What happened
On 15 March 2011, an ATR 72-202, registration EI-SLG, was conducting a non-revenue commercial flight near Edinburgh Airport. During the climb to FL 230, the crew noticed the aircraft rolling to the left by approximately 5 to 10 degrees as the speed increased through 185 kt.
As the flight progressed, the co-pilot attempted to correct the sideslip by applying right rudder and aileron. However, the rudder felt unusually spongy and failed to respond effectively to inputs. The crew eventually declared a PAN and requested a return to Edinburgh. During the procedure to troubleshoot the issue, the crew manually adjusted the Travel Limitation Unit (TLU) switch, which caused the aircraft to temporarily lose roll control authority.
Upon approaching Runway 24, the co-pilot had to use both hands on the control wheel to maintain directional control. The aircraft landed just left of the runway centerline and drifted toward the left side of the runway during the rollout. No injuries were reported, and there was no damage to the aircraft.
The investigation
The investigation, conducted in collaboration with Irish and French authorities, focused on the rudder Travel Limitation Unit (TLU). This system is designed to mechanically limit rudder deflection at higher speeds to prevent structural stress.
Investigators examined recent maintenance performed at Edinburgh Airport, which had involved the disassembly of the TLU mechanism. It was discovered that during reassembly, the right-hand cam had been installed in the wrong orientation. Because of a specific offset in the cam's design, the incorrect installation allowed the cam to be fitted onto the splined shaft, but it prevented the rollers from properly engaging the vee grooves. This misalignment caused an uncommanded rudder input and physical resistance when the TLU activated at high speeds.
Findings
- The primary cause of the incident was the incorrect orientation of the right-hand cam during maintenance reassembly.
- The maintenance task did not include a warning that the cams could be installed incorrectly.
- An operational test of the TLU system was not performed following the maintenance work.
- The existing maintenance manual did not specify a required duration for the TLU operational test button depression.
- The incorrect cam installation caused an asymmetric restriction in rudder travel, leading to the observed control difficulties.