What happened
On 15 March 2011, an ATR 72-202, registration EI-SLG, was conducting a non-revenue commercial flight near Edinburgh Airport. After departing from Runway 24, the aircraft climbed to FL 230. As the aircraft accelerated through approximately 185 kt, the crew experienced a sudden roll to the left of about 5 to 10 degrees. The co-pilot noted that the rudder felt "spongy" and that the aircraft was difficult to control, with the slip ball and rudder trim both indicating a full right deflection.
In response to the flight control issues, the crew manually adjusted the Travel Limitation Unit (TLU) switch to the LO SPD position. While this reduced the roll difficulty, the green LO SPD indicator light failed to illuminate. During the approach to Runway 24, the aircraft required significant control inputs to maintain heading. Upon landing, the aircraft drifted left of the runway centerline, and the application of reverse thrust caused the aircraft to diverge further towards the left side of the runway. No injuries were reported, and there was no damage to the aircraft.
The investigation
The investigation, conducted by the AAIB with assistance from the French BEA, focused on the mechanical state of the rudder system. Investigators found that the aircraft had recently undergone routine maintenance at an engineering facility in Edinburgh, which included work on the rudder system.
Testing revealed that the right-hand cam of the TLU had been installed in the incorrect orientation (rotated 180 degrees). Because the master locating spline on the shaft is offset, the cam could be fitted backwards without preventing assembly. This incorrect orientation caused the right-hand roller to encounter resistance from the upper lobe of the cam rather than sliding into the intended groove. This mechanical interference resulted in uncommanded rudder input when the TLU mechanism attempted to transition between authority positions.
Findings
- The right-hand cam on the TLU was installed 180 degrees from its correct orientation during recent maintenance.
- The incorrect installation caused an asymmetric restriction in rudder travel and uncommanded deflection during speed changes.
- The Aircraft Maintenance Manual (AMM) task for the TLU assembly did not warn that the cams could be installed in the wrong direction.
- Neither an independent inspection nor an operational test of the TLU system was performed following the maintenance work to detect the error.