Narrative:

A 3 mile 'bat' had already been placed on the B737. [I was] briefed that the B737 was reduced from 180 KTS to 160 KTS; as was the preceding aircraft; a B757; until 5.3 mile final. The ground speed showed 190 KTS; just as other aircraft on approach had indicated 190 KTS ground speed when at 180 KTS. Watching the replay; this speed was assigned approximately 17 miles from airport and ground speed remained constant at 190 KTS for almost 8 miles. At this point I had just taken sector and issued 160 KTS to the B737 approximately 10 miles from airport and it was acknowledged. I was basing my separation on the 3 mile 'bat' and sufficient distance remained for more than 3 mile spacing at the threshold. I changed the B737 frequency to sfo tower approximately 6 miles from the airport and they immediately issued 'go-around' instructions. At this point I saw the spacing was adequate for 3 miles but saw that 4 miles was needed. I was fully aware the preceding aircraft was a B757 and 4 miles was required. I was not aware; and did not notice; that the 'bat' was incorrectly applied by the previous controller. Again; I was basing separation with the incorrect 3 mile 'bat'. As I received the hand-offs at this sector I determine the spacing needed by applying the 'bat'. Only 2 'bats' were displayed as I took the sector. Obviously; I should have checked the previous controller's use of the 'bat'. The previous controller had issued the approach clearance. I just worked the aircraft for about 4 or 5 miles; watching the speeds closely; before changing the frequency. It is obvious now that the required distance was not maintained.

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Original NASA ASRS Text

Title: NCT Controller described a loss of B757 separation event on final to SFO when he/she failed to note the 'BAT'; (an electronic RADAR display separation device); was set at 3 miles instead of the required 4 miles in this instance.

Narrative: A 3 mile 'BAT' had already been placed on the B737. [I was] briefed that the B737 was reduced from 180 KTS to 160 KTS; as was the preceding aircraft; a B757; until 5.3 mile final. The ground speed showed 190 KTS; just as other aircraft on approach had indicated 190 KTS ground speed when at 180 KTS. Watching the replay; this speed was assigned approximately 17 miles from airport and ground speed remained constant at 190 KTS for almost 8 miles. At this point I had just taken sector and issued 160 KTS to the B737 approximately 10 miles from airport and it was acknowledged. I was basing my separation on the 3 mile 'BAT' and sufficient distance remained for more than 3 mile spacing at the threshold. I changed the B737 frequency to SFO Tower approximately 6 miles from the airport and they immediately issued 'go-around' instructions. At this point I saw the spacing was adequate for 3 miles but saw that 4 miles was needed. I was fully aware the preceding aircraft was a B757 and 4 miles was required. I was not aware; and did not notice; that the 'BAT' was incorrectly applied by the previous controller. Again; I was basing separation with the incorrect 3 mile 'BAT'. As I received the hand-offs at this sector I determine the spacing needed by applying the 'BAT'. Only 2 'BATs' were displayed as I took the sector. Obviously; I should have checked the previous controller's use of the 'BAT'. The previous controller had issued the approach clearance. I just worked the aircraft for about 4 or 5 miles; watching the speeds closely; before changing the frequency. It is obvious now that the required distance was not maintained.

Data retrieved from NASA's ASRS site as of April 2012 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.