Narrative:

We had been cleared for the FMS bridge visual runway 28R at sfo airport by sfo approach control and advised to contact sfo tower. Sfo tower subsequently gave us clearance to land on runway 28R. We were between garow and samul intxns where the published minimum crossing altitude for samul intersection on the FMS visual approach was 1800 ft MSL. The first officer, who was the PF at the time, had set the altitude on the MCP at 1000 ft MSL which was approximately 1000 ft AGL. During our descent on the approach, we were advised that we would have traffic on the parallel runway, runway 28L. At the time we were trying to obtain visual contact with the reported traffic and pick up the airport visually. My first officer had informed me that he had never flown into sfo before so I was trying to assist him in making visual contact with the airport by reference to other visual clues. Before reaching samul intersection, I noticed that we had descended several hundred ft below the 1800 ft MSL required crossing altitude for samul intersection. I brought this to the attention of my first officer and he immediately began making a correction to get back onto the proper descent profile. Sfo tower then called us to inform us they had a low altitude alert on our aircraft. We acknowledged their report and proceeded with our approach to a normal landing. There are several factors that I believe led us into this situation. After landing, I discussed what had happened with my first officer. I learned that in order to obtain a higher rate of descent that he had selected the flight level change mode on the vertical navigation portion of the MCP rather than VNAV. In VNAV mode, all of the altitude constraints on the approach would have been honored by the FMC, whereas in the flight level change mode, only the altitude selected in the altitude window of the MCP would be honored. This was set at 1000 ft MSL. Distrs such as reported traffic approaching the parallel runway, responding to confign changes and checklist requests by the PF, and trying to help the first officer visually pick up the runway all contributed to my lack of altitude awareness, but perhaps the biggest factor was from the complacency of being in visual conditions, knowing we had safe ground clearance, having been cleared to land, and being focused mainly and establishing visual contact and alignment with the runway of intended landing. One factor that would have prevented our descent below 1800 ft MSL is if our altitude window would have been set to 1800 ft MSL on the MCP. Other than that, altitude awareness has to be maintained as a priority on visual approachs as if they are being flown in instrument conditions.

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Original NASA ASRS Text

Title: A B757 FLC DSNDS BELOW THE PLANNED ALT XING RESTR AT SAMUL INTXN DURING THEIR FMS BRIDGE VISUAL APCH TO SFO, CA.

Narrative: WE HAD BEEN CLRED FOR THE FMS BRIDGE VISUAL RWY 28R AT SFO ARPT BY SFO APCH CTL AND ADVISED TO CONTACT SFO TWR. SFO TWR SUBSEQUENTLY GAVE US CLRNC TO LAND ON RWY 28R. WE WERE BTWN GAROW AND SAMUL INTXNS WHERE THE PUBLISHED MINIMUM XING ALT FOR SAMUL INTXN ON THE FMS VISUAL APCH WAS 1800 FT MSL. THE FO, WHO WAS THE PF AT THE TIME, HAD SET THE ALT ON THE MCP AT 1000 FT MSL WHICH WAS APPROX 1000 FT AGL. DURING OUR DSCNT ON THE APCH, WE WERE ADVISED THAT WE WOULD HAVE TFC ON THE PARALLEL RWY, RWY 28L. AT THE TIME WE WERE TRYING TO OBTAIN VISUAL CONTACT WITH THE RPTED TFC AND PICK UP THE ARPT VISUALLY. MY FO HAD INFORMED ME THAT HE HAD NEVER FLOWN INTO SFO BEFORE SO I WAS TRYING TO ASSIST HIM IN MAKING VISUAL CONTACT WITH THE ARPT BY REF TO OTHER VISUAL CLUES. BEFORE REACHING SAMUL INTXN, I NOTICED THAT WE HAD DSNDED SEVERAL HUNDRED FT BELOW THE 1800 FT MSL REQUIRED XING ALT FOR SAMUL INTXN. I BROUGHT THIS TO THE ATTN OF MY FO AND HE IMMEDIATELY BEGAN MAKING A CORRECTION TO GET BACK ONTO THE PROPER DSCNT PROFILE. SFO TWR THEN CALLED US TO INFORM US THEY HAD A LOW ALT ALERT ON OUR ACFT. WE ACKNOWLEDGED THEIR RPT AND PROCEEDED WITH OUR APCH TO A NORMAL LNDG. THERE ARE SEVERAL FACTORS THAT I BELIEVE LED US INTO THIS SIT. AFTER LNDG, I DISCUSSED WHAT HAD HAPPENED WITH MY FO. I LEARNED THAT IN ORDER TO OBTAIN A HIGHER RATE OF DSCNT THAT HE HAD SELECTED THE FLT LEVEL CHANGE MODE ON THE VERT NAV PORTION OF THE MCP RATHER THAN VNAV. IN VNAV MODE, ALL OF THE ALT CONSTRAINTS ON THE APCH WOULD HAVE BEEN HONORED BY THE FMC, WHEREAS IN THE FLT LEVEL CHANGE MODE, ONLY THE ALT SELECTED IN THE ALT WINDOW OF THE MCP WOULD BE HONORED. THIS WAS SET AT 1000 FT MSL. DISTRS SUCH AS RPTED TFC APCHING THE PARALLEL RWY, RESPONDING TO CONFIGN CHANGES AND CHKLIST REQUESTS BY THE PF, AND TRYING TO HELP THE FO VISUALLY PICK UP THE RWY ALL CONTRIBUTED TO MY LACK OF ALT AWARENESS, BUT PERHAPS THE BIGGEST FACTOR WAS FROM THE COMPLACENCY OF BEING IN VISUAL CONDITIONS, KNOWING WE HAD SAFE GND CLRNC, HAVING BEEN CLRED TO LAND, AND BEING FOCUSED MAINLY AND ESTABLISHING VISUAL CONTACT AND ALIGNMENT WITH THE RWY OF INTENDED LNDG. ONE FACTOR THAT WOULD HAVE PREVENTED OUR DSCNT BELOW 1800 FT MSL IS IF OUR ALT WINDOW WOULD HAVE BEEN SET TO 1800 FT MSL ON THE MCP. OTHER THAN THAT, ALT AWARENESS HAS TO BE MAINTAINED AS A PRIORITY ON VISUAL APCHS AS IF THEY ARE BEING FLOWN IN INST CONDITIONS.

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.