Narrative:

While en route from mia to jacksonville, the ZMA advised us that our transponder was not reporting our altitude. He also wanted us to verify that we were at FL190 and cycle our transponder. After confirming that we were leveled at FL190, we cycled our transponder code. 2 or 3 mins passed before ZMA advised us again that he was still not receiving our altitude, we then switched our transponder from #1 to #2. As we got closer to jacksonville, we were able to recycle #1 transponder 3 to 4 times, after which, it appears that #1 transponder was transmitting our altitude and our TCASII was also back in service. After being switched to jacksonville approach, we were given the localizer (back course) 31 at jacksonville. The jacksonville ATIS was reporting a ceiling of 800 ft and 3 mi visibility. The jacksonville approach vectored us to join the localizer (back course) 31 in the vicinity of craig VOR. We used the DME portion for our approach. We were cleared down to 3000 ft to join the localizer (back course) and cleared for the approach. After completion of our checklist and localizer idented, we descended to 1700 ft when we were inside the 10 NM. We then configured the aircraft for landing. When we reached 7.2 DME from craig, we selected a descent rate of 1200 FPM in our autoplt, started our time, and then started a descent to a MDA of 440 ft with an airspeed of 120 KTS. At approximately 850 ft with intermittent ground contact, approach control advised us of a low altitude alert and to check our altimeter. I disconnected the autoplt and leveled off at that altitude. After verifying that all 3 altimeters were correct, we then continued our approach. The probable cause for the low altitude alert was caused by the erroneous report from the transponder.

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Original NASA ASRS Text

Title: LOW ALT ALERT WARNING.

Narrative: WHILE ENRTE FROM MIA TO JACKSONVILLE, THE ZMA ADVISED US THAT OUR XPONDER WAS NOT RPTING OUR ALT. HE ALSO WANTED US TO VERIFY THAT WE WERE AT FL190 AND CYCLE OUR XPONDER. AFTER CONFIRMING THAT WE WERE LEVELED AT FL190, WE CYCLED OUR XPONDER CODE. 2 OR 3 MINS PASSED BEFORE ZMA ADVISED US AGAIN THAT HE WAS STILL NOT RECEIVING OUR ALT, WE THEN SWITCHED OUR XPONDER FROM #1 TO #2. AS WE GOT CLOSER TO JACKSONVILLE, WE WERE ABLE TO RECYCLE #1 XPONDER 3 TO 4 TIMES, AFTER WHICH, IT APPEARS THAT #1 XPONDER WAS XMITTING OUR ALT AND OUR TCASII WAS ALSO BACK IN SVC. AFTER BEING SWITCHED TO JACKSONVILLE APCH, WE WERE GIVEN THE LOC (BACK COURSE) 31 AT JACKSONVILLE. THE JACKSONVILLE ATIS WAS RPTING A CEILING OF 800 FT AND 3 MI VISIBILITY. THE JACKSONVILLE APCH VECTORED US TO JOIN THE LOC (BACK COURSE) 31 IN THE VICINITY OF CRAIG VOR. WE USED THE DME PORTION FOR OUR APCH. WE WERE CLRED DOWN TO 3000 FT TO JOIN THE LOC (BACK COURSE) AND CLRED FOR THE APCH. AFTER COMPLETION OF OUR CHKLIST AND LOC IDENTED, WE DSNDED TO 1700 FT WHEN WE WERE INSIDE THE 10 NM. WE THEN CONFIGURED THE ACFT FOR LNDG. WHEN WE REACHED 7.2 DME FROM CRAIG, WE SELECTED A DSCNT RATE OF 1200 FPM IN OUR AUTOPLT, STARTED OUR TIME, AND THEN STARTED A DSCNT TO A MDA OF 440 FT WITH AN AIRSPD OF 120 KTS. AT APPROX 850 FT WITH INTERMITTENT GND CONTACT, APCH CTL ADVISED US OF A LOW ALT ALERT AND TO CHK OUR ALTIMETER. I DISCONNECTED THE AUTOPLT AND LEVELED OFF AT THAT ALT. AFTER VERIFYING THAT ALL 3 ALTIMETERS WERE CORRECT, WE THEN CONTINUED OUR APCH. THE PROBABLE CAUSE FOR THE LOW ALT ALERT WAS CAUSED BY THE ERRONEOUS RPT FROM THE XPONDER.

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.