What happened
On 25 February 2000, a Boeing 747-436, registration G-BNLB, was operating a scheduled passenger flight from Orlando to London Gatwick. While cruising at FL370 over the North Atlantic, the flight crew observed vertical speed indications fluctuating on the primary flight displays. Initially, the crew attributed these movements to clear air turbulence, especially as passengers reported symptoms of airsickness.
As the flight progressed, the oscillations became more pronounced. The commander attempted to resolve the issue by cycling through all three available autopilot systems, but the pitch disturbances persisted at irregular intervals. When the crew attempted manual flight, they encountered a "notchy" and stiff control column, which led to overcontrolling the aircraft.
To ensure a safe arrival, the commander declared a 'PAN' status with Air Traffic Control to secure a priority approach. During the landing at Gatwick, the crew implemented the Emergency Landing Checklist as a precaution. Although the touchdown was smooth, smoke was observed coming from the number 4 engine during taxi, and a burning smell was noted in the cabin, which was later attributed to tyre smoke from a heavy braking event.
The investigation
Investigators analyzed data from the aircraft's flight recorders, which revealed that the pitch oscillations were present during several flights in the preceding 55 hours of operation. The investigation focused on the aircraft's pitch control system, which utilizes elevators and a horizontal stabilizer.
Initial mechanical inspections of the control surfaces and electronic modules yielded no defects. After replacing the stabilizer RVDT and flap computer, the problem persisted. Subsequent testing led investigators to examine the inboard elevator Power Control Units (PCUs).
Upon disassembly of the malfunctioning PCU, investigators discovered that the upper ball-bearing was almost entirely seized due to severe corrosion. The investigation established that condensation had penetrated the dust seals, washing lubricant away and allowing the bearings to corrode and eventually lock.