AN ACR LGT CREW RPTS THAT THEY GOT BELOW THE FMS VISUAL APCH PATH AT SFO. ERROR ADMITTED. ARPT PROB ILS OTS.

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CLRED FOR FMS BRIDGE VISUAL RWY 28R. CAPT IS FLYING USING VNAV AND USING FLAPS AND SPD BRAKES TO GET BACK ON PATH. CROSSED GAROW AT 4000 FT WHICH IS NORMAL. HE MUST HAVE SELECTED FLT LEVEL CHANGE BECAUSE THE AIRPLANE DSNDED BELOW VNAV PATH. JUST OUTSIDE OF BRIJJ; WHICH WE SHOULD HAVE CROSSED AT 1800 FT; I NOTICED WE WERE AT APPROX 1200 FT VISUALLY. I CONFIRMED THAT WE WERE LOW BY LOOKING AT THE RWYS AND THE BRIDGE. I IMMEDIATELY INDICATED THIS TO THE CAPT WHO LEVELED OFF MANUALLY. THE ACFT GOT A LITTLE BELOW SPD. ONCE BACK ON PATH; THE REST OF APCH WAS NORMAL. CONTRIBUTING TO PROB WAS THE ILS RWY 28R WAS OTS. ALSO; THE CAPT WAS VERY EXHAUSTED DUE TO NUMEROUS ALL NIGHT FLTS PRIOR TO THIS FLT AND IS IN THE MIDDLE OF SOME FAMILY PROBS. I WAS TIRED ALSO. I LEARNED TO STAY IN VNAV ON THIS TYPE OF APCH OR DON'T RUN THE ALT SETTING ALL THE WAY DOWN AND SINCE IT'S A VISUAL APCH; DON'T FORGOT TO LOOK OUTSIDE A LOT (EVEN IN A GLASS COCKPIT AIRPLANE). SUPPLEMENTAL INFO FROM ACN 348390: I HAD SET MY ALT ALERT TO FIELD ELEVATION INSTEAD OF EACH XING FIX. I WAS HAND FLYING AFTER GAROW.