Narrative:

During flight planning, we observed APU was deferred inoperative due to negative bleed air. We read operations placard and discussed course of action for inoperative APU. We arrived at aircraft at XA45 and were met by a technician who briefed us that APU was inoperative, but due to failure of fire testing system, not bleed air. Technician was very pleasant and we thanked him for his efforts. Dispatch sent a new flight plan based on new MEL item and maintenance generated a new maintenance release document. We read operations placard. Remainder of departure sequence was normal. At approximately XC10, at FL310, abeam ZZZ airport, we preparing to descend into ZZZ2, the APU fire warning system activated. We accomplished emergency QRH checklist and flight manual reference items. An emergency was declared with ATC, dispatch notified via ACARS, flight attendants briefed on cabin advisory and a PA announcement was made to passenger. We believed that the warning was false due to the fact that: 1) APU had never been operated, and 2) maintenance had worked on system the night before. However, all appropriate SOP items were accomplished for APU in-flight fire. We determined that ZZZ2 was nearest suitable airport. Uneventful approach and landing. We asked ZZZ2 fire department to scan tail area after landing for any evidence of fire. They responded that there was no fire. We taxied to gate.

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Original NASA ASRS Text

Title: A B737-300 IN CRUISE AT FL310 DECLARED AN EMER DUE TO APU FIRE WARNING. APU PREVIOUSLY DEFERRED INOP.

Narrative: DURING FLT PLANNING, WE OBSERVED APU WAS DEFERRED INOP DUE TO NEGATIVE BLEED AIR. WE READ OPS PLACARD AND DISCUSSED COURSE OF ACTION FOR INOP APU. WE ARRIVED AT ACFT AT XA45 AND WERE MET BY A TECHNICIAN WHO BRIEFED US THAT APU WAS INOP, BUT DUE TO FAILURE OF FIRE TESTING SYS, NOT BLEED AIR. TECHNICIAN WAS VERY PLEASANT AND WE THANKED HIM FOR HIS EFFORTS. DISPATCH SENT A NEW FLT PLAN BASED ON NEW MEL ITEM AND MAINT GENERATED A NEW MAINT RELEASE DOCUMENT. WE READ OPS PLACARD. REMAINDER OF DEP SEQUENCE WAS NORMAL. AT APPROX XC10, AT FL310, ABEAM ZZZ ARPT, WE PREPARING TO DSND INTO ZZZ2, THE APU FIRE WARNING SYS ACTIVATED. WE ACCOMPLISHED EMER QRH CHKLIST AND FLT MANUAL REF ITEMS. AN EMER WAS DECLARED WITH ATC, DISPATCH NOTIFIED VIA ACARS, FLT ATTENDANTS BRIEFED ON CABIN ADVISORY AND A PA ANNOUNCEMENT WAS MADE TO PAX. WE BELIEVED THAT THE WARNING WAS FALSE DUE TO THE FACT THAT: 1) APU HAD NEVER BEEN OPERATED, AND 2) MAINT HAD WORKED ON SYS THE NIGHT BEFORE. HOWEVER, ALL APPROPRIATE SOP ITEMS WERE ACCOMPLISHED FOR APU INFLT FIRE. WE DETERMINED THAT ZZZ2 WAS NEAREST SUITABLE ARPT. UNEVENTFUL APCH AND LNDG. WE ASKED ZZZ2 FIRE DEPT TO SCAN TAIL AREA AFTER LNDG FOR ANY EVIDENCE OF FIRE. THEY RESPONDED THAT THERE WAS NO FIRE. WE TAXIED TO GATE.

Data retrieved from NASA's ASRS site as of July 2007 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.