Incident Details
After departure, the captain reported that the first officer needed excessive right wing down trim to maintain a wings level attitude. The left outboard roll spoiler indicated approximately one-half deployment on the Powered Flight Control Surfaces (PFCS). The crew maintained control of the airplane with trim and control wheel inputs, declared an emergency, returned to the departure airport, and landed without further incident.
Post-Incident Inspection
A postincident inspection revealed that the left outboard roll spoiler was deployed approximately halfway. Examination and disassembly of the roll spoiler servoactuator disclosed that a plug (part number A50993-1), which had been fusion-welded into the base of the piston (part number A50992-1), had separated and lodged itself between the base of the piston and the shims, restricting the piston's movement.
Manufacturer History
Dowty Aerospace, the manufacturer of the roll spoiler servoactuator, had recorded seven previous failures of fusion welds in pistons on DHC-8 roll spoiler servoactuators before this incident.