Narrative:

I was conducting OJT on the sfo finals during very low ceilings weather; rvrs were fluctuating between 1;000 and 6;500. Sfo was IFR with 1/4 mile visibility so we were conducting ILS approaches to runway 28R. A heavy B767 was cleared for approach and issued a speed to axmul. An A319 was cleared 6 plus miles behind the B767 and given the same speed. When the B767 hit axmul he slowed to his final and the A319 closed due to compression. Both the supervisor and I; along with the trainee; were watching the distance between the two aircraft closely using the prm scopes located next to the woodside/foster scopes. The prm gives a more accurate measure of distance than the woodside/foster scopes do. Before they closed to 5 miles the trainee issued a go-around to the A319. It was later determined that the two aircraft were 4.95 miles apart when the go-around was issued and 4.93 miles apart when the A319 acknowledged the instructions. It was observed that the closest vertical separation was 1;900 ft at the time the A319 went around but 4.93 miles lateral. In my opinion there was never a safety issue with the operation. The problem I have is that we; the controllers; are being held to a standard that we can't meet as the people watching can. Our equipment does not show .05 miles separation whereas the equipment that the people watching us does. Even our prm scopes which are set about 8 ft from our primary scope is not as accurate as the equipment that others are using to determine separation errors. If we are to be held to such fine standards we should be given the equipment necessary to achieve such standards.

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Original NASA ASRS Text

Title: NCT Controller described a heavy separation error detected by after the fact computer reviews noting that even the very precise PRM equipment was unable to detect the apparent 'tenths' of a mile error.

Narrative: I was conducting OJT on the SFO Finals during very low ceilings weather; RVRs were fluctuating between 1;000 and 6;500. SFO was IFR with 1/4 mile visibility so we were conducting ILS Approaches to Runway 28R. A Heavy B767 was cleared for approach and issued a speed to AXMUL. An A319 was cleared 6 plus miles behind the B767 and given the same speed. When the B767 hit AXMUL he slowed to his final and the A319 closed due to compression. Both the Supervisor and I; along with the Trainee; were watching the distance between the two aircraft closely using the PRM scopes located next to the Woodside/Foster scopes. The PRM gives a more accurate measure of distance than the Woodside/Foster scopes do. Before they closed to 5 miles the Trainee issued a Go-Around to the A319. It was later determined that the two aircraft were 4.95 miles apart when the go-around was issued and 4.93 miles apart when the A319 acknowledged the instructions. It was observed that the closest vertical separation was 1;900 FT at the time the A319 went around but 4.93 miles lateral. In my opinion there was never a safety issue with the operation. The problem I have is that we; the controllers; are being held to a standard that we can't meet as the people watching can. Our equipment does not show .05 miles separation whereas the equipment that the people watching us does. Even our PRM scopes which are set about 8 FT from our primary scope is not as accurate as the equipment that others are using to determine separation errors. If we are to be held to such fine standards we should be given the equipment necessary to achieve such standards.

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.