What happened
On 15 March 2011, an ATR 72-202, registration EI-SLG, was performing a non-revenue commercial flight near Edinburgh Airport. The aircraft had recently completed a heavy maintenance check at an engineering facility. During the flight, as the aircraft accelerated through 185 kt, the crew experienced an uncommanded roll to the left of approximately 5° to 10°.
The co-pilot noted that the rudder pedals felt "spongy" and that the aircraft lacked sufficient control authority to make right turns. Due to this restriction, the commander requested air traffic control to provide radar vectors for only left turns. The crew performed the required Quick Reference Handbook (QRH) procedures, attempting to restore control by switching the Rudder Travel Limitation Unit (TLU) to the low speed position. The aircraft eventually returned to Edinburgh, landing safely, though the rudder continued to show significant deflection upon touchdown.
The investigation
The investigation, conducted alongside Irish and French authorities, focused on the maintenance performed prior to the flight. It was discovered that during a recent replacement of the TLU actuator, a cam on the rudder T/LU mechanism had been removed and subsequently refitted in the incorrect orientation.
Investigators found that the maintenance engineer, working under significant time pressure and heavy workload, had been interrupted frequently while reassembling the mechanism. The production manager had also pressured the engineer to proceed with an unapproved and unrecorded repair to a TLU support arm to avoid delaying the aircraft's release. Crucially, the required independent inspection and functional tests of the TLU were not performed following the reassembly.
Findings
- The primary cause of the incident was the incorrect installation of a cam on the rudder TLU mechanism.
- The design of the TLU cams allowed for incorrect orientation without obvious physical resistance during assembly.
- Commercial pressure within the maintenance organisation led to the decision to perform unrecorded and unapproved maintenance tasks.
- Inadequate staffing levels and high workload contributed to the maintenance error and the failure to perform necessary inspections.
- The maintenance task instructions lacked specific warnings or markings to ensure the correct orientation of the cams.