What happened
On October 1, 2000, a Canadair CL-600-2B19 departed Düsseldorf for a scheduled flight to Gothenburg, Sweden, carrying 43 passengers and 4 crew members. During the climb to flight level 260, the crew noticed an autopilot warning indicating a right trim imbalance, accompanied by a mistrim flag on the primary flight display. Upon disabling the autopilot to investigate, the crew found the aircraft was not actually trimmed to the right during level flight; however, re-engaging the autopilot triggered the same warning.
Following established abnormal procedures, the crew continued the flight using manual controls. During a right turn directed by Air Traffic Control, the crew discovered that the right aileron was significantly restricted, making the controls difficult to move. The electronic flight instrument system indicated that while the left aileron functioned normally, the right side was not deflecting. Before the crew could execute the jammed aileron procedure, a sudden movement released the right aileron, and the autopilot became functional again. As a precaution, the commander elected to divert the flight to Cologne-Bonn for an immediate landing.
The investigation
The BFU investigation was conducted without the use of the flight data recorder, as the operator had not removed it following the event. Post-flight inspections and functional tests of the aileron system by the operator's technicians yielded no defects. The investigation noted that the aircraft had departed from a wet runway with standing water and encountered moderate icing conditions during the climb, with the ice detector activating at approximately flight level 80.
While the aircraft had been previously retrofitted with a splash shield in the main landing gear bay to protect the aileron components from slush and snow, the investigation looked into whether this protection was sufficient. The BFU also coordinated with the Canadian Transportation Safety Board, noting that similar aileron stiffness issues had been reported in the United States earlier that year, which had led to a Canadian Airworthiness Directive (AD).
Findings
- The investigation determined that the primary cause of the malfunction was the freezing of the aileron pulleys and control cables within the main landing gear bay.
- During takeoff, water from puddles on the wet runway accumulated in the gear bay area.
- As the aircraft climbed through freezing temperatures, this accumulated water froze, causing a temporary mechanical blockage of the right aileron.
- The existing splash shield, installed via a previous service bulletin, proved insufficient to prevent water ingress and subsequent freezing in these specific weather conditions.