Narrative:

During cruise flight en route from anchorage to narita, approximately 1 hour 30 mins after departure, the flight engineer announced that the lower aft cargo compartment (class east) fire detector indicated a fault in 1 of the 2 fire detector loops. The captain gave control of the airplane to me and he and the flight engineer performed the abnormal checklist. After completing the checklist and troubleshooting the indications. We could not make a positive determination as to whether the problem was an actual fire or a malfunctioning system. The flight engineer inspected the area as best he could but there was no access to the compartment, he could only feel the outside of the compartment. There was no positive indication of a fire, nor were there combustible materials in the area. Following the procedures in the company checklist, we determined the best course of action was to return to anchorage. At that point, we had lost radio contact with zan (we were out of VHF range) so we executed the procedures published on the pacific orientation chart. Another aircraft relayed our intentions to zan on 121.5. We declared an emergency, turned 90 degrees south to vacate the track, descended to FL315 initially, then to FL250 when clear of the track. We flew direct to amott intersection and landed at anc without further incident. Maintenance corrected the problem and the flight continued normally.

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Original NASA ASRS Text

Title: A B747-200F DIVERTED TO ANC DUE TO AN INDICATED FAULT IN THE LOWER AFT CARGO COMPARTMENT FIRE DETECTOR LOOP.

Narrative: DURING CRUISE FLT ENRTE FROM ANCHORAGE TO NARITA, APPROX 1 HR 30 MINS AFTER DEP, THE FE ANNOUNCED THAT THE LOWER AFT CARGO COMPARTMENT (CLASS E) FIRE DETECTOR INDICATED A FAULT IN 1 OF THE 2 FIRE DETECTOR LOOPS. THE CAPT GAVE CTL OF THE AIRPLANE TO ME AND HE AND THE FE PERFORMED THE ABNORMAL CHKLIST. AFTER COMPLETING THE CHKLIST AND TROUBLESHOOTING THE INDICATIONS. WE COULD NOT MAKE A POSITIVE DETERMINATION AS TO WHETHER THE PROB WAS AN ACTUAL FIRE OR A MALFUNCTIONING SYS. THE FE INSPECTED THE AREA AS BEST HE COULD BUT THERE WAS NO ACCESS TO THE COMPARTMENT, HE COULD ONLY FEEL THE OUTSIDE OF THE COMPARTMENT. THERE WAS NO POSITIVE INDICATION OF A FIRE, NOR WERE THERE COMBUSTIBLE MATERIALS IN THE AREA. FOLLOWING THE PROCS IN THE COMPANY CHKLIST, WE DETERMINED THE BEST COURSE OF ACTION WAS TO RETURN TO ANCHORAGE. AT THAT POINT, WE HAD LOST RADIO CONTACT WITH ZAN (WE WERE OUT OF VHF RANGE) SO WE EXECUTED THE PROCS PUBLISHED ON THE PACIFIC ORIENTATION CHART. ANOTHER ACFT RELAYED OUR INTENTIONS TO ZAN ON 121.5. WE DECLARED AN EMER, TURNED 90 DEGS S TO VACATE THE TRACK, DSNDED TO FL315 INITIALLY, THEN TO FL250 WHEN CLR OF THE TRACK. WE FLEW DIRECT TO AMOTT INTXN AND LANDED AT ANC WITHOUT FURTHER INCIDENT. MAINT CORRECTED THE PROB AND THE FLT CONTINUED NORMALLY.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.