What happened
On 12 December 2002, a BAe 1s6-200, registration G-JEAX, was operating a passenger flight from Birmingham to Belfast City Airport. Prior to departure from Birmingham, the aircraft had been treated with undiluted Type II+ de-icing fluid. Although snow was present on the ground, the crew decided de-icing was unnecessary as the snow was not accumulating on the airframe.
During the climb, the flight crew observed unusual pitch oscillations while the autopilot was engaged. Upon reaching flight level 240, the crew transitioned to manual flight, but the aircraft began to deviate from its assigned altitude. The commander disengaged the autopilot and encountered a significant pitch-up tendency, requiring heavy forward pressure and electric elevator trim to maintain control. As the aircraft pitched down, the commander attempted to counter the descent with rearward force, noting that the controls felt extremely heavy.
During this period of instability, the aircraft experienced a violent shudder. The sudden movement caused two cabin crew members to fall in the aisle, resulting in two serious injuries (one broken leg and one fractured ankle), and one passenger and one cabin attendant sustained minor head injuries. The crew eventually stabilized the aircraft at flight level 240 and completed a safe landing at Belfast City Airport.
The investigation
The AAIB examined the aircraft's flight data recorder, which revealed that elevator trim inputs were cycling every 12 to 15 seconds during the climb. The data showed that after the autopilot was disconnected, the elevators moved rapidly to a nose-up position, causing the aircraft to experience a vertical acceleration of approximately +3g.
Physical inspections of the elevator control system revealed traces of de-icing fluid residue in the gaps between the elevators and the servo tabs. While this residue had a low freezing point, the investigation also focused on the mechanical components of the control system. Investigators discovered that the bearings in the rod ends attaching to the servo tabs were extremely stiff and lacked lubrication, containing only a dry, powdery residue.
Findings
- The aircraft experienced severe pitch oscillations and heavy control forces during the climb phase.
- The stiffness in the elevator servo tabs was a primary factor, potentially caused by either natural icing or the freezing of de-icing fluid residues.
- The bearings in the rod ends attaching to the servo tabs were found to be devoid of grease and were operating with significant stiffness.
- The presence of moisture ingress into the bearing cage void may have caused the bearings to seize when frozen during flight.