A B747-200 ON INITIAL CLB DECLARED AN EMER AND DIVERTED DUE TO EXCESSIVE RUDDER TRIM AND UNCOMMANDED FLT CTL INPUTS CAUSED BY BOTH YAW DAMPER COMPUTER FAILURES WITH NO COCKPIT WARNINGS OR INDICATIONS.

Narrative

AFTER TKOF FROM NRT; COPLT (WHO WAS FLYING) NOTICED AN ABNORMAL AMOUNT OF R RUDDER TRIM WAS REQUIRED TO MAINTAIN A TRIMMED CONDITION (APPROX 2 1/2 UNITS OF R RUDDER TRIM). DURING ACCELERATION TO ENRTE CLB SPD OUT OF 10000 FT; UNCOMMANDED FLT CTL INPUTS WERE NOTED. THESE INPUTS BECAME MORE INTENSE AS AIRSPD INCREASED FROM 250 KIAS TO 290 KIAS. NO INDICATIONS OF EQUIP FAILURE WERE EVIDENT IN COCKPIT. CAPT DECLARED EMER AND VECTORS TO AN UNEVENTFUL LNDG WERE PROVIDED BY NARITA APCH CTL. AFTER LNDG; IT WAS DISCOVERED THAT BOTH YAW DAMPER COMPUTERS HAD FAILED; AND IT WAS DETERMINED THAT THOSE COMPUTER FAILURES CAUSED THE UNCOMMANDED FLT CTL INPUTS. THE COMPUTERS WERE REPLACED AND THE FLT RESUMED WITH NO REPEAT OF PROB. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE ACFT WAS A B747-200 AND BOTH UPPER AND LOWER YAW DAMPERS TESTED OK DURING THE PREFLT CHK. THE RPTR SAID AT NO TIME DURING THE FLT DID ANY YAW DAMPER WARNING LIGHTS ILLUMINATE AND THE CIRCUIT BREAKERS WERE CHKED AND FOUND CLOSED. THE RPTR SAID THE FIX WAS THE REPLACEMENT OF BOTH YAW DAMPER COMPUTERS.