DHC8 CREW USES THE APU FOR HEAT; UNAWARE THAT IT HAS BEEN MEL'ED. APU HAS A STACK FIRE AND SHUTS DOWN AUTOMATICALLY. A MECH SUGGESTED TO THE CREW THAT IT WAS OK TO USE THE APU.

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WHEN MECHS DROPPED OFF ACFT 1 OF THEM TOLD MY FO 'THE APU WOULD WORK FINE.' UNFORTUNATELY; THE APU WAS MEL'ED; AND I WAS NOT PRESENT TO QUESTION THE MECH ON THIS. LATER; THE FO SHARED WITH ME THE MECH'S COMMENT. WHILE PARKED IN A WINTER WONDERLAND THE ACFT BECAME VERY COLD. I DECIDED TO TRY THE APU FOR HEAT. APU STARTED AND RAN NORMALLY FOR 5-10 MINS; THEN FAILED. A SMALL EXHAUST-PIPE FIRE OCCURRED ON SHUTDOWN. IT WAS QUICKLY EXTINGUISHED; WITH NO DAMAGE TO ACFT. NO PAX WERE ON BOARD. THE USE OF MEL'ED EQUIP AT MY AIRLINE HAS BECOME COMMON PLACE. I ALLOWED MY DECISION MAKING TO BE INFLUENCED BY THIS; BY MY DESIRE TO MAINTAIN A COMFORTABLE ENVIRONMENT; AND BY THE 'OFF THE RECORD' SUGGESTION OF A MECH. SUPPLEMENTAL INFO FROM ACN 354708: THE APU CAUGHT FIRE AND SHUT ITSELF DOWN BUT DID NOT FIRE THE FIRE BOTTLE OR GIVE ANY WARNING LIGHTS. PEOPLE FROM THE TERMINAL LET US KNOW ABOUT IT. I THINK PEOPLE SHOULD BE AWARE THAT IF AN APU SHUTS ITSELF DOWN WITH NO WARNING LIGHTS THERE STILL COULD BE A PROB. IF SOMEONE HAD NOT TOLD US; WE MAY NOT HAVE BECOME AWARE OF THE PROB FOR A COUPLE MORE MINS.