B737-800
Flight Crew Air Transport Pilot (ATP)
Deviation - Altitude Crossing Restriction Not Met; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Flight Deck / Cabin / Aircraft Event Other / Unknown; Inflight Event / Encounter CFTT / CFIT
Air Traffic Control Issued Advisory / Alert; Flight Crew Became Reoriented; Flight Crew Returned To Clearance
Aircraft; Chart Or Publication; Human Factors; Procedure
On the FMS bridge visual 28R approach; on dogleg to final; between SAMUL and F101D; I was flying the approach manually; using the HUD; with LNAV and VNAV engaged. I followed the HUD commands closely and as I realized we were lower than normal; the Tower notified us that they were getting an altitude alert. I recall being above 500 FT. At that time; I was turning to line up with the runway. I leveled off and continued the approach to intercept the PAPI glide path; and landed the airplane. I do not have access to the FMC as I write this; but I do recall that there was a published glide path on this approach. I thought that this would allow the use of VNAV. I am not certain why the HUD commanded a greater than normal descent; but in the future this could be avoided by either engaging the ILS approach mode after SAMUL; or maintaining visual contact with the PAPI after SAMUL.
[No substantive additional information was included in the secondary narrative.]
A B737-800 flight crew received a low altitude alert from SFO Tower when they improperly used VNAV guidance during a FMS Bridge Visual to Runway 28R.