A B747-400 Flight Crew returned to their departure airport when EICAS revealed three of the eight engine fire detector loops had failed.

Narrative

Our aircraft had multiple MEL's including: Fire Detector Loop B #1. Climbing through 5000 MSL; 25 NM East of the airport; observed the 'status' cue illuminated on primary EICAS. Upon selecting status; observed: Fire Detector Loop B; #4; Fire Detector Loop A; #1; and the existing MEL Fire Detector Loop B #1. Seconds later on primary EICAS observed: Detector/OHT 1. Completed the QRH for the Detector Fire/OHT 1 procedure. After all four pilots reviewed what we had left for fire detection on the number one system discussions were had about an obvious return to departure airport so as not to fly an ocean crossing with no fire detection on an engine. We decided to test the fire and overheat test system since three of the eight detectors were malfunctioning. Fire test failure was indicated and the ENG Fire 1 warning remained illuminated with bell and lights. Completed the QRH for ENG Fire 1 and five to eight seconds after pulling the #1 fire switch the fire warning extinguished. Coordination with Dispatch; Maintenance Control; flight attendants; ATC; and FO operations; 199;000 lbs of fuel was jettisoned; an emergency was declared with ATC; a yellow emergency was accomplished with the cabin crew and an uneventful three engine landing was completed back the departure airport.