What was found
APU IS BLOWING SMOKE OUT EXHAUST. ACFT WAS INSPECTED FOR FIRE DAMAGE. SHUT DOWN APU, HAD FIRE IN TAIL PIPE, TRIED RESTART TO BLOW OUT NO LUCK. FIRE DEPT SHOT WATER IN TAIL PIPE TO EXTINGUISH FLAMES, NO FIRE ALARM, DECIDED TO BLOW FIRE BOTTLE DUE SOME SMOKE OUT OF APU DOOR VENT AS PRECAUTION BEFORE FIRE DEPT OPENED COWLING. REMOVED AND REPLACED APU FIRE BOTTLE AND BOTTLE CARTRIDGE IAW AMM 26-22-01. SQUIB TESTED NORMAL.