RETURN LAND MANDATED BY INTERMITTENT ENG FIRE WARNING.

Narrative

CLBING/ACCELERATING OUT OF 10000 FT AND INTERMITTENT FIRE WARNING OCCURRED ON ENG #2 -- PWR WAS IMMEDIATELY REDUCED ON THAT ENG AND THE FIRE WARNING DISCONTINUED. PWR WAS ADVANCED ABOVE 400 DEGS CENTIGRADE EGT AND THE FIRE WARNING BEGAN INTERMITTENTLY AGAIN. PWR WAS REDUCED TO IDLE THRUST AND NO FIRE WARNING INDICATIONS WERE ENCOUNTERED -- SUSPECTED A BLEED-AIR DUCT FAILURE AND RETURNED IMMEDIATELY TO MSP. LANDED UNEVENTFULLY; VISUAL EXTERNAL INSPECTION OK; SHUT DOWN ENG. AFTER CLRING RWY AND WHEN COWLING WAS OPENED; FOUND A BLEED-AIR DUCT HAD BROKEN AT THE FIREWALL CLAMP PUTTING HOT BLEED-AIR ON THE FIRE DETECTION SYS. ALL SOPS ADHERED TO. SUPPLEMENTAL INFO FROM ACN 285156: INITIALLY THE CREW THOUGHT WE HAD A MALFUNCTIONING FIRE DETECT SYS. WHEN IT SEEMED TO RESPOND TO PWR REDUCTIONS; WE THEN THOUGHT IT WAS A BLEED PROB. SUPPLEMENTAL INFO FROM ACN 285414: NO SECONDARY FIRE INDICATIONS (HIGH EGT; HIGH OIL TEMP) WERE NOTED. ADDITIONALLY; THERE WERE NO INDICATIONS OF AN AFT BODY OVERHEAT. OUR CREW (INCLUDING THE CHK AIRMAN) BELIEVED THIS TO BE AN INDICATION PROB. NO EMER WAS DECLARED. THE #2 FIREWALL HAD ALSO SUSTAINED DAMAGE. THE CREW INCORRECTLY DETERMINED IT WAS A FAULTY INDICATION. THE INDICATION; HOWEVER; WAS VALID AND WAS ALERTING OUR CREW TO A PROB WITH OUR #2 ENG.