B777-300
Air Traffic Control Fully Certified
ATC Issue All Types; Inflight Event / Encounter Unstabilized Approach
General None Reported / Taken
ATC Equipment / Nav Facility / Buildings; Human Factors; Procedure
Air Carrier X was cleared for a Visual Approach to Runway 28L on a 16 mile base to final at 4;300 FT. He was 8 miles behind an EMB-120 to the same runway. I assigned him 160 KTS or greater to a 7 mile final. At 9 mile final I switched the aircraft to SFO Tower frequency. He was established on final at 2;900 FT at that point and everything appeared normal. At about 6 mile final the aircraft started descending rapidly; as quick as 200 feet per RADAR update. The 'Low Altitude Alert' went off at about a 5 mile final. The aircraft was then at 700 FT. I immediately called the Tower and issued a 'Low Altitude Alert.' Subsequently; the aircraft went around. I decided that for the next approach I would give the aircraft a LOC/DME Y Runway 28L approach since both glideslopes are out of service. The second approach terminated without incident. Having a functional glideslope to at least one of the active runways could potentially have prevented this situation.
NCT Controller described a go-around event after an air carrier was observed descending rapidly and issued a 'low altitude alert;' the reporter noted both Glide Slopes to the parallel runways were inoperative.