A B747-400 MAKING AN APCH INTO JFK LOSES THE L FMC WHICH FAILS ALL THE ILS INSTS. THE ACFT EXECUTES A MISSED APCH BECAUSE OF THE LOW WX AND JFK AND DIVERTS TO EWR.

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ACR FLT XX APR/XA/98. DURING THE INITIAL APCH PHASE TO RWY 31R AT JFK AT 4000 FT; EXPERIENCED INTERMITTENT FAILURE OF BOTH FMC'S AND SUBSEQUENT INTERMITTENT VHF FAILURE (PARTIAL). AT THIS POINT; WE DISCONTINUED THE APCH; CLBED TO 8000 FT MSL AND HELD APPROX 20 MI NE; SE OF JFK TO RATIFY SIT. UPON HEARING FROM THE CABIN CHIEF PURSER ABOUT AN ODOR OF BURNING WIRES IN THE AFT CABIN; WE DISCONTINUED HOLDING AND PROCEEDED TO EWR FOR AN UNEVENTFUL APCH AND LNDG IN VISUAL CONDITIONS. THE FLC DETERMINED THAT ELECTRICAL EQUIP FAILURE WAS DUE IN PART TO A MALFUNCTION IN THE ELECTRICAL EQUIP COOLING SYS. I DO NOT RECALL THE CAPT DECLARING AN EMER; BUT ATC DID PROVIDE PRIORITY HANDLING FOR OUR APCH AND LNDG AT EWR. MY PRIMARY ROLE DURING THIS INCIDENT WAS IN MAINTAINING CONTACT WITH MAINT TO TRY AND FIND A CAUSE FOR THE ACFT'S PROBS. I ALSO WAS RESPONSIBLE FOR COORDINATING OUR DIVERSION FOR LNDG AT EWR INSTEAD OF JFK. THIS WAS DONE THROUGH COMPANY COMS. I ALSO KEPT THE PAX AWARE OF OUR SIT THROUGHOUT THE ENTIRE EPISODE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THAT IT IS NOW KNOWN THAT THERE WAS A FAILURE OF THE L FMC. IT IS FELT THAT THE FAILURE WAS INTERMITTENT WHICH WOULD EXPLAIN WHY THE FLC NEVER RECEIVED AN EICAS WARNING. WHEN A FLT ATTENDANT RPTED THE SMELL OF HOT BURNING WIRES ANOTHER RELIEF FLT OFFICER WENT TO THE BACK TO INVESTIGATE; BUT HE COULD NOT DETECT ANY BURNING WIRE ODOR. THE COOLING FAN WAS ALSO OPERATING ON AN INTERMITTENT BASIS AS WAS DETERMINED LATER. WITH THE LOSS OF THE FMC; THEY COULD NOT KEEP THE ILS AVAILABLE FOR THE APCH. SINCE THE FLC COULD VISUALLY IDENT EWR THEY MADE A DECISION TO GO TO THAT VFR ARPT. FLC AND MAINT PERSONNEL DETERMINED TOGETHER THAT BECAUSE THE FMC WAS INTERMITTENT IT WOULD BE REPLACED. HOWEVER; AS THE ACFT WAS FERRIED BACK TO JFK BY ANOTHER FLC; THE FMC FAILED AGAIN. IT WAS FELT THAT A COOLING FAN THAT WAS FAILING SPORADICALLY PLAYED A PART IN THE FMC FAILING BECAUSE OF INADEQUATE COOLING. THAT WAS ANOTHER FAILURE THAT WAS NEVER DISPLAYED ON THE EICAS PANEL. THE ACFT TYPE WAS A B747- 400. THE ACFT MANUFACTURER AND ACR MAINT ARE WORKING ON WHY THE FAILURE WAS NEVER ANNUNCIATED ON THE EICAS SCREEN. SUPPLEMENTAL INFO FROM ACN 398505: THE UPPER EICAS DISPLAYED 'AUTOTHROTTLE DISCONNECT' AND 'FMC MESSAGE.' THE L AND R CDU'S WERE BLINKING 'RESYNCHING' AND THE 'MAP FAIL' AND 'VTK' WARNINGS WERE ON THE NAV DISPLAYS. THE L 'ND' RESTORED ABOUT 30 SECONDS LATER AND I SAID 'I GOT THE MAP BACK.' ONE MIN OR SO LATER; IT FAILED AGAIN WITH THE 'RESYNCHING' MESSAGE ON THE CDU'S. JFK MAINT WAS CONTACTED AND WE WERE GIVEN SOME GUIDANCE THAT POSSIBLY WE HAD A MAJOR ELECTRICAL OVERLOAD AND THEY SUGGESTED TURNING 'OFF' THE UTILITY BUS SWITCHES TO LOWER THE ELECTRICAL LOAD. THIS WAS DONE AND IMMEDIATELY THE UPPER EICAS FILLED WITH MESSAGES ABOUT SYS THAT WERE NOW UNPWRED; AND THE 'AUTOTHROTTLE' AND 'FMS MESSAGES' WERE STILL AT THE TOP OF THE PAGE. WE CONTINUED IN THAT CONFIGN FOR A FEW MORE MINS UNTIL ONE OF THE EICAS MESSAGES CAME UP 'EQUIP COOLING.' THE PNF FO'S REMARKED 'FINALLY THERE'S SOMETHING I HAVE A PROC FOR.' THE L CDU WOULD ACCEPT NOTHING. I MOMENTARILY SELECTED THE 'NAV SELECTOR' TO 'CDU-L' TO TRY TO HARD TUNE THE L ILS AND THE CDU WENT BLANK. ABOUT 4 MI OUT ON FINAL; THE FO'S WERE ABLE TO GET THE ILS ON THE RADIO RAW DATA DISPLAYS AND THE ILS WAS USED FOR GUIDANCE. THE APPROX TIME FROM INITIAL PROB TO LNDG AT EWR WAS ABOUT 25 MINS. THE 'MASTER FMC' WAS NOT TRIED BECAUSE BOTH FMC'S APPEARED TO HAVE PROBS -- BUT NO EICAS MESSAGE EVER SAID 'FMC FAILURE.' THIS WAS ALL AT THE END OF A 12 HR FLT! FATIGUE CERTAINLY DIDN'T HELP! SUPPLEMENTAL INFO FROM ACN 399265: NAV INFO FROM FMC'S NIL TO UNRELIABLE.