DASH 8 ROLL CONTROL FUNCTION FROZE AFTER DISCONNECTING AUTOPILOT. FLT CREW WAS ABLE TO OVERPOWER ANOMALY AND CONTROL WAS REGAINED.

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WE WERE ABOUT 6000 FT ON A LEFT DOWNWIND. WEATHER WAS FEW CLOUDS ABOUT 10;000 FT OR SO. CAPT WAS FLYING. WE WERE CLEARED FOR A VISUAL APPROACH AND CLEARED TO A LOWER ALTITUDE. CAPT DISENGAGED THE AUTOPLT TO HAND FLY. AS SHE TRIED TO ROLL THE AIRCRAFT TO LEVEL OUT; SHE SAID SHE WAS UNABLE TO MOVE THE CONTROL YOKE TO MAKE THE AIRCRAFT ROLL. SHE ASKED IF I HAD THE SAME PROBLEM; I DID. WE HAD PITCH BUT DIDN'T HAVE ROLL. WE WERE DISCUSSING PULLING THE ROLL DISCONNECT; WHEN WE HEARD A LOUD POP AND HAD FULL CONTROL OF THE CONTROLS. ENTIRE EPISODE LASTED APPROX 5 SEC OR SO. AT NO TIME DID WE HAVE ANY CAUTION OR WARNING LIGHTS. WE WERE ALREADY CLEARED FOR THE VISUAL; ABOUT TO TURN BASE; WITHIN SIGHT OF THE FIELD; AND #1 FOR THE AIRPORT. WE DID NOT DECLARE AN EMERGENCY. WE HAD A SUCCESSFUL LANDING WITH NO OTHER PROBLEMS. MAINT WAS NOTIFIED AND ACFT WAS GROUNDED.CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: REPORTER STATED THAT THEY HAD PITCH CONTROL BUT ROLL CONTROL WAS FROZEN. IT WAS SPECULATED THAT EVEN THOUGH COCKPIT INDICATIONS VERIFIED AUTOPILOT DISCONNECT; THE ROLL FUNCTION REMAINED ENGAGED UNTIL SUFFICIENT FORCE WAS APPLIED TO THE YOKE. MAINT COULD NOT DUPLICATE THE MALFUNCTION. AFTER ALL FLT CONTROL COMPONENTS WERE INSPECTED; AUTOPILOT COMPUTER WAS REPLACED AS A PRECAUTIONARY MEASURE.