What happened
During a flight phase where the aircraft had climbed to and leveled off at an altitude of 37,000 feet, the crew operating the DC-10-30F observed an anomaly in the cockpit controls. Specifically, the yaw control wheels for the captain and the first officer moved laterally independent of each other by approximately two inches. Despite this mechanical discrepancy, the flight continued without incident, resulting in an unscheduled but uneventful landing at Dulles International Airport.
The investigation
Following the arrival at the airport, maintenance personnel examined the aircraft's control systems. Inspections revealed that the left aileron cable tension regulator assembly had experienced a decrease in cable tension. Technicians applied lubrication to the unit, which caused the mechanism to jump to a new position. This adjustment successfully restored proper cable tension between the control wheel and the aileron control loop, resolving the issue.
Findings
The investigation determined that the root cause was mechanical wear or friction within the tension regulator assembly that necessitated maintenance intervention. In response to this finding, the operator revised their maintenance protocols. The lubrication schedule for these assemblies was changed from an 'on condition' basis to a fixed interval of every 2,500 hours, which equates to more than once a year.