B737-800 FLT CREW IS ISSUED A LOW ALT ADVISORY DURING VISUAL APCH TO MEM.

Narrative

WHILE BEING VECTORED BY MEMPHIS APCH CTL FOR FINAL APCH INTO MEMPHIS; LNDG TO THE N; WE WERE GIVEN THREE DIFFERENT RWYS TO EXPECT TO LAND ON RWYS 36R; 36C; AND FINALLY RWY 36L. IN ADDITION WE HAD BEEN GIVEN DSCNT CLRNCS TO BE AT 3000 FT MSL APPROX 25 MI FROM THE ARPT AND AN ASSIGNED SPD OF 170 KIAS. WHEN STILL OUTSIDE OF FREAZ WE WERE CLRED FOR THE VISUAL APCH TO RWY 36L; WE WERE ALSO CLRED TO 2000 FT MSL. THE ALT AT FREAZ WAS CHARTED AT 4000 FT MSL WITH 3000 FT MSL BY 'ATC AUTH.' THE LOWER THAN CHARTED ALT CLRNC SEEMED ACCEPTABLE SINCE THE WX WAS VMC WITH EXCELLENT VISIBILITY (35+ MI). THE FINAL APCH FIX KOLEY HAD 1800 FOR ITS CHARTED ALT (1733 FT ON GS). SINCE WE WERE OVER 15 MI FROM THE ARPT I INITIATED A SLOW DSCNT OF 500 FPM TO THE ASSIGNED 2000 FT MSL. WHEN PAST FREAZ I SET 1800 FT MSL (THE ALT AT KOLEY) AND ARMED THE LOC ON THE MODE CTL PANEL. WHEN THE LOC WAS CAPTURED BY THE AUTOPLT I ARMED THE APCH MODE (FOR EVENTUAL GS CAPTURE) AND SET THE MISSED APCH ALT OF 5000 FT MSL. MY MISTAKE WAS I HAD NOT CONFIRMED THAT ALT HOLD WAS ANNUNCIATED INDICATING WE HAD LEVELED OFF AT 1800 FT MSL UNTIL GS CAPTURE. AS A RESULT WE CONTINUED OUR SLOW DSCNT THROUGH 1800 FT MSL TO APPROX 1500 FT MSL WHEN WE NOTICED THE DEV AND STARTED A CLB BACK UP TO 1800 FT MSL. AS WE STARTED CLBING BACK UP THE TWR/APCH CTLR GOT A LOW ALT ALERT AND INFORMED US OF SUCH; ASKING US TO CHK OUR ALT AND ALTIMETER SETTING. AFTER CORRECTING THE DEV WE INTERCEPTED THE GS AND COMPLETED THE FLT UNEVENTFULLY.