Narrative:

I was descending the southbound A319 to 6;000 ft into tampa and instructed him to turn left 15 degrees for traffic that would cross out in front at 10;000 ft; northwestbound. I turned on my 'min' key to verify that my turn would be enough to sustain horizontal separation. I witnessed the 'min' alert go from approximately 1.59 miles to greater than 3 miles after the A319 completed his turn. After performing some other controller task; I went back to the A319 and witnessed the 'min' alert go down to 3.02 miles in separation. I then looked down to turn off the 'min' key and as I looked back up I noticed that the separation went down to 2.99 miles at the same time I hit the enter button. I went to call traffic and inadvertently knocked my headset off my ear. After which the aircraft passed each other without incident. I then turned the A319 back toward tampa and proceeded the descent to a further altitude. Neither aircraft were made aware of each other. And neither aircraft were in a position of compromise. I use the 'min' key a lot and have never noticed separation decrease after all turns were completed. I never turned the A319 or the PA46 after my initial turn. It is possible that either aircraft deviated slightly for a cloud without my knowledge. I acknowledge that I should have called traffic earlier; which may have averted any concern should they had been in visual contact with each other.

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Original NASA ASRS Text

Title: TPA TRACON Controller described proximity event at 10;000 FT; relying on automation assist tool to determine separation; resulting in minimum spacing event.

Narrative: I was descending the southbound A319 to 6;000 FT into Tampa and instructed him to turn left 15 degrees for traffic that would cross out in front at 10;000 FT; northwestbound. I turned on my 'MIN' key to verify that my turn would be enough to sustain horizontal separation. I witnessed the 'MIN' alert go from approximately 1.59 miles to greater than 3 miles after the A319 completed his turn. After performing some other controller task; I went back to the A319 and witnessed the 'MIN' alert go down to 3.02 miles in separation. I then looked down to turn off the 'MIN' key and as I looked back up I noticed that the separation went down to 2.99 miles at the same time I hit the enter button. I went to call traffic and inadvertently knocked my headset off my ear. After which the aircraft passed each other without incident. I then turned the A319 back toward Tampa and proceeded the descent to a further altitude. Neither aircraft were made aware of each other. And neither aircraft were in a position of compromise. I use the 'MIN' key a lot and have never noticed separation decrease after all turns were completed. I never turned the A319 or the PA46 after my initial turn. It is possible that either aircraft deviated slightly for a cloud without my knowledge. I acknowledge that I should have called traffic earlier; which may have averted any concern should they had been in visual contact with each other.

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.