Narrative:

While being vectored by memphis approach control for final approach into memphis; landing to the north; we were given three different runways to expect to land on runways 36R; 36C; and finally runway 36L. In addition we had been given descent clrncs to be at 3000 ft MSL approximately 25 mi from the airport and an assigned speed of 170 KIAS. When still outside of freaz we were cleared for the visual approach to runway 36L; we were also cleared to 2000 ft MSL. The altitude at freaz was charted at 4000 ft MSL with 3000 ft MSL by 'ATC authority/authorized.' the lower than charted altitude clearance seemed acceptable since the WX was VMC with excellent visibility (35+ mi). The final approach fix koley had 1800 for its charted altitude (1733 ft on GS). Since we were over 15 mi from the airport I initiated a slow descent of 500 FPM to the assigned 2000 ft MSL. When past freaz I set 1800 ft MSL (the altitude at koley) and armed the localizer on the mode control panel. When the localizer was captured by the autoplt I armed the approach mode (for eventual GS capture) and set the missed approach altitude of 5000 ft MSL. My mistake was I had not confirmed that altitude hold was annunciated indicating we had leveled off at 1800 ft MSL until GS capture. As a result we continued our slow descent through 1800 ft MSL to approximately 1500 ft MSL when we noticed the deviation and started a climb back up to 1800 ft MSL. As we started climbing back up the tower/approach controller got a low altitude alert and informed us of such; asking us to check our altitude and altimeter setting. After correcting the deviation we intercepted the GS and completed the flight uneventfully.

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Original NASA ASRS Text

Title: 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.

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.