What happened
Following departure, the captain reported that the first officer was required to use excessive right wing down trim to keep the wings level. The crew observed that the left outboard roll spoiler indicated approximately one-half deployment on the Powered Flight Controlled Surfaces (PFCS) display.
Despite the asymmetric lift, the crew maintained control of the DHC-8 using control wheel inputs and trim adjustments. The crew declared an emergency, returned to the airport, and landed the aircraft without further incident. There were no injuries during the event.
The investigation
A postincident inspection confirmed that the left outboard roll spoiler was indeed deployed approximately halfway. Investigators performed an examination and disassembly of the roll spoiler servoactuator to identify the source of the malfunction.
The investigation revealed that a plug (PN A50993-1), which had been fusion-welded into the base of the piston (PN A50992-1), had separated. This component became lodged between the base of the piston and the shims, which restricted the movement of the piston.