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.