ZAU CTLR DESCRIBED LOSS OF SEPARATION AT FL270 WHEN MD83 LNDG ORD DSNDED BELOW ASSIGNED ALT; CONFLICTING WITH OVERFLT TFC.

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ACR X WAS INBOUND TO ORD AT FL330. ACFT Y WAS OVERFLYING AT FL270. I CLRED ACR X DIRECT ORD; AND ISSUED A DSCNT TO FL280; ADDING THAT ACR X COULD EXPECT TO CROSS 90 MI SW OF ORD AT FL240. SOMETIME LATER; ACR X ASKED IF THE EXPECTED XING WAS 50 MI SW OF BENKY INTXN. I REITERATED THAT THE EXPECTED XING WAS 90 MI SW OF ORD. WHEN THE 2 ACFT WERE APPROX 25 MI APART; I ISSUED TFC TO BOTH; NOTING THAT ACR X WAS DSNDING TO FL280. WHEN THE ACFT WERE APPROX 7 MI APART; I RE-ISSUED TFC. AS I ISSUED TFC TO ACR X; I SAW THE MODE C READOUT GO TO FL278; AND IMMEDIATELY TOLD ACR X TO MAINTAIN FL280. THE PLT RESPONDED THAT THEY WERE 'CLBING BACK' TO FL280. (I OBSERVED THE MODE C GO AS LOW AS FL274 BEFORE IT WENT BACK UP; WHICH IS NOT UNCOMMON AS OUR READOUTS CAN BE WELL BEHIND ACTUAL FLT CONDITIONS). I INQUIRED IF ACR X HAD RECEIVED THE ALT TO MAINTAIN OF FL280. HE REPLIED THAT HE HAD INDEED GOTTEN THE CLRNC. BUT HAD GOTTEN THE 'EXPECTED' ALT IN HIS HEAD AND DSNDED THROUGH THE ASSIGNED ALT. SINCE THE ADVENT OF 'COMPUTERIZED' ACFT; I HAVE ALWAYS ATTEMPTED TO PROVIDE PLTS WITH AS MUCH INFO AS I CAN ABOUT MY FUTURE PLANS FOR THEIR ACFT -- PARTICULARLY WHEN I AM DSNDING THEM TO AN ALT OTHER THAN THAT WHICH THE PUBLISHED ARR PROCS FOR ORD TELL THEM THEY WILL BE GOING TO. THUS; MY USE OF THE 'EXPECT' CLRNC IN THIS CASE. I HAVE ALWAYS ASSUMED (DREADED WORD) THAT PLTS CAN USE AS MUCH ADVANCE NOTICE AS POSSIBLE SO AS TO ALLOW THEM TO PROGRAM FUTURE CLRNCS INTO THEIR FMS'S; ONLY HAVING TO EXECUTE THE PROGRAMMING ONCE THE ACTUAL CLRNC HAS BEEN ISSUED. THIS ALSO HAS ALWAYS LED ME TO BE EXTRA VIGILANT; SINCE ISSUING A CLRNC THAT CONTAINS 2 (OR MORE) ALTS CAN CAUSE CONFUSION. IN CONCLUSION; I WOULD LIKE TO ADD THAT I FEEL TERRIBLE ABOUT WHAT I KNOW WILL PROBABLY HAPPEN TO THE CREW INVOLVED IN THIS INCIDENT. SINCE SEPARATION WAS LOST (BY 3/10 OF A MI); THE SUPVRS AND MGRS IN MY FACILITY HAD NO CHOICE BUT TO PROCESS THIS AS A PLTDEV (WHICH LED TO A LOSS OF STANDARD SEPARATION). DUE TO A SIMPLE HUMAN MISTAKE -- WHICH I MAY HAVE INSTIGATED THROUGH THE USE OF AN 'EXPECT' CLRNC -- THIS CREW MUST NOW FACE THE WRATH OF FSDO AND THEIR OWN COMPANY'S POLICIES. AND ALL FOR WANT OF 3/10 OF A MI IN A CASE WHERE NEITHER ACFT HAD TO TAKE EVASIVE ACTION; TCAS WAS SATISFIED THAT THE ACFT WERE CLR; AND NEITHER ACFT EVEN SAW THE OTHER. SUPPLEMENTAL INFO FROM ACN 706883: ACFT CLRED TO FL280; EXPECT FL240 AT BDF. FL280 WAS SELECTED IN THE ALT WINDOW. PRIOR TO ATTAINMENT OF FL280; FL240 WAS SELECTED; LEADING TO THE ACFT DSNDING APPROX 450 FT LOW. ACFT CONTINUED BACK TO FL280 WITH NO FURTHER INCIDENT. SUPPLEMENTAL INFO FROM ACN 707046: LEVEL AT FL330 WITH CLRNC TO FL280 AND EXPECT CLRNC TO FL240 90 DME S OF ORD VOR. APCHING FL280 I RESET THE ALT SELECT WINDOW TO FL240 THINKING WE HAD RECEIVED THE CLRNC TO FL240 90 MI S OF ORD. AT ABOUT FL275 ATC CALLED AND WE CORRECTED OUR ALT.