B737-800
Flight Crew Air Transport Pilot (ATP)
Communication Breakdown; Situational Awareness
Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Issued Advisory / Alert; Flight Crew Became Reoriented; Flight Crew Returned To Clearance
Human Factors
We were told to expect the FMS Bridge Visual (RNAV-F) Rwy 28R to SFO outside of ARCHI. We were cleared for the visual approach to 28R. Both the Captain and I heard cleared for the Visual approach to 28R; not cleared FMS Bridge Visual (RNAV-F) Rwy 28R.  I was flying pilot and told the captain I was going to hand fly the approach since it was VFR and we had the bridge and the airport in sight. I disengaged the autopilot and started a gradual descent. Somewhere after JANYY the captain said I was low and I initiated a climb (I saw 1200 feet) to 1500 feet. Right after I started the climb we got a low altitude alert from Tower. We landed at SFO.We should have verified the clearance of 'Visual approach' versus the FMS Bridge Visual Rwy 28R. Since the FMS approach is an instrument approach versus a visual approach. Use VNAV/LNAV until established on straight in final. Better scan of instruments (altimeter) on a visual approach. Not hand flying an unfamiliar approach even in severe clear conditions.
I was PM and the FO was PF arriving at SFO on the DYAMD2; FMS BRIDGE VISUAL Rwy 28R. At about 15 miles from the airport; after calling the airport and the San Mateo bridge in sight; we were cleared the FMS Bridge Visual approach RWY 28R . I mistakenly thought that we were cleared a visual approach and were cleared off of the FMS BRIDGE VISUAL 28R approach. I did not think that we had to meet the remaining altitude restrictions which applied to the FMS BRIDGE VISUAL. At about 15 miles out; the FO disengaged the autopilot and flew the rest of the approach by hand; beginning a shallow descent. At one point I noticed that we were at about 1;200 feet between JANYY (10DME) and SAMUL (7DME). I also noticed that we were below both the ILS 28R glide slope and the VNAV PATH. I thought this was too low even for a visual approach since we were nearing final configuration altitude of 1;000 AGL; so I told the FO that we were too low. Just a moment later approach control issued a Low Altitude Warning to us and we climbed back up to 1;500 feet and complied with the remaining crossing altitude of 1;200 feet at F101D. We prematurely descended through the altitudes of 2;500 feet at JANYY and 1;500 feet at SAMUL.I have flown into SFO enough to be familiar with the FMC BRIDGE VISUAL Rwy 28R approach. I have never before descended below any of the altitude restrictions. Somehow; late during the approach; I thought we were cleared a normal visual approach. This was unusual enough so that I should have confirmed our clearance. By neglecting to be 100% sure what our clearance was; I allowed a large possibility of error to develop. I also knew that the FO hadn't been to SFO in years; so I should have flown the approach myself and let the FO observe. Even if we had been cleared a generic visual approach instead of the FMC BRIDGE VISUAL; I should have known the parameters of the Class B airspace and not have let us descend out of it.
B737-800 flight crew reported being cleared for the visual approach to Runway 28R at SFO then learned later they were actually cleared for the FMS BRIDGE VISUAL Runway 28R. Between JANYY and SAMUL the flying First Officer descended to 1200 feet causing a low altitude alert from ATC and a climb back to 1500 feet.