Nct controller described a loss of wake separation event on the sfo final when after issuing a go ar...

Narrative:

Aircraft Y; a B757; was following aircraft X a heavy B767 on final. Winds were a factor and aircraft were slow to slow. Aircraft X was inside the final approach fix and the ground speed dropped to 110. This was 20-30 KTS slower than all other aircraft had shown inside the marker. My attention was on the two aircraft the whole time. I slowed aircraft Y down to avoid a go around situation. The controller I relieved was on his two minute overlap and was monitoring the prm scope and counting down the separation and advising me of the 1 second update on speeds and separation. I canceled the approach clearance on aircraft Y; climbed him to 4;000 via runway heading; in which I thought was a timely manner according to the prm readout. The go around was reviewed by the supervisor performing omic duties and found that I only had 4.84 miles between the aircraft when aircraft X read back the clearance. I was never less than 1;000 ft from the preceding aircraft; but when on an instrument approach procedure; wake turbulence cannot be compromised. I have advised my supervisor that there may be a discrepancy between the prm readout on mileage and/or time. He agrees and is looking into it. Be that as it may; I accept responsibility for running them too tight and will give a little more of a buffer when dealing with overtakes in the future; while running things more conservatively. Also; I could have canceled aircraft Y's clearance earlier and stepped them down on the ILS behind the heavy jet until I was sure the spacing was/was not there.

Original NASA ASRS Text

Title: NCT Controller described a loss of wake separation event on the SFO final when after issuing a go around; radar data indicated the separation was less than required prior to the go around instruction; contrary to PRM Data.

Narrative: Aircraft Y; a B757; was following Aircraft X a Heavy B767 on final. Winds were a factor and aircraft were slow to slow. Aircraft X was inside the final approach fix and the ground speed dropped to 110. This was 20-30 KTS slower than all other aircraft had shown inside the marker. My attention was on the two aircraft the whole time. I slowed Aircraft Y down to avoid a go around situation. The Controller I relieved was on his two minute overlap and was monitoring the PRM scope and counting down the separation and advising me of the 1 second update on speeds and separation. I canceled the approach clearance on Aircraft Y; climbed him to 4;000 via runway heading; in which I thought was a timely manner according to the PRM readout. The go around was reviewed by the supervisor performing OMIC duties and found that I only had 4.84 miles between the aircraft when Aircraft X read back the clearance. I was never less than 1;000 FT from the preceding aircraft; but when on an instrument approach procedure; wake turbulence cannot be compromised. I have advised my supervisor that there may be a discrepancy between the PRM readout on mileage and/or time. He agrees and is looking into it. Be that as it may; I accept responsibility for running them too tight and will give a little more of a buffer when dealing with overtakes in the future; while running things more conservatively. Also; I could have canceled Aircraft Y's clearance earlier and stepped them down on the ILS behind the heavy jet until I was sure the spacing was/was not there.

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.

Attributes
ACN	966665
Time
Date	201108
Local Time Of Day	0001-0600
Place
Locale Reference	SFO.Airport
State Reference	CA
Aircraft 1
Make Model Name	B767 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	Final Approach
Flight Plan	IFR
Aircraft 2
Make Model Name	B757 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	Final Approach
Flight Plan	IFR
Person 1
Function	Approach
Qualification	Air Traffic Control Fully Certified
Events
Anomaly	ATC Issue All Types Deviation - Procedural Published Material / Policy

Nct controller described a loss of wake separation event on the sfo final when after issuing a go around; radar data indicated the separation was less than required prior to the go around instruction; contrary to prm data.