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Approximately 90 minutes into the flight at FL350, the crew received autopilot caution alerts while responding to an air traffic control heading instruction. The aircraft failed to turn as commanded, and subsequent attempts using an alternate autopilot channel were also unsuccessful.
When the autopilot was disengaged, the crew identified that the control wheel had little to no movement in roll, indicating a control restriction.
Pitch and yaw control remained available, allowing the aircraft to remain stable.
A MAYDAY was declared, and the aircraft descended to FL200. After reviewing the Quick Reference Handbook (QRH) procedures, both pilots applied significant, coordinated force to the control column, which successfully released the restriction and restored full roll control. The flight continued uneventfully to destination.
Flight data indicated a progressive asymmetry in aileron movement prior to the event, consistent with increasing restriction within the system. The investigation concluded that the most likely cause was ice formation somewhere within the aileron control system, restricting movement. In this case, wet cargo loading conditions and very low cruise temperatures may have provided favourable conditions for ice formation, although no definitive source or location was identified.
The safe outcome was primarily attributable to measured decision making, strong crew coordination, and adherence to established procedures.
The operator carried out several post event safety actions.
Photo: Dltl2010 Wikimedia Creative Commons 1.0