Incident Overview
During takeoff, the flight crew reported that significant aileron and rudder input was required to maintain straight and level flight. When aileron input was reduced to zero, at least five units of left rudder trim were needed to keep the wings level. The crew decided to return to the departure airport, where the aircraft landed without further incident.
Postflight Findings
A postflight examination revealed that the ABSB-4 right aileron bus cable was broken, and the ABSA-4 right aileron bus cable was frayed. Both failures occurred where the cables passed over the right wing/body joint aileron pulleys.
Laboratory Analysis
The cables were sent to the NTSB materials laboratory for analysis. Examiners concluded that the ABSB cable failed due to severe internal wear, which resulted from the normal movement of the cable over a pulley over an extended period. The inspection also identified several areas of severe external wear on the cables.
Maintenance Background
The aircraft's maintenance manual specifies that exposed cables should be inspected at each 'C' check, which occurs approximately once per year. The manual includes procedures for checking cables for broken wires and measuring the extent of external wear. An 'optional as needed' step also recommends cable replacement based on a reduction in the cable's cross-sectional area.
Approximately six weeks before the incident, the aircraft had undergone a segmented 'C' check that included inspection of the aileron cables.