B737-800
Air Traffic Control Fully Certified
Airspace Violation All Types; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter CFTT / CFIT
Air Traffic Control Issued Advisory / Alert; Flight Crew FLC complied w / Automation / Advisory
Human Factors
Aircraft X was on a visual approach just checking in around the SFO 127/9 DME for the visual approach to Runway 28 Left. The Local Controller cleared them to land; and shortly thereafter the MSAW/LA warning went off. The Local Controller gave them the 'Low Altitude Alert; Aircraft X check your altitude immediately.' Their altitude dropped them to 1200 feet; which placed them outside of the Bravo airspace into San Carlos (SQL) airport traffic area; while SQL Tower was closed. I suggested to the Local Controller that an aircraft on a visual approach should be given distance from touchdown and altitude so that the pilot can quickly relate their descent rate and adjust as necessary for the glide path. Additionally; we need to tell them when they have left Bravo airspace. There were no outages of the PAPI or glide slope; so it makes me wonder what the pilots were navigating off of. Maintain enough altitude to cross the San Mateo Bridge at 1800 feet as recommended on the charted visual approaches.
SFO Controller reported a B737-800 on a visual approach descended out of Class B and activated a Low Altitude Alert.