SFO Tower Controller and an air carrier flight crew reported the aircraft descended below the assigned altitude; left the Class Bravo; and the climbed back to assigned altitude.
Synopsis
SFO Tower Controller and an air carrier flight crew reported the aircraft descended below the assigned altitude; left the Class Bravo; and the climbed back to assigned altitude.
Narrative
We were landing Runways 28 and departing Runways 1. Light rain had been falling for a while and the aircraft were still able to visual in. The finals started to show 'I8L' and '+VSL' indicating that they were setting up for visuals 28R and ILS Runway 28 Left. Also the TRACONs measures were up. Aircraft X checked in for the right and were cleared to land. Then Aircraft Y joined final for the left and were cleared to land. Aircraft Y was above and to the left of Aircraft X probably a mile and a half stagger. The altitudes looked normal until I was talking to Aircraft Y and the Low Altitude Alert went off. I gave them a safety alert stating they were 6 and 1/2 miles final and 1;100 above plus the altimeter. I did not give a reference to SAMML crossing altitude as it is not depicted on the STARS map; and I was not sure if they had crossed it yet. They then showed 1;000 as they acknowledged my transmission. It almost seemed like I surprised them. A short time later I advised back in class bravo; and asked if they had a TCAS RA; no response. I asked if all was good. They said all is good. They continued to a normal landing.About fifteen minutes later we were on parallel ILS approaches. Up until the end of my session (40 minutes later) I could see all of the arrivals visually so I do not think they were descending to avoid weather on a shaky visual or anything like that. I am not sure what to recommend.
Second reporter narrative
While flying the FMS Bridge Visual to [Runway] 28R ATC requested we expedite our descent. We were above 3;000 ft; but not sure exactly where. I selected LVL CHG with the touchdown elevation set in the MCP (Mode Control Panel). We were distracted by the proximity to the traffic on [Runway] 28L. I left LVL CHG engaged longer than necessary. ATC issued a low altitude alert. I disengaged the autopilot; climbed slightly and then remained level to re-intercept the correct glide path; stabilizing the approach and landing uneventfully.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.