A B727-200 ACR FLC EXPERIENCES A YAWING MOVEMENT AND DISCOVERS LATER THAT THE LOWER RUDDER YAW DAMPER'S COMPUTER HAD MALFUNCTIONED CAUSING YAW TO BE INDUCED. WHEN THE LOWER YAW DAMPER SWITCH WAS SHUT OFF THE ACFT FLEW NORMALLY.

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ON FLT FROM ORD TO FLL ON APR/MON/97; AT 33000 FT .82 MACH; ACFT BEGAN TO YAW EXCESSIVELY AND SPONTANEOUSLY. I REQUESTED A LOWER ALT AND WE BEGAN A DSCNT TO FL240 AS CLRED BY ATC. WE SLOWED OUR SPD TO 250 KTS. SOMEWHERE AROUND FL260 I REQUESTED LOWER. WE WERE CLRED TO 10000 FT. AT ABOUT FL250 WE NOTICED THE LOWER RUDDER INDICES WERE DEFLECTING L AND R. SUSPECTING LOWER YAW DAMPER MALFUNCTION; WE DISCONNECTED LOWER YAW DAMPER. OSCILLATIONS CEASED AND WE LEVELED AT FL240 AND REQUESTED CLRNC TO ORD. ALL APPROPRIATE CHKLISTS; PROCS AND LIMITATIONS WERE FOLLOWED. LNDG WAS MADE ORD RWY 4R WITHOUT FURTHER INCIDENT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE FO OF THIS B727-200 SAID THAT THEY WERE IN SMOOTH FLT CONDITIONS WHEN THE ACFT BEGAN TO YAW EXCESSIVELY. AFTER THEY DSNDED TO A LOWER ALT THEY FOUND THAT THE LOWER RUDDER WAS MOVING L AND R WHILE THE UPPER RUDDER WAS STABLE. AFTER SWITCHING THE LOWER YAW DAMPER TO OFF THE MOVEMENT OF THE LOWER CONFORMED TO THAT OF THE UPPER RUDDER AND THE YAWING MOVEMENT STOPPED. THE RPTR SAID THAT THEY WERE ABOUT 100 NM N OF BNA AND AFTER TALKING WITH THE COMPANY THEY DECIDED TO RETURN TO ORD FOR ANOTHER ACFT. MAINT FOUND THAT THE LOWER RUDDER'S YAW DAMPER COMPUTER MALFUNCTIONED AND WAS INDUCING YAW. AFTER THE COMPUTER WAS REPLACED THE ACFT FLEW NORMALLY.