Narrative:

Working an aircraft into lex from west at 11000 ft wanted lower, coordinated (my D side) for 8000 ft with respect to eastbound aircraft at 10000 ft. Other controller at sdf advised my D side that 10000 ft traffic was turning north. My D side to me he was turning 'a little north.' I did a flight plan readout and saw that the 10000 ft was a C550, my 11000 ft traffic was a pazt. The C550 was 8 mi south of 11000 ft traffic and in front by 2-3 mi. Thinking that C550 would only be turning a 'little north' ie, maybe 30-40 degrees left, I descended my 11000 ft traffic to 8000 ft. C550 turned sharply north (90 degree turn) and separation was lost. I should have questioned 'how far north?' and also taken note of route of flight of C550 before issuing lower to my aircraft.

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Original NASA ASRS Text

Title: LTSS WHEN RPTR DSNDS PAZT THROUGH ALT OF C550 ON A CONVERGING COURSE.

Narrative: WORKING AN ACFT INTO LEX FROM W AT 11000 FT WANTED LOWER, COORDINATED (MY D SIDE) FOR 8000 FT WITH RESPECT TO EBOUND ACFT AT 10000 FT. OTHER CTLR AT SDF ADVISED MY D SIDE THAT 10000 FT TFC WAS TURNING N. MY D SIDE TO ME HE WAS TURNING 'A LITTLE N.' I DID A FLT PLAN READOUT AND SAW THAT THE 10000 FT WAS A C550, MY 11000 FT TFC WAS A PAZT. THE C550 WAS 8 MI S OF 11000 FT TFC AND IN FRONT BY 2-3 MI. THINKING THAT C550 WOULD ONLY BE TURNING A 'LITTLE N' IE, MAYBE 30-40 DEGS L, I DSNDED MY 11000 FT TFC TO 8000 FT. C550 TURNED SHARPLY N (90 DEG TURN) AND SEPARATION WAS LOST. I SHOULD HAVE QUESTIONED 'HOW FAR N?' AND ALSO TAKEN NOTE OF RTE OF FLT OF C550 BEFORE ISSUING LOWER TO MY ACFT.

Data retrieved from NASA's ASRS site as of July 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.