A B727-200 CLBING AT FL280 EXPERIENCED AN APU FIRE WARNING DUE TO FAILURE OF THE APU TO SHUT DOWN PRIOR TO TKOF.

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WE RECEIVED OUR TAXI CLRNC AND PROCEEDED OUT OF THE CARGO RAMP AT SJU. AT THIS TIME I BEGAN MY TAXI FLOWS AS PER COMPANY PROCS WHICH INCLUDES SHUTTING DOWN OF THE APU. I REACHED FOR THE APU START SWITCH; TURNED IT OFF AND OBSERVED THE APU GENERATOR BREAKER LIGHT SLOWLY EXTINGUISHED. THERE WAS AN INITIAL DECREASE IN EGT AND THE APU FUEL LIGHT ON THE FE DOOR WARNING ANNUNCIATOR PANEL WENT OUT. FO XXX TAXIED ONTO THE ACTIVE TXWY AND CALLED FOR FLAPS 15 DEGS; TAXI CHKLIST. I READ THE CHKLIST AND RECEIVED ALL THE APPROPRIATE RESPONSES. INCLUDED IN THE CHKLIST WAS APU OFF; LIGHT OUT. I RECALLED REACHING FOR THE APU START SWITCH AND VERIFYING IT WAS IN THE OFF POS. BY THIS TIME; THE GENERATOR BREAKER LIGHT FOR THE APU WAS EXTINGUISHED; THE EGT WAS BTWN 200-300 DEGS C AND APU LIGHT ON THE FE DOOR WARNING ANNUNCIATOR PANEL WAS EXTINGUISHED. FROM THESE INDICATIONS; THERE WAS NO DOUBT IN MY MIND THAT THE APU WAS SHUT DOWN. WE COMPLETED ALL THE NECESSARY CHKLISTS AND DEPARTED SJU. ON THE CLBOUT SOMEWHERE BTWN FL230-FL250; WE HEARD A THUMPING NOISE AT APPROX 10 SECOND INTERVALS. THE CAPT ELECTED TO CONTINUE THE CLB TO FL280 LEVELOFF AND THEN WE BEGAN TO INVESTIGATE THE SOURCE OF THE NOISE. WE LOOKED AT THE FORWARD ENG INST PANEL AND I MONITORED THE FE PANELS FOR ABNORMAL AND IRREGULAR INDICATIONS. I OBSERVED NO IRREGULARITIES AND ON 2 OCCASIONS WAS INSTRUCTED BY THE CAPT TO LEAVE MY SEAT SO AS TO DETERMINE WHERE THE NOISE WAS COMING FROM. A FEW MINS AFTER RETURNING TO MY SEAT THE APU FIRE WARNING LIGHT ILLUMINATED AND THE BELL SOUNDED. I REACHED UP; PULLED THE APU FIRE HANDLE (EGT AT THIS TIME RISING THROUGH 550 DEGS) AND DISCHARGED THE FIRE BOTTLE AS PER COMPANY PROCS. THE LIGHT IMMEDIATELY WENT OUT AND THE EGT BEGAN TO DECREASE. AFTER COMPLETING THE EXPANDED PORTION OF THE APU FIRE CHKLIST; I NOTICED THE EGT HAD STABILIZED AROUND 240 DEGS AND DROPPING SLOWLY. BY THIS TIME; WE WERE IN THE VICINITY OF GRAND TURK AND HAD AGREED THAT AN EMER SIT HAD NO LONGER EXISTED. WE DECIDED TO CONTINUE TO MIAMI AS IT SEEMED TO BE THE MOST SUITABLE ARPT AT THAT TIME. ON OUR POSTFLT INSPECTION WE NOTICED THE UPPER SURFACE OF THE R WING AFT OF THE APU EXHAUST VENT HAD BEEN DAMAGED DUE TO HEAT EXPOSURE FROM THE APU EXHAUST. IT WAS NOTICED THAT SEVERAL RIVETS HAD BEEN MISSING FROM THE EXHAUST DUCT AND AS A RESULT THE EXHAUST AIR WAS DEFLECTED DIRECTLY ON THE ACFT SHEET METAL INSTEAD OF THROUGH THE EXHAUST VENT. I WOULD ALSO LIKE TO ADD THAT THIS AIRPLANE XYZ IS SUPPOSEDLY EQUIPPED WITH A TKOF WARNING HORN WHICH SOUNDS IF THE APU FUEL SHUTOFF VALVE IS IN THE OPEN POS WHEN THE THROTTLES ARE ADVANCED. WE GOT NO SUCH WARNING ON TKOF THE DAY THE INCIDENT OCCURRED (CAN BE VERIFIED BY CAPT AND FO). PRIOR TO TKOF; THERE WAS ALSO NO APU LIGHT ON THE FE ANNUNCIATOR PANEL INDICATING THAT THE APU FUEL SHUTOFF VALVE SHOULD HAVE BEEN CLOSED. IT MUST BE NOTED WHEN THE APU FIRE HANDLE WAS PULLED; ONLY THE GENERATOR FIELD LIGHT ON THE APU PANEL WAS ILLUMINATED (CAN BE CONFIRMED BY THE FO). THIS IS A NORMAL INDICATION IF THE APU START SWITCH IS IN THE OFF POS AND THE FIRE HANDLE IS PULLED. IN CONCLUSION; I WOULD LIKE TO SAY THAT AT NO TIME WAS THERE ANY DOUBT IN MY MIND THAT THE APU WAS NOT SHUT DOWN. HOWEVER; FOR SOME UNKNOWN REASON TO ME THE APU REMAINED HUNG UP OR DID NOT SHUT DOWN COMPLETELY. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE APU APPEARED TO SHUT DOWN NORMALLY; EGT DECREASED; FUEL VALVE LIGHT WENT OUT AND APU EXHAUST DOOR LIGHT WENT OUT. THE RPTR SAID IN CLB TO CRUISE HE NOTED THE APU EXHAUST GAS TEMP WAS CLBING TO 550 DEGS. THE RPTR SAID THE APU FIRE WARNING SOUNDED AND THE FIREWALL HANDLE WAS PULLED AND THE BOTTLE FIRED. THE RPTR STATED ON THE GND SOME WING SHEET METAL AFT OF THE EXHAUST DOOR WAS DAMAGED. THE RPTR SAID THE APU WAS REPLACED.