B777 CREW HAS SECONDARY ATTITUDE AND AIR DATA REF UNIT FAILURE AND RETURNS TO SFO.

Narrative

AT APPROX 45 MINS AFTER DEP THE STANDBY ATTITUDE INDICATOR FAILED AND WE RECEIVED A NO LAND 3 EICAS MESSAGE. WE WERE A REDISPATCH FLAG OP PLANNED IN ETOPS AND CLASS 2 AIRSPACE. WE DETERMINED THAT WE HAD LOST THE BACKUP NAV SYS; THE SAARU. THE PRIMARY ATTITUDE AND NAV SYS; THE ADIRU WAS OPERATING NORMALLY AT THIS TIME. WE WERE ABOUT 0+50 MINS FROM ENTERING ETOPS AIRSPACE UNDER 180 ETOPS. WE CONTACTED DISPATCH AND MAINT TO RELAY OUR PROB AND DETERMINE IF ANY RESETS WERE POSSIBLE. WE DETERMINED THAT WE WERE LEGAL TO CONTINUE INTO ETOPS; CLASS 2; AND RVSM AIRSPACE; WITHOUT THE BACKUP NAV PLATFORM. HOWEVER; WE WERE PROCEEDING INTO LESS THAN 1 MI FORECAST WX AT OUR DEST OF NGO WITH NO BACKUP ATTITUDE INFO. WE DETERMINED THAT WHILE LEGAL; OUR ISSUE IN CONTINUING WAS BEING COMFORTABLE IN PROCEEDING INTO THE IMC WX IN A DEGRADED NAV AND ATTITUDE CONDITION. EVEN THOUGH NOT CTLING; I PULLED UP THE MEL CARD FOR THE INOP SAARU CONDITION AND IT WAS VERY RESTRICTIVE FOR DISPATCH; INCLUDING NO IMC. I ASKED TO BRING THE DUTY MANAGER IN ON THE CALL; AND BRIEFED HIM ON OUR SITUATION TO RECEIVE AN OPINION FROM FLT OPS. THE FEELING IN THE COCKPIT WAS THAT WE WERE VERY UNCOMFORTABLE WITH PROCEEDING TO DEST IN THE DEGRADED CONDITION. WE INQUIRED AND WERE TOLD THAT A REPLACEMENT PART WAS LOCATED IN NRT; SEA; SFO. I TO CONTACT THE FLEET TO VERIFY A DECISION BEFORE COMMITTING TO A DIVERT. THE B777 FLEET CAPT AGREED WITH OUR DECISION THAT THE SAFEST COURSE OF ACTION WAS NOT TO CONTINUE INTO ETOPS AIRSPACE TOWARDS A LOW MINIMUM APCH AFTER A LONG INTL DUTY DAY. WE DIVERTED BACK TO SFO AND PLANNED FOR AN OVERWT LNDG AT SFO; FOR THIS UNPLANNED DIVERSION. WE CONTACTED MAINT CTL; AND ALERTED THEM TO THE OVERWT CONDITION. WE CONSULTED THE LNDG DISTANCES; NON NORMAL OVERWT LNDG CHKLIST AND CONSIDERATION FOR AUTOBRAKES WITH THE OVERWT CONDITION ON BRAKE TEMPS. WE SENT THE APPROPRIATE CODES FOR THE SAARU FAILURE AND AFTER LNDG THE OVERWT CODE. WE PLANNED FOR A 540000 LB LNDG AFTER DETERMINING THE PERFORMANCE AND RWY LIMITS FOR THE LONGEST RWY AVAIL AT SFO. WE NOTIFIED THE FLT ATTENDANTS AND PAX OF THE REASON FOR THE RETURN TO SFO AND THAT THEY REMAIN ON THE ACFT AND TO WAIT FOR THE ANNOUNCEMENT AFTER WE ARRIVED AT THE GATE. UNEVENTFUL LNDG; AND FOLLOWED THE DIVERSION CHKLIST IN FOM.