Narrative:

I was working all of radar combined in the tower on the d-brite while light; moderate and occasional heavy precipitation was moving through the area from west to east. About 20 minutes prior to receiving my first report of turbulence and erratic winds; a baron transited the same area with no complaints at all; leading me to believe this disturbance was small in area and moved in quite rapidly. Roughly 35 minutes preceding the subject of this event; I began receiving several pilot reports of continuous moderate chop from 6;000 ft-2;500 ft with multiple reports of gains and losses of 25 KTS in airspeed in that altitude range. Additionally; on the base turn for my pattern; the wind would vary from a headwind for the westbound base leg to a headwind for the eastbound base leg with single aircraft sometimes experiencing both a headwind and tailwind on the base leg. Indeed; the winds were incredibly erratic as several pilots said they had never experienced winds like this. A falcon was on roughly a 6 mile final when I turned a global express onto roughly a 14 mile final at 4;000. As the global express made the turn northbound and joined the final; the global express' ground speed increased from about 210 KTS to 280 KTS. With the falcon indicating about 130 KTS ground speed on final I knew the global express would have to slow significantly for the sequence to work. I told the global express to descend and maintain 3;000 to join the localizer in the hope that the tailwind he was in would shift to a headwind at the lower altitude. The global express lost very little ground speed and closed the gap quickly with the falcon. I knew the spacing was not going to work in enough time that I estimate the two aircraft had approximately 3.5 miles laterally when I initiated the missed approach for the global express. The global express was slow to execute the climb and turn I issued to maintain separation; and in the time it took the global express to make the turn and climb; the remaining distance closed and I suspect that the closest lateral distance the two aircraft came was approximately 2.8 miles. At no point were the two aircraft in unsafe proximity. Without corrective action; however; the situation would have become much more serious. Prior to this event I had never experienced weather phenomena similar to this. Every technique that I rely upon as a safe and effective controller seemed not to work as the wind and atmospheric conditions were so erratic that I could not develop any sort of a consistent picture of what the winds aloft were doing. It seemed that one turn to final would result in a ground speed gain and the aircraft would fly through final and the next turn the aircraft would lose speed and the turn would be too tight. My best suggestion for working in these conditions is; weather permitting; force oneself to slow down the scenario; and if the pilots will accept it; resort to unusually long finals and plenty of spacing to buffer and allow compensation time for what the weather/wind is doing that particular moment. I thought I was doing that with the subject scenario; but what I thought was plenty of extra spacing ended up being insufficient when coupled with the 60-70 KT increase in ground speed that was experienced when turning final.

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Original NASA ASRS Text

Title: TRACON Controller described an over take event on final and subsequent loss of separation when go around instructions were issued; the reporter listing varying wind conditions accompanying thunderstorm activity as contributing to the event.

Narrative: I was working all of RADAR combined in the Tower on the D-BRITE while light; moderate and occasional heavy precipitation was moving through the area from west to east. About 20 minutes prior to receiving my first report of turbulence and erratic winds; a Baron transited the same area with no complaints at all; leading me to believe this disturbance was small in area and moved in quite rapidly. Roughly 35 minutes preceding the subject of this event; I began receiving several pilot reports of continuous moderate chop from 6;000 FT-2;500 FT with multiple reports of gains and losses of 25 KTS in airspeed in that altitude range. Additionally; on the base turn for my pattern; the wind would vary from a headwind for the westbound base leg to a headwind for the eastbound base leg with single aircraft sometimes experiencing both a headwind and tailwind on the base leg. Indeed; the winds were incredibly erratic as several pilots said they had never experienced winds like this. A Falcon was on roughly a 6 mile final when I turned a Global Express onto roughly a 14 mile final at 4;000. As the Global Express made the turn northbound and joined the final; the Global Express' ground speed increased from about 210 KTS to 280 KTS. With the Falcon indicating about 130 KTS ground speed on final I knew the Global Express would have to slow significantly for the sequence to work. I told the Global Express to descend and maintain 3;000 to join the localizer in the hope that the tailwind he was in would shift to a headwind at the lower altitude. The Global Express lost very little ground speed and closed the gap quickly with the Falcon. I knew the spacing was not going to work in enough time that I estimate the two aircraft had approximately 3.5 miles laterally when I initiated the missed approach for the Global Express. The Global Express was slow to execute the climb and turn I issued to maintain separation; and in the time it took the Global Express to make the turn and climb; the remaining distance closed and I suspect that the closest lateral distance the two aircraft came was approximately 2.8 miles. At no point were the two aircraft in unsafe proximity. Without corrective action; however; the situation would have become much more serious. Prior to this event I had never experienced weather phenomena similar to this. Every technique that I rely upon as a safe and effective controller seemed not to work as the wind and atmospheric conditions were so erratic that I could not develop any sort of a consistent picture of what the winds aloft were doing. It seemed that one turn to final would result in a ground speed gain and the aircraft would fly through final and the next turn the aircraft would lose speed and the turn would be too tight. My best suggestion for working in these conditions is; weather permitting; force oneself to slow down the scenario; and if the pilots will accept it; resort to unusually long finals and plenty of spacing to buffer and allow compensation time for what the weather/wind is doing that particular moment. I thought I was doing that with the subject scenario; but what I thought was plenty of extra spacing ended up being insufficient when coupled with the 60-70 KT increase in ground speed that was experienced when turning final.

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.