Narrative:

Final vector scenario: a B757 was cleared for the approach and assigned 170 KTS to the outer marker; an A319 was 6 miles in trail also cleared and assigned 170 KTS to the outer marker with the wake turbulence advisor for the boeing 757. Using the distance measuring tools; 4.02 miles was shown at the outer marker so the A319 was told to; 'cancel approach clearance; turn left heading three-one-zero; climb and maintain three thousand.' at the point the pilot was still reading back the cancellation of the approach; 3.92 miles was recorded between the two aircraft for one sweep of the radar. It is unknown when the pilot flying started to take action to cancel the approach and adhere to the climb out instructions; it may have occurred before the loss of separation; however we only have the transmissions to go by and a delayed radar sweep to show the trend of the aircraft. Recommendation; I would recommend taking away the immense pressure put on the final vector controllers on strong weather days such as today. The controller is expected to make every aircraft exactly minimally spaced in order to reduce delay in the ATC system; however that is an extremely fine line. 4.0 mile spacing behind a boeing 757 is what is expected but if the separation becomes anything less than that; it is now deemed a major error with wake turbulence and the controller just went from performing his job correctly to now jeopardizing the safety of the aircraft. There has to be an exact line of what is safe and what isn't but to ask someone to juggle that fine line with every aircraft is pushing the envelope; thus causing these errors to occur more often than they need to be. If capacity was based off practical capacity and not theoretical capacity; it might make it easier on everybody working the system because in the real world; theoretical capacity can never be reached. I don't think that is necessarily a solution that can be put into practice as I am sure it varies between each facility but that seems to fit this case at least.

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Original NASA ASRS Text

Title: A90 Controller described a possible loss of separation between two aircraft on approach to BOS.

Narrative: Final vector scenario: a B757 was cleared for the approach and assigned 170 KTS to the Outer Marker; an A319 was 6 miles in trail also cleared and assigned 170 KTS to the Outer Marker with the wake turbulence advisor for the Boeing 757. Using the distance measuring tools; 4.02 miles was shown at the Outer Marker so the A319 was told to; 'Cancel approach clearance; turn left heading three-one-zero; climb and maintain three thousand.' At the point the pilot was still reading back the cancellation of the approach; 3.92 miles was recorded between the two aircraft for one sweep of the RADAR. It is unknown when the pilot flying started to take action to cancel the approach and adhere to the climb out instructions; it may have occurred before the loss of separation; however we only have the transmissions to go by and a delayed RADAR sweep to show the trend of the aircraft. Recommendation; I would recommend taking away the immense pressure put on the Final Vector Controllers on strong weather days such as today. The Controller is expected to make every aircraft exactly minimally spaced in order to reduce delay in the ATC system; however that is an extremely fine line. 4.0 mile spacing behind a Boeing 757 is what is expected but if the separation becomes anything less than that; it is now deemed a major error with wake turbulence and the Controller just went from performing his job correctly to now jeopardizing the safety of the aircraft. There has to be an exact line of what is safe and what isn't but to ask someone to juggle that fine line with every aircraft is pushing the envelope; thus causing these errors to occur more often than they need to be. If capacity was based off practical capacity and not theoretical capacity; it might make it easier on everybody working the system because in the real world; theoretical capacity can never be reached. I don't think that is necessarily a solution that can be put into practice as I am sure it varies between each facility but that seems to fit this case at least.

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.