Incident Description
On 22 August 1999, a Boeing 747-136 (registration G-AWNF) was in flight at approximately 0600 UTC while operating as a public transport passenger flight. Approaching a waypoint at 54°N 020°W, the flight crew performed all relevant company drills, but the aircraft did not turn onto the next track towards 54° N 015° W as expected. Navigation selections were double-checked and found normal except that the Flight Director displayed a fly left demand. Heading mode was selected but had no effect.
Crew Actions
The autopilot was disengaged, but the roll flight controls were 'seized solid'. The checklist was consulted, and extreme force was used to free the roll controls. The crew advised ATC of the situation and informed them that the aircraft was 10 nm south of track but regaining the intended track. Control appeared normal for a time, but the problem re-occurred approaching the English coast. The same drill was carried out, and the captain requested a lower level, suspecting ice on the control runs. The problem persisted and worsened as roll controls became more difficult to move. A total freeze seemed imminent. The crew informed ATC and issued a PAN call to London ATC. London ATC directed the aircraft straight into Heathrow. At 11,000 feet, the problem eased, but the captain elected to continue with the PAN in case the improvement was temporary. A normal landing was made.
Post-Flight Findings
After landing, approximately 2 gallons of water was drained from the area between the canted pressure deck and the top surface of the wing centre section. Water had apparently frozen and jammed the aileron cables. The water entered from a leak in one of the galley drains, which seeped under the floor and accumulated, forming ice at altitude. The leak was repaired; cables were sprayed with a water displacement fluid, re-greased, and the floor resealed.
Previous Occurrence
A similar incident had occurred on G-AWNF just five days earlier, on 17 August 1999. The aircraft had departed after heavy rain, which was thought to have accumulated on the canted pressure deck, so the drains were cleared. The problem of water accumulation in this area had been experienced on 14 previous occasions by the operator and was addressed by Boeing through Service Bulletin SB 747-51 2044 (Revision 1). The operator used the bulletin to develop an engineering order that installed more drains than required. This order was embodied on all of the company's B747-200s by mid-1998 and was expected to be completed on later aircraft (including B747-400s) by July 2001. The operator decided not to embody the engineering order on their B747-136 aircraft as their retirement was anticipated.