct the imbalance, the pilot adjusted the rudder trim to its limit. While this partially centered the ball, the flight crew remained concerned about the aircraft's controllability, particularly in the event of a failure of engine number one. Fearing a loss of directional control, the crew decided to abort the flight and returned to the departure aerodrome. During the approach and landing, the crew noted that the slip decreased as airspeed was reduced, and the aircraft landed safely without further incident.
The investigation
Post-flight maintenance inspections revealed that the rudder trim tab control linkage was disconnected. The investigation traced the error back to a heavy maintenance visit occurring between May 3/May 30 and June 24, 2011.
During this period, corrosion was discovered on the rudder torque tube, necessitating additional work. A technician was assigned to remove the lower portion of the torque tube. During this process, the technician disconnected the rudder trim tab control linkage. However, because the maintenance software only allowed for recording the start and end of a task rather than individual steps, the disconnection was not documented.
Furthermore, the technician responsible for the subsequent reinstallation only focused on the lower portion of the torque tube and did not perform the reconnection of the trim tab linkage, which was part of the upper tube assembly. The inspector did not check the rudder area because the work order did not indicate that any work had been performed on the upper section of the torque tube. Additionally, the standard functional test of the rudder control failed to detect the issue because the test relied on monitoring the actuator position rather than a direct visual inspection of the trim tab's actual movement.
Findings
- The primary cause of the incident was the incomplete execution and verification of maintenance tasks, which resulted in the unrecorded disconnection of the rudder trim tab linkage.
- The airline's maintenance IT system contributed to the error by lacking the granularity to track specific sub-steps within a larger work order.
- A lack of individual task sign-offs by technicians and inspectors prevented the detection of the missed reconnection step.
- The functional test procedures were insufficient, as they monitored the trim actuator position rather than the physical deflection of the trim tab itself.