What happened
On 4 October 1999, a BAe 146-300, registration G-UKID, was performing a scheduled public transport flight from Aberdeen to Amsterdam. During the climb through approximately 11,000 feet, the crew attempted to engage the autopilot, which resulted in an immediate nose-down pitch and a left roll. Upon disengaging the system, the crew noted that the aileron controls were becoming increasingly difficult to move, eventually reaching a state where they felt 'locked solid'.
To avoid a violent roll, the commander opted to use aileron trim for lateral control rather than applying heavy force to break the jam. Suspecting that moisture from overnight rain had frozen within the control runs, the crew requested a diversion to Stansted and descended to 5,000 feet. As the aircraft reached 2,500 feet, the control stiffness gradually subsided. The crew performed a low-speed handling check and completed a safe landing at Stansted with no injuries to the 5 crew members or 94 passengers.
The investigation
The AAIB examined the aircraft's flight data recorder, which confirmed the uncommanded rolls upon autopilot engagement. Subsequent inspections of the aircraft in a heated hangar found no evidence of a mechanical jam. While an avionics engineer identified a fault in the autopilot computer, this was deemed unrelated to the control stiffness.
Further investigation focused on the aileron components. Although the aileron servo motor was replaced, the problem persisted in subsequent flights. Detailed disassembly of the aileron structure eventually revealed that the back-to-back bearing assembly on the upstream end of the torsion bar was operating with significant resistance. The investigation found that the retaining plate was not sufficiently sealed, allowing external contaminants, specifically water, to enter the bearing area. This ingress led to the washing out of grease and the accumulation of solid residues and corrosion.
Findings
- The primary cause of the control stiffness was degraded bearings within the aileron torsion bar assembly.
- The bearing housing was susceptible to water ingress from rain, which caused the grease to wash out and left solid residues that hindered smooth operation.
- The formation of ice crystals within the contaminated bearing area during the climb likely exacerbated the rolling resistance.
- The crew's decision to use aileron trim was a measured response to avoid a sudden, uncontrolled roll, though the aircraft's manual provided conflicting guidance regarding the 'break-out' of the lateral disconnect.
- Inconsistency in the terminology used in the aircraft's checklists regarding the aileron disconnect system may have contributed to crew confusion during the incident.