What happened
On 22 August 1999, a Boeing 747-136, registration G-AWNF, was operating a passenger flight as BA 202 when the crew encountered significant flight control difficulties. While approaching a waypoint at 54° N 020° W, the aircraft failed to follow its programmed track. Although the crew verified all navigation settings, they noted the Flight Director was requesting a left turn, yet the aircraft would not respond to heading mode selections. Upon disengaging the autopilot, the crew discovered the roll flight controls were completely seized.
After using extreme force to free the controls, the aircraft briefly regained its intended track. However, as the flight approached the English coast, the seizure recurred and intensified. Suspecting ice buildup on the control runs, the pilot requested a lower altitude. Despite the descent to 11,000 feet, the difficulty in moving the controls persisted, prompting the crew to declare a PAN to London ATC. The aircraft was subsequently directed to land at Heathrow, where it completed a normal landing without zero injuries or further damage.
The investigation
Following the landing, investigators discovered approximately 2 gallons of water had drained from the area between the canted pressure deck and the upper surface of the wing centre section. This water had frozen, causing the aileron cables to jam. The investigation established that the water originated from a leak in a galley drain, which had seeped under the floor and accumulated in the wing area.
It was noted that a nearly identical incident had occurred on G-AWNF only five days prior. Furthermore, the operator had experienced 14 previous instances of water accumulation in this specific area. While Boeing had issued a Service Bulletin (SB 747-51 2044) to address this, the operator had implemented an enhanced engineering order involving additional drains on their B747-200 fleet. However, the operator had chosen not to apply this specific engineering order to this Boeing 747-136 because the aircraft was nearing retirement.