What was found
PRIOR TO DEPARTURE, ENG 4 FIRE DET LOOP B WAS DEFERRED. SHORTLY AFTER TAKEOFF, LOOP A ON THE SAME ENGINE BEGAN FLAGGING IN AHM. QRH PROCEDURES FOR DUAL LOOP FAILURE INDICATES FIRE DETECTION AND OVERHEAT DETECTION MAY BE INOP FOR THAT ENGINE. DUE TO CLOSE PROXIMITY TO ANC AND LACK OF FIRE PROTECTION, CREW ELECTED TO RETURN TO ANC REFERENCE ITEMS 1, M2 AND M3 ON LP 1540181. FOUND DAMAGE WIRE ON ENGINE 4 AND PERFORMED REPAIR PER AMM 20-10. PERFORMED THROUGH CMC, TEST PASSED PER AMM 26-10-00. UPPER AFT FIRE DETECTOR ASSEMBLY WAS REPLACED PRECAUTIONARY DUE TO CYCLES, IN ACCORDANCE WITH 26-11-02. LOWER AFT FIRE DETECTOR ASSEMBLY WAS REPLACED PRECAUTIONARY IAW AMM 26-11-02.</P>)