B747 FLT CREW REPORTS 'STAB TRIM UNSCHD' EICAS MESSAGE IN CRUISE; WHICH HAS BEEN CHRONIC IN THIS ACFT. EMERGENCY DECLARED AND FLIGHT DIVERTS TO NEAREST SUITABLE ARPT FOR NORMAL LANDING WITH STAB TRIM DEACTIVATED.

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ON OUR ARR TO THE ACFT LCL MECH BRIEFED US ON THE ACFT STATUS. HE STATED THAT IT CAME IN WITH A LOG PAGE WRITE-UP; EICAS MESSAGE 'STABILIZER TRIM UNSCHD' IN CRUISE AND THAT THIS ITEM HAD BEEN PREVIOUSLY WRITTEN UP NOW 4 TIMES. ADDITIONALLY; ONE OTHER RECENT LOG PAGE WRITE-UP SHOWED AN ITEM THAT THE AUTOPLT DISCONNECTED TWICE IN CRUISE. HE STATED THAT HE WAS CONCERNED AND WANTED TO TROUBLESHOOT THE ISSUE USING THE BOEING TROUBLESHOOTING GUIDANCE AND IT MAY TAKE SOME TIME TO RESOLVE THIS ISSUE. THE LCL MECH HAD NUMEROUS CONVERSATIONS WITH ACR MAINT. THE LCL MECH STATED THAT ACR WANTED HIM TO CHANGE AN FCC AND DO A TEST. MECH EXPLAINED TO US THAT HE CHANGED OUT AN FCC AND RAN 3 SEPARATE TESTS; ISOLATING EACH ONE OF THE 3 FCC'S BY PULLING A CIRCUIT BREAKER. THE TEST CAME OUT GOOD AND LCL MECH CLRED THE LOGBOOK ITEM AS PER ACR MAINT. THE TKOF AND CLBOUT WAS NORMAL. FO DID THE TKOF AND HAND FLEW THE ACFT TO THE TOP OF CLB AND THEN REQUESTED THE R AUTOPLT TO COMMAND. APPROX 5 MINS AFTER THE TOP OF CLB; AN EICAS MESSAGE 'STABILIZER TRIM UNSCHD' APPEARED. WE FOLLOWED THE QRH; NO PITCH CHANGE WAS DETECTED; REESTABLISHING SYS #2; CHK PITCH TRIM OP AND FOUND TO BE WORKING NORMALLY. ALL OPS WERE NORMAL EXPECT HYD STABILIZER SYS #3 WAS NOW ISOLATED IN CUTOUT AS PER THE QRH. AFTER ESTABLISHING COMS WITH ACR; WHICH INCLUDED DISPATCH; MAINT; AND THE CHIEF PLT; I FULLY EXPLAINED THE ISSUES; THE PRIOR LOG PAGE WRITE-UPS AND REQUESTED THEIR INPUT AND DIRECTIONS FOR US TO TAKE. AFTER EXPLAINING TO ACR THE ISSUES; THEY SAID MAINT WOULD CONTACT BOEING AND THEN WOULD ACARS US WITH THEIR RECOMMENDATIONS. SHORTLY AFTER THE COM WITH ACR; WE GOT ANOTHER EICAS MESSAGE 'STABILIZER TRIM UNSCHD.' THIS TIME WE HAD A PITCH UP OF 800 FPM. I SLOWED THE ACFT TO AN IN TRIM SPD OF 248 KTS; REQUESTED AND GOT A CLRNC TO DESCEND TO FL300 AND DIVERT TO ZZZZ. WE NOTIFIED ACR OPS OF THE LATEST ISSUES AND THAT WE WERE DIVERTING TO ZZZZ. WE DECLARED AN EMER; BUT ADVISED ATC THAT WE HAD THE ISSUE UNDER CTL. WE REQUESTED PRIORITY HANDLING AND EQUIP STANDING BY AT ZZZZ AS A PRECAUTIONARY MEASURE. 15 MINS AFTER LEVELING AT FL300 WHILE HAND-FLYING THE ACFT; WE GOT OUR THIRD EICAS MESSAGE 'STABILIZER TRIM UNSCHD' BUT WITH NO PITCH CHANGE. WE CONFIRMED THAT BOTH STABILIZER TRIM SWITCHES WERE STILL IN CUTOFF AS PER THE QRH AND CONTINUED HAND-FLYING THE ACFT. ONCE ESTABLISHED ON FINAL APCH; FULLY CONFIGURED FOR LNDG AND WITH THE RADIO ALTIMETER ALIVE; WE GOT A STRANGE EICAS MESSAGE 'DARG REQUIRED.' WE CONTINUED WITH A NORMAL LNDG WITH THE EXCEPTION OF HVY AFT CTL FORCE NECESSARY TO MAINTAIN THE APPROPRIATE APCH PATH. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: MAINTENANCE DISCOVERED; AFTER CONSULTING WITH THE MANUFACTURER; THAT A POSITION INDICATING TRANSDUCER LOCATED IN THE TAIL OF THE ACFT WAS ARCING AND PRODUCING ERRONEOUS INDICATIONS OF STAB MOVEMENT. THIS INFORMATION IS ALSO SUPPLIED TO FLIGHT CONTROL COMPUTERS AND MAY HAVE CAUSED THE AUTOPILOT DISCONNECTS AND PITCH UP.