Narrative:

When we received aircraft at the gate; we noticed that the APU detector inoperative light was MEL'ed. After speaking with inbound crew; captain became convinced it was ok to use APU. I noticed that the APU fire detection circuit breaker was collared during preflight duties and did not understand the MEL application. Captain said we would have maintenance in our next city meet us to explain better. Upon arrival; we requested maintenance and were met at the gate by a company mechanic. We questioned the method of deferral and MEL note. After speaking with maintenance control; company mechanic removed the circuit breaker collar on the APU fire detection system. Flight departed normally using APU for engine start. Flight arrived in ont and departed the gate on time. During taxi out approximately 1 min after APU was shut down; we received an APU fire warning. We performed QRH items and fired APU extinguisher. We then asked tower if there was any visual smoke indication coming from our APU. They said there was nothing. We requested crash fire rescue equipment to inspect and returned to the gate for an evacuate/evacuation via the jetway. Subsequent maintenance inspection revealed no fire had occurred. We MEL'ed the APU and the flight departed about 2 hours after our original departure time. Upon further investigation of the MEL; we noticed that the APU detector inoperative light has 2 different notes that apply to 2 different forms of mechanical irregularity. We originally believed that the light was deferred for not illuminating; via note 1 of MEL which would allow the APU to be used. Apparently it was actually deferred under note 2 of MEL for coming on spuriously and that condition forbids the use of the APU altogether. When company maintenance personnel removed the circuit breaker collar; that enabled the use of the APU; which caused the subsequent false fire indication. A more thorough review of the MEL notes and discussions with maintenance control would have possibly helped to understand the true nature of the deferral. A more thorough review of MEL notes that are applicable with deferral. This should include a conversation with maintenance control for exact explanations. Supplemental information from acn 734053: the aircraft had 1 MEL in the logbook and on the release. (I also added another MEL relating to a leading edge device light; not related to this incident.) it was MEL APU det inoperative light inoperative. I reviewed MEL under the wording in the logbook and release: APU det inoperative light inoperative. It states it may be inoperative; extinguished; provided: the APU fire detection system operates normally and a fire warning test is performed before each start. It goes on to state in a note: if the APU det inoperative light fails to illuminate during overheat/inoperative or fault/inoperative test; but APU fire warning switch illuminates during fire test; APU fire detection system is operating normally; and the APU may be used provided fire warning test is conducted before each APU start. The first officer and myself checked these 2 items and found the APU det inoperative light failed to illuminate and the APU fire warning switch illuminated during the fire test. I; therefore; concluded the APU could be used. I had a mechanic clear the other MEL from the aircraft and he said he would clear both MEL's; but he didn't have the time to do it. He never mentioned not to use the APU. We flew on to 3 more cities and used the APU for start each time. Looking back; I can only assume the write-up in the logbook and the release should have been APU det inoperative light inoperative and illuminated. I also should have called maintenance to ensure I had a correct understanding of this deferred item.

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Original NASA ASRS Text

Title: B737 CREW USES MEL'ED APU TO START ENGINES AND GETS A FALSE APU FIRE WARNING 1 MINUTE AFTER APU SHUT DOWN.

Narrative: WHEN WE RECEIVED ACFT AT THE GATE; WE NOTICED THAT THE APU DETECTOR INOP LIGHT WAS MEL'ED. AFTER SPEAKING WITH INBOUND CREW; CAPT BECAME CONVINCED IT WAS OK TO USE APU. I NOTICED THAT THE APU FIRE DETECTION CIRCUIT BREAKER WAS COLLARED DURING PREFLT DUTIES AND DID NOT UNDERSTAND THE MEL APPLICATION. CAPT SAID WE WOULD HAVE MAINT IN OUR NEXT CITY MEET US TO EXPLAIN BETTER. UPON ARR; WE REQUESTED MAINT AND WERE MET AT THE GATE BY A COMPANY MECH. WE QUESTIONED THE METHOD OF DEFERRAL AND MEL NOTE. AFTER SPEAKING WITH MAINT CTL; COMPANY MECH REMOVED THE CIRCUIT BREAKER COLLAR ON THE APU FIRE DETECTION SYS. FLT DEPARTED NORMALLY USING APU FOR ENG START. FLT ARRIVED IN ONT AND DEPARTED THE GATE ON TIME. DURING TAXI OUT APPROX 1 MIN AFTER APU WAS SHUT DOWN; WE RECEIVED AN APU FIRE WARNING. WE PERFORMED QRH ITEMS AND FIRED APU EXTINGUISHER. WE THEN ASKED TWR IF THERE WAS ANY VISUAL SMOKE INDICATION COMING FROM OUR APU. THEY SAID THERE WAS NOTHING. WE REQUESTED CFR TO INSPECT AND RETURNED TO THE GATE FOR AN EVAC VIA THE JETWAY. SUBSEQUENT MAINT INSPECTION REVEALED NO FIRE HAD OCCURRED. WE MEL'ED THE APU AND THE FLT DEPARTED ABOUT 2 HRS AFTER OUR ORIGINAL DEP TIME. UPON FURTHER INVESTIGATION OF THE MEL; WE NOTICED THAT THE APU DETECTOR INOP LIGHT HAS 2 DIFFERENT NOTES THAT APPLY TO 2 DIFFERENT FORMS OF MECHANICAL IRREGULARITY. WE ORIGINALLY BELIEVED THAT THE LIGHT WAS DEFERRED FOR NOT ILLUMINATING; VIA NOTE 1 OF MEL WHICH WOULD ALLOW THE APU TO BE USED. APPARENTLY IT WAS ACTUALLY DEFERRED UNDER NOTE 2 OF MEL FOR COMING ON SPURIOUSLY AND THAT CONDITION FORBIDS THE USE OF THE APU ALTOGETHER. WHEN COMPANY MAINT PERSONNEL REMOVED THE CIRCUIT BREAKER COLLAR; THAT ENABLED THE USE OF THE APU; WHICH CAUSED THE SUBSEQUENT FALSE FIRE INDICATION. A MORE THOROUGH REVIEW OF THE MEL NOTES AND DISCUSSIONS WITH MAINT CTL WOULD HAVE POSSIBLY HELPED TO UNDERSTAND THE TRUE NATURE OF THE DEFERRAL. A MORE THOROUGH REVIEW OF MEL NOTES THAT ARE APPLICABLE WITH DEFERRAL. THIS SHOULD INCLUDE A CONVERSATION WITH MAINT CTL FOR EXACT EXPLANATIONS. SUPPLEMENTAL INFO FROM ACN 734053: THE ACFT HAD 1 MEL IN THE LOGBOOK AND ON THE RELEASE. (I ALSO ADDED ANOTHER MEL RELATING TO A LEADING EDGE DEVICE LIGHT; NOT RELATED TO THIS INCIDENT.) IT WAS MEL APU DET INOP LIGHT INOP. I REVIEWED MEL UNDER THE WORDING IN THE LOGBOOK AND RELEASE: APU DET INOP LIGHT INOP. IT STATES IT MAY BE INOP; EXTINGUISHED; PROVIDED: THE APU FIRE DETECTION SYS OPERATES NORMALLY AND A FIRE WARNING TEST IS PERFORMED BEFORE EACH START. IT GOES ON TO STATE IN A NOTE: IF THE APU DET INOP LIGHT FAILS TO ILLUMINATE DURING OVERHEAT/INOP OR FAULT/INOP TEST; BUT APU FIRE WARNING SWITCH ILLUMINATES DURING FIRE TEST; APU FIRE DETECTION SYS IS OPERATING NORMALLY; AND THE APU MAY BE USED PROVIDED FIRE WARNING TEST IS CONDUCTED BEFORE EACH APU START. THE FO AND MYSELF CHKED THESE 2 ITEMS AND FOUND THE APU DET INOP LIGHT FAILED TO ILLUMINATE AND THE APU FIRE WARNING SWITCH ILLUMINATED DURING THE FIRE TEST. I; THEREFORE; CONCLUDED THE APU COULD BE USED. I HAD A MECH CLR THE OTHER MEL FROM THE ACFT AND HE SAID HE WOULD CLR BOTH MEL'S; BUT HE DIDN'T HAVE THE TIME TO DO IT. HE NEVER MENTIONED NOT TO USE THE APU. WE FLEW ON TO 3 MORE CITIES AND USED THE APU FOR START EACH TIME. LOOKING BACK; I CAN ONLY ASSUME THE WRITE-UP IN THE LOGBOOK AND THE RELEASE SHOULD HAVE BEEN APU DET INOP LIGHT INOP AND ILLUMINATED. I ALSO SHOULD HAVE CALLED MAINT TO ENSURE I HAD A CORRECT UNDERSTANDING OF THIS DEFERRED ITEM.

Data retrieved from NASA's ASRS site as of January 2009 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.