Narrative:

First officer flying; captain was pilot monitoring on the mia localizer 8L approach. First officer had briefed that he would utilize VNAV speed intervene during the localizer non-precision approach. Mia approach cleared us for the visual approach and to cross 10 mi west of mia at or above 3000 ft; then switched us to mia tower. I ensured we met the 3000 ft restr while checking in with tower. They cleared us to land #2 behind a learjet 4 mi ahead. I told the first officer to continue to slow while I looked for the traffic. I guess I was distraction while looking for traffic and talking to tower; and I missed that the first officer stated the autoplt was off. I did hear when he asked for VNAV speed 140 KTS; but I thought he was merely informing me that he had selected it (because I was under the impression the autoplt was still on). Consequently; neither of us selected VNAV. As we passed through 3000 ft; I said 'I'll set 500 ft for the MDA.' at approximately 6.5 mi from mia; I noticed we were descending through 1200 ft. I challenged him on it and he leveled at 1100 ft and stayed there. Mia tower called us shortly after that stating; 'low altitude warning.' by that time we were nearly back on the VNAV path again. I figured out what had happened; selected VNAV and the first officer intercepted the VNAV path and flew the rest of the approach without further incident. The 'low altitude alert' call from mia tower was a safety call considering we were cleared for the visual approach. Next time I'll ensure I am better aware of the AFDS mode currently selected and that we maintain at or above the VNAV path on a visual approach if one is available in the FMS database.

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Original NASA ASRS Text

Title: A B757-200 FLT CREW DESCENDED BELOW CHARTED ALTITUDE ON A NON-PRECISION APPROACH TO MIA.

Narrative: FO FLYING; CAPT WAS PLT MONITORING ON THE MIA LOC 8L APCH. FO HAD BRIEFED THAT HE WOULD UTILIZE VNAV SPD INTERVENE DURING THE LOC NON-PRECISION APCH. MIA APCH CLRED US FOR THE VISUAL APCH AND TO CROSS 10 MI W OF MIA AT OR ABOVE 3000 FT; THEN SWITCHED US TO MIA TWR. I ENSURED WE MET THE 3000 FT RESTR WHILE CHKING IN WITH TWR. THEY CLRED US TO LAND #2 BEHIND A LEARJET 4 MI AHEAD. I TOLD THE FO TO CONTINUE TO SLOW WHILE I LOOKED FOR THE TFC. I GUESS I WAS DISTR WHILE LOOKING FOR TFC AND TALKING TO TWR; AND I MISSED THAT THE FO STATED THE AUTOPLT WAS OFF. I DID HEAR WHEN HE ASKED FOR VNAV SPD 140 KTS; BUT I THOUGHT HE WAS MERELY INFORMING ME THAT HE HAD SELECTED IT (BECAUSE I WAS UNDER THE IMPRESSION THE AUTOPLT WAS STILL ON). CONSEQUENTLY; NEITHER OF US SELECTED VNAV. AS WE PASSED THROUGH 3000 FT; I SAID 'I'LL SET 500 FT FOR THE MDA.' AT APPROX 6.5 MI FROM MIA; I NOTICED WE WERE DSNDING THROUGH 1200 FT. I CHALLENGED HIM ON IT AND HE LEVELED AT 1100 FT AND STAYED THERE. MIA TWR CALLED US SHORTLY AFTER THAT STATING; 'LOW ALT WARNING.' BY THAT TIME WE WERE NEARLY BACK ON THE VNAV PATH AGAIN. I FIGURED OUT WHAT HAD HAPPENED; SELECTED VNAV AND THE FO INTERCEPTED THE VNAV PATH AND FLEW THE REST OF THE APCH WITHOUT FURTHER INCIDENT. THE 'LOW ALT ALERT' CALL FROM MIA TWR WAS A SAFETY CALL CONSIDERING WE WERE CLRED FOR THE VISUAL APCH. NEXT TIME I'LL ENSURE I AM BETTER AWARE OF THE AFDS MODE CURRENTLY SELECTED AND THAT WE MAINTAIN AT OR ABOVE THE VNAV PATH ON A VISUAL APCH IF ONE IS AVAILABLE IN THE FMS DATABASE.

Data retrieved from NASA's ASRS site as of May 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.