What happened
On 29 March 2000, a Jetstream 4100, registration G-MAJD, was conducting a post-maintenance test flight at Manchester Aerodrome (Woodford). The aircraft was undergoing a wing splice replacement as part of a scheduled service bulletin. During the flight, the pilot attempted to adjust the aileron trim to correct a roll bias. However, the application of the trim instead worsened the roll. Upon realizing the trim was operating in the wrong direction, the pilot reversed the input, stabilized the aircraft, and returned to the airfield for a safe landing. No injuries were reported, and no damage was sustained by the aircraft.
The investigation
An investigation by the AAIB revealed that the ailerly trim cables had been incorrectly routed around a pair of co-axial pulleys at the wing root. This error caused the connections to the trim screw jack chain to be reversed. The investigation found that the technician responsible for reinstalling the cables did not use specific instructions, instead relying on what appeared to be the natural routing. Furthermore, the mechanical design feature intended to prevent such misassembly—differing nipple and block sizes—proved ineffective in this instance.
Maintenance oversight was also a significant factor. The project leader, who was managing multiple responsibilities and working long hours, performed the functional checks alone. During this process, he failed to detect that the trim movement was reversed. Additionally, the contract engineer failed to perform a required duplicate functional inspection, and the maintenance documentation was found to be fragmented, incomplete, and lacking clear instructions for the specific task of cable installation.
Findings
- The primary cause was the incorrect routing of aileron trim cables around the wing root pulleys.
- The technician relied on visual estimation rather than following technical instructions.
- The mechanical safeguards designed to prevent incorrect assembly were ineffective.
- Inspection failures occurred because the project leader did not recognize the reversed direction of movement during functional testing.
- Maintenance documentation was deficient, lacking specific instructions for cable reconnection and using consolidated work cards that obscured individual task accountability.