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.