Gentlemen,When receiving this warning: DET FIRE/OHT3, the Abnormal Procedures QRH states:Condition: Engine fire/overheat detection loops A and B have failedNo action for the crew is required.The question I had regarding this was what happens when only one loop has failed? Also, according to the 744 MEL, one loop per engine can be inoperative. Is it correct to assume that if two are inoperative you must divert to closest airport?Thanks,Mark

The answer is simple: you won't notice anything.But if you do get this warning then I suggest you divert to the nearest available airport cause you don't have any sort of warning of indication of a possible fire left.

This sort of thing is covered in the MEL (minimum equipment list) and DDG (dispatch deviation guide)The Boeing 747-400 MEL says one of the two fire & overheat detection loops may be inoperative per engine for dispatch. If both are inoperative, the plane will not be dispatched.Note that "divert to [the] closest suitable alternate" is done ONLY when specified by the QRH in a time critical emergency (ie depressurization or engine fire)

Thanks guys for your replies regarding this issue. You have cleareed this up for me.Mark