What was found
AFTER LEVEL OFF FROM CLIMB, NR 2 ENGINE "B" LOOP BEGAN OSCILLATING TO TOP OF SCALE AND BACK AND ILLUMINATED THE FIRE DETECTION AMBER LIGHT INTERMITTENTLY; THE "A" LOOP WAS SELECTED AND FOUND TO BE NORMAL. THE FLIGHT PROCEEDED NORMALLY FOR APPROX 1 HOUR WHEN THE "A" LOOP BEGAN TO INDICATE A FIRE ALSO, CAUSING THE RED ENGINE FIRE WARNING LIGHTS TO ILLUMINATE. THE CREW WAS FORCED TO TREAT THE EVENT AS A FIRE AND PERFORMED THE ENGINE FIRE CHECKLIST RETURNING TO DEPARTURE. DURING INSP OF NR2 ENGINE, A TURBINE MID FRAME COOLING DUCT WAS FOUND BROKEN. THE DUCT WAS R & R WITH A SERVICEABLE UNIT IAW AMM 75-25-00. A MAX POWER ASSURANCE CHECK WAS ACCOMPLISHED IAW AMM 71-00-00, ALL PARAMETERS NORMAL, NO LEAKS EVIDENT.