B737-500 FLC FAILED TO LEVEL AT ASSIGNED ALT WHILE LOOKING FOR ARPT AT SFO; CA.

Narrative

THE FLT; ACR XYZ; FROM LAS VEGAS TO SAN FRANCISCO WAS PLANNED WITH A FUEL LOAD TO LAND AT SFO AT MAX ALLOWABLE LNDG WT. THE ENTIRE SFO BAY AREA HAD EXPERIENCED 2 DAYS OF DENSE FOG AND SUBSEQUENT ARPT CLOSURES AND DELAYS. SHORTLY AFTER LEVELOFF IT BECAME APPARENT WE WOULD LAND OVERWT AT THE FUEL BURN RATE WHICH EXISTED. I DEPLOYED PARTIAL SPD BRAKES AND GOT A LOWER CRUISE ALT. THE COPLT AND I AGREED THAT EVEN AFTER THIS ACTION WE WOULD STILL NEED TO LOWER THE LNDG GEAR EARLY ON THE APCH TO ENSURE THE ACFT WAS BELOW THE MAX ALLOWABLE LNDG WT AT TOUCHDOWN. THIS WAS DONE AT APPROX 25 MI FROM SFO. APCH CTL GAVE US A DSCNT TO 6000 FT FROM 10000 FT. A FEW MOMENTS BEFORE REACHING 6000 FT; WE WERE ASKED TO RPT THE FIELD IN SIGHT. WE WERE GIVEN CLRNC FOR THE QUIET BRIDGE VISUAL APCH BY APCH. ALL 3 OF US (THERE WAS ANOTHER PLT ON THE JUMP SEAT) AGREED THAT THE PREVIOUSLY FOGGY CONDITIONS WERE DISSIPATING RAPIDLY; YET THE FIELD WAS STILL NOT IN SIGHT. WE DID SEE THE SAN MATEO BRIDGE BUT A FOG BANK WAS BTWN THAT AND THE FIELD. AS THIS WAS GOING ON I HEARD THE ALT ALERT SOUND. I GLANCED AT THE ALTIMETER AND SAW US AT 5700 FT AND DSNDING. I IMMEDIATELY RAISED THE NOSE I WAS HAND-FLYING AND APPLIED PWR AND RETURNED TO 6000 FT. THE UNUSUAL CONFIGN OF GEAR DOWN EARLY IN THE APCH CONTRIBUTED TO THE ALTDEV BUT THE FACT THAT I WAS NOT CONCENTRATING ON MY BASIC RESPONSIBILITY TO FLY THE AIRPLANE CAUSED THE DEV. HAD I WAITED FOR THE COPLT TO SIGHT THE FIELD AND THEN LOOKED; THE LENGTHY TIME I SPENT LOOKING OUTSIDE AND THE ALTDEV; WOULD NOT HAVE OCCURRED.