A B747-400 IN CRUISE AT FL370 DECLARED AN EMER AUTOPLT DISCONNECTED BUT NOT DISENGAGED. STIFF AILERONS LIMITED TO 2-3 DEGS OF BANK.

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I HAD JUST AWAKENED FROM BREAK. FO WAS RELIEF PF. ATC CLRED US DIRECT TO VOR. ACFT WOULD NOT RESPOND. RECEIVED AUTOPLT EICAS. FO ATTEMPTED TO DISENGAGE; AUTOPLT DISCONNECTED BUT WOULD NOT DISENGAGE. HAD TO BE OVERPWRED. WE FOUND WE COULD TURN R; BUT VERY STIFF 2-3 DEG BANK L TURNS. SATCOM TO LINE MAINT AND DISPATCH NO HELP. DECIDED TO DECLARE EMER; PREPARE FOR EVAC AND REQUESTED STRAIGHT-IN TO RWY. I RETURNED TO L SEAT AND FO RETURNED TO R SEAT. WE DSNDED AND TRIED TO MAKE TURNS AT FL290 AND LOWER. SAME RESULT. WE PREPPED FOR EVAC (EXCELLENT JOB BY THE CREW; FLT ATTENDANT; AND FO). LNDG WAS UNEVENTFUL; FLAPS 30 DEGS; SPD BRAKES WERE NOT ARMED. ON FINAL; TURNS IN BOTH DIRECTIONS WERE VERY STIFF AND DIFFICULT. 2-3 DEGS L AND 5 DEGS R BANK WERE ABOUT MAX. ON TAXI IN; CTLS WERE SMOOTH AND AILERON DEFLECTION WAS OK. TAXIED TO GATE; DEBRIEFED MAINT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE AUTOPLT EICAS WARNING CAME FIRST AND THE FO DISCONNECTED THE AUTOPLT BUT IT REMAINED ENGAGED. THE RPTR SAID AT FIRST THE AILERONS WERE LIMITED TO R TURNS ONLY AND BANK WAS LIMITED TO 2-3 DEGS. THE RPTR STATED ON FINAL APCH; L AND R BANKS WERE AVAILABLE; BUT AGAIN WERE LIMITED TO 2-3 DEGS WITH VERY STIFF AILERONS. THE RPTR SAID ON LNDG; ALL CTLS WERE NORMAL. THE RPTR STATED MAINT BELIEVES A POTABLE WATER LEAK INTO THE MAIN WHEEL WELL FREEZING THE CENTRAL CTL ACTUATOR MAY BE THE CAUSE. THE RPTR SAID THE INCIDENT IS STILL UNDER INVESTIGATION AND NO FINDINGS HAVE BEEN RELEASED.