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.