Narrative:

Air carrier X was eastbound, going to dtw over the lan VORTAC. Air carrier Y was sebnd, going to dtw, north of air carrier X on a converging course. Air carrier X was slowed to 270 KTS for traffic in front of him. Air carrier Y was later slowed to 250 KTS and turned to a 180 degree heading for spacing. Both aircraft were descending to 10000'. After turning air carrier Y south, I observed the speed drop. According to our upper wind reports that we have in the computer, air carrier X had a 30-50 KT tailwind. Winds showed 270 at 57 KTS at FL240. Afterward I was told I could not rely upon this information as accurate. By relying upon the winds, the difference in assigned speeds, and what I thought I observed on the ground speed readouts in the data block. I felt these aircraft were separated. I looked away from the situation to take care of something else, and when I looked back, I realized the situation was not working. I then stopped air carrier X at 13000', but it was not in time to provide sep. I was also told that I could not have seen what I said I saw in the data block of air carrier Y speed-wise. The way to prevent this type error is to always use altitudes when vectoring for spacing. If an aircraft cannot meet any restrictions to meet letters of agreement, spin them!

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Original NASA ASRS Text

Title: ACFT ON CONVERGING COURSES CAME IN CLOSE PROX WHILE IN DESCENT.

Narrative: ACR X WAS EBND, GOING TO DTW OVER THE LAN VORTAC. ACR Y WAS SEBND, GOING TO DTW, N OF ACR X ON A CONVERGING COURSE. ACR X WAS SLOWED TO 270 KTS FOR TFC IN FRONT OF HIM. ACR Y WAS LATER SLOWED TO 250 KTS AND TURNED TO A 180 DEG HDG FOR SPACING. BOTH ACFT WERE DSNDING TO 10000'. AFTER TURNING ACR Y S, I OBSERVED THE SPD DROP. ACCORDING TO OUR UPPER WIND RPTS THAT WE HAVE IN THE COMPUTER, ACR X HAD A 30-50 KT TAILWIND. WINDS SHOWED 270 AT 57 KTS AT FL240. AFTERWARD I WAS TOLD I COULD NOT RELY UPON THIS INFO AS ACCURATE. BY RELYING UPON THE WINDS, THE DIFFERENCE IN ASSIGNED SPDS, AND WHAT I THOUGHT I OBSERVED ON THE GND SPD READOUTS IN THE DATA BLOCK. I FELT THESE ACFT WERE SEPARATED. I LOOKED AWAY FROM THE SITUATION TO TAKE CARE OF SOMETHING ELSE, AND WHEN I LOOKED BACK, I REALIZED THE SITUATION WAS NOT WORKING. I THEN STOPPED ACR X AT 13000', BUT IT WAS NOT IN TIME TO PROVIDE SEP. I WAS ALSO TOLD THAT I COULD NOT HAVE SEEN WHAT I SAID I SAW IN THE DATA BLOCK OF ACR Y SPD-WISE. THE WAY TO PREVENT THIS TYPE ERROR IS TO ALWAYS USE ALTS WHEN VECTORING FOR SPACING. IF AN ACFT CANNOT MEET ANY RESTRICTIONS TO MEET LETTERS OF AGREEMENT, SPIN THEM!

Data retrieved from NASA's ASRS site as of August 2007 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.