Narrative:

Aircraft #1 departed ord wbound climbing to 13000 ft when I assumed control. Aircraft #1 was climbed to FL230; supposedly on 270 degree vector as per local LOA. Aircraft #2 checked on climbing to 13000 ft (departed ord) 5+ mi south of aircraft #1; also supposedly on 270 degree heading per LOA; and was climbed to FL230 and turned to 260 degree heading to increase divergence. On initial contact; aircraft were both supposed to be on parallel 270 degree headings and remain that way. However; even after assigning a heading to aircraft #2 to further diverge; the 2 aircraft actually continued to converge. Upon reviewing available radar data; it appears aircraft #1 was not on a 270 degree heading; but possibly a 260 degree or 250 degree heading; something not anticipated. Separation was lost near 15000N ft; at which time aircraft #1 was turned to 280 degrees to diverge. Now; 1 week later; upon further review of radar data from both center and TRACON; it appears aircraft #1 initially flew 270 dg heading; then subsequently turned direct iow (for unknown reason); thereby making it a probable pilot deviation. Due to extremely busy and complex departure rush and the slow convergence rate; I was unable to detect the discrepancy until separation was lost. It has been classified as a low severity/controled operational error; but may be reclassified as a pilot deviation.

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Original NASA ASRS Text

Title: ZAU CTLR EXPERIENCED OPERROR AT 15000 FT AS DEP ACFT MADE AN UNEXPECTED TURN CONFLICTING WITH PREVIOUS DEP FROM ORD.

Narrative: ACFT #1 DEPARTED ORD WBOUND CLBING TO 13000 FT WHEN I ASSUMED CTL. ACFT #1 WAS CLBED TO FL230; SUPPOSEDLY ON 270 DEG VECTOR AS PER LCL LOA. ACFT #2 CHKED ON CLBING TO 13000 FT (DEPARTED ORD) 5+ MI S OF ACFT #1; ALSO SUPPOSEDLY ON 270 DEG HDG PER LOA; AND WAS CLBED TO FL230 AND TURNED TO 260 DEG HDG TO INCREASE DIVERGENCE. ON INITIAL CONTACT; ACFT WERE BOTH SUPPOSED TO BE ON PARALLEL 270 DEG HDGS AND REMAIN THAT WAY. HOWEVER; EVEN AFTER ASSIGNING A HDG TO ACFT #2 TO FURTHER DIVERGE; THE 2 ACFT ACTUALLY CONTINUED TO CONVERGE. UPON REVIEWING AVAILABLE RADAR DATA; IT APPEARS ACFT #1 WAS NOT ON A 270 DEG HDG; BUT POSSIBLY A 260 DEG OR 250 DEG HDG; SOMETHING NOT ANTICIPATED. SEPARATION WAS LOST NEAR 15000N FT; AT WHICH TIME ACFT #1 WAS TURNED TO 280 DEGS TO DIVERGE. NOW; 1 WK LATER; UPON FURTHER REVIEW OF RADAR DATA FROM BOTH CTR AND TRACON; IT APPEARS ACFT #1 INITIALLY FLEW 270 DG HDG; THEN SUBSEQUENTLY TURNED DIRECT IOW (FOR UNKNOWN REASON); THEREBY MAKING IT A PROBABLE PLTDEV. DUE TO EXTREMELY BUSY AND COMPLEX DEP RUSH AND THE SLOW CONVERGENCE RATE; I WAS UNABLE TO DETECT THE DISCREPANCY UNTIL SEPARATION WAS LOST. IT HAS BEEN CLASSIFIED AS A LOW SEVERITY/CTLED OPERROR; BUT MAY BE RECLASSIFIED AS A PLTDEV.

Data retrieved from NASA's ASRS site as of January 2009 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.