ZAU controller instructed a descending aircraft to maintain 13;000 feet for opposite direction traff...

Narrative:

Aircraft X was being worked by me descending. I had a trainee as my radar assist. Aircraft X was pointed out to the appropriate sectors. TRACON had released to 7000 feet. Aircraft X had also been pointed out to ARTCC sectors. ARTCC sector had referenced aircraft Y who was on the arrival. On my end; coordination was incomplete as it did not establish which aircraft would be lower. I had assumed my aircraft X would be lower as he was closer to his destination. I issued a descent to 7000 feet. I had to amend the descent to 12000 feet for another inbound. I applied visual separation and then descended aircraft X to 070. I then notice another aircraft descending out of 10000 feet. I amended the altitude again to 10000 feet. The arrival traffic had not been referenced by any facility as being traffic. TRACON should have perhaps referenced their arrival traffic but would not have known about that aircraft at the time they took the original point out and released 7000 feet. Regardless; this again was not the specific issue. After clearing the inbound; I again descended aircraft X to 7000 feet. Aircraft was at about 11500 at this point. Although aircraft Y had already been cleared to 11000 feet by ARTCC; the radar controller there saw the situation developing and amended aircraft Y to 13000 feet. At some point; I had noticed aircraft X descending slowly and asked him to expedite to avoid the inbound stream. I heard the ARTCC controller issue the amendment and saw the change to aircraft Y data block. I assumed at this point that the intent for both me and the ARTCC controller was for me to continue the descent below aircraft Y. Unfortunately; aircraft Y had never read back the amendment to ARTCC and descended below 13000 feet. Separation may have been lost.vector aircraft X in such a manner as to avoid multiple point-outs in a busy airport arrival corridor. Better coordination on my part with other ARTCC sectors. Ensure prompt compliance with clearances; particularly altitude amendments and descent rates.

Original NASA ASRS Text

Title: ZAU Controller instructed a descending aircraft to maintain 13;000 feet for opposite direction traffic. The Controller did not realize the flight crew did not hear and acknowledge the clearance resulting in a loss of required separation.

Narrative: Aircraft X was being worked by me descending. I had a trainee as my Radar Assist. Aircraft X was pointed out to the appropriate sectors. TRACON had released to 7000 feet. Aircraft X had also been pointed out to ARTCC sectors. ARTCC Sector had referenced Aircraft Y who was on the arrival. On my end; coordination was incomplete as it did not establish which aircraft would be lower. I had assumed my Aircraft X would be lower as he was closer to his destination. I issued a descent to 7000 feet. I had to amend the descent to 12000 feet for another inbound. I applied visual separation and then descended Aircraft X to 070. I then notice another aircraft descending out of 10000 feet. I amended the altitude again to 10000 feet. The arrival traffic had not been referenced by any facility as being traffic. TRACON should have perhaps referenced their arrival traffic but would not have known about that aircraft at the time they took the original point out and released 7000 feet. Regardless; this again was not the specific issue. After clearing the inbound; I again descended Aircraft X to 7000 feet. Aircraft was at about 11500 at this point. Although Aircraft Y had already been cleared to 11000 feet by ARTCC; the radar controller there saw the situation developing and amended Aircraft Y to 13000 feet. At some point; I had noticed Aircraft X descending slowly and asked him to expedite to avoid the inbound stream. I heard the ARTCC controller issue the amendment and saw the change to Aircraft Y data block. I assumed at this point that the intent for both me and the ARTCC Controller was for me to continue the descent below Aircraft Y. Unfortunately; Aircraft Y had never read back the amendment to ARTCC and descended below 13000 feet. Separation may have been lost.Vector Aircraft X in such a manner as to avoid multiple point-outs in a busy airport arrival corridor. Better coordination on my part with other ARTCC sectors. Ensure prompt compliance with clearances; particularly altitude amendments and descent rates.

Data retrieved from NASA's ASRS site 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.

Attributes
ACN	1347767
Time
Date	201604
Local Time Of Day	0601-1200
Place
Locale Reference	ZAU.ARTCC
State Reference	IL
Environment
Light	Daylight
Aircraft 1
Make Model Name	Light Transport Low Wing 2 Turbojet Eng
Operating Under FAR Part	Part 135
Flight Phase	Descent
Flight Plan	IFR
Aircraft 2
Make Model Name	Medium Large Transport Low Wing 2 Turbojet Eng
Operating Under FAR Part	Part 121
Flight Phase	Descent
Route In Use	Direct
Flight Plan	IFR
Person 1
Function	Enroute
Qualification	Air Traffic Control Fully Certified
Experience	Air Traffic Control Time Certified In Pos 1 (yrs) 16.9
Person 2
Function	Enroute
Qualification	Air Traffic Control Fully Certified
Experience	Air Traffic Control Time Certified In Pos 1 (yrs) 25
Events
Anomaly	ATC Issue All Types Conflict Airborne Conflict Deviation - Procedural Clearance Deviation - Procedural Published Material / Policy

ZAU controller instructed a descending aircraft to maintain 13;000 feet for opposite direction traffic. The controller did not realize the flight crew did not hear and acknowledge the clearance resulting in a loss of required separation.