Date: 201309
Local time of day: 1201-1800
Location: ZZZ.TRACON, US
Aircraft: Dash 8-400
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Final Approach
ATC / advisory: TRACON ZZZ
Second aircraft: Dash 8-400
Reporter organization: Government
Reporter function: Approach; Departure
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Situational Awareness
Anomaly: ATC Issue All Types; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Air Traffic Control
Result: General None Reported / Taken
Contributing factors: ATC Equipment / Nav Facility / Buildings; Human Factors; Procedure
Primary problem: ATC Equipment / Nav Facility / Buildings

Narrative: I was working monitor for Runway XXL. The visibility was low and there was a 40 knot tail wind until about the final approach fix where it switched to a head wind. Five to six mile hits from FR1 was compressing to 3 miles by the runway requiring me to assign several speed restrictions to almost every aircraft. The first Dash 8-400 was slowed early to 150 knots following traffic around 10 mile final. I slowed the first Dash 8-400 to 150 knots when the spacing compressed to 4 miles. Several seconds later; when the first Dash 8-400 crossed the FAF; I slowed the second Dash 8-400 to final approach speed. When both aircraft were inside the marker; the first Dash 8-400 was indicating 130 knots and the second Dash 8-400 was indicating 140 knots. My TPA 'bat' showed that I still had a little over 3 miles and I was anticipating that the second Dash 8-400 would slow further to match the first Dash 8-400's speed. I called Local to see if maybe she could see the high speed taxiway momentarily to allow me to go to 2.5 miles; but she said no. I got off the line to make one last judgment call. It became apparent that I would not be able to maintain 3 miles between the two; so I made the decision to pull the second Dash 8-400 out. At this time; a B737-700 checked in. I was waiting for Local to respond; then I would immediately send the second Dash 8-400 around as it appeared that I still had 3 miles and I did not need to override the frequency.  Local instead sent the second Dash 8-400 around before answering the B737-700. Recommendation; although the TPA bat made it appear that I still had 3 miles; the replay showed that as the B737-700 was checking on separation diminished to 2.94 miles. I waited until the last second to allow the second Dash 8-400 a chance to slow 10 knots further to hold the 3 miles.  I should have sent the second Dash 8-400 around without regard to putting another plane in the pattern. It has been suggested that I use the 'splat T' function to give further separation guidance which I will do. I also refrained from requesting a slower speed at the trips bar from Final 1. I knew that if all aircraft came over at 150 knots at [an airspace fix] vs. 170; compression would not have been such a factor. I again declined to request this as I saw how heavy the flow was from [Center] and did not want to put undue strain on an already busy Final Controller. In the future I shall be more proactive. I do feel that the rate was set too high for the weather conditions putting several controllers in situations where they faced situations that were more complex than normal. The TRACON team as a whole; to include supervisors and TMU; should have been more proactive and called for a lower rate from [the Center]. There were several go-arounds that morning; both before this incident and after; indicating that the existing configuration was not working.




Synopsis: Monitor Controller experienced a loss of separation event when the required 3 mile separation reduced to 2.94; the reporter noting the TPA 'Bat' tool is less than precise in measuring distance.