Nct controller described a go-around event after an air carrier was observed descending rapidly and ...

Narrative:

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 localizer/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.

Original NASA ASRS Text

Title: 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.

Narrative: 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.

Data retrieved from NASA's ASRS site as of July 2013 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.

Attributes
ACN	1103708
Time
Date	201307
Local Time Of Day	0001-0600
Place
Locale Reference	NCT.TRACON
State Reference	CA
Aircraft 1
Make Model Name	B777-300
Operating Under FAR Part	Part 121
Flight Phase	Final Approach
Route In Use	Visual Approach
Flight Plan	IFR
Person 1
Function	Approach
Qualification	Air Traffic Control Fully Certified
Events
Anomaly	ATC Issue All Types Inflight Event / Encounter Unstabilized Approach

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.