Date: 201604
Local time of day: 0601-1200
Location: ZAU.ARTCC, IL
Light: Daylight
Aircraft: Light Transport; Low Wing; 2 Turbojet Eng
Operator: Air Taxi
Operating under: Part 135
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
Airspace: Class E ZAU
ATC / advisory: Center ZAU
Second aircraft: Medium Large Transport; Low Wing; 2 Turbojet Eng
Reporter organization: Government
Reporter function: Enroute
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Communication Breakdown; Confusion; Situational Awareness
Second reporter function: Enroute
Anomaly: ATC Issue All Types; Conflict Airborne Conflict; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Air Traffic Control Separated Traffic; Flight Crew Requested ATC Assistance / Clarification
Contributing factors: Human Factors; Procedure; Airspace Structure
Primary problem: Procedure

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.


Second narrative: I had three aircraft direct [fix]; cleared to cross [fix] at 11000 feet. ARTCC Sector 1 had displayed an aircraft on my radar. The aircraft was stuck at 16;000 feet since TRACON will not work overflights. ARTCC Sector 1 and 2 are adjacent; with the radar controllers next to each other and the radar assists outside. I overheard the ARTCC Sector 1 Assist trying to coordinate a point out with TRACON. I glanced at the ARTCC Sector 1 display. The ARTCC Sector 1 controller leaned over and tried to point out Aircraft X; twice we were interrupted by calls on my frequency. He then leaned over; again and said 'watch Aircraft X reference Aircraft Y.' There were arrivals in the way and the Aircraft X did not show any interest in descending. The data block on Aircraft Y showed an assigned altitude of 11;000 feet; I think the mode C might've been 16;000-ish. The ARTCC Sector 1 controller was trying to expedite Aircraft X to 7;000 feet but it appeared that plan was not going to be successful. I amended Aircraft Y's altitude to 13;000 feet (still above 15;700; I think) but received no readback. Aircraft X is descending through 12;000 feet (super poorly) and then Aircraft Y's mode C showed 12;700 feet. I amended Aircraft Y's altitude to 12;000 feet. Aircraft X continued his descent but Aircraft Y responded he'd climb back to 12;000 feet. He continued to descend to 11;400 feet then climbed and reported 12;000 feet and said he'd been descending at 3000 feet per minute to make the crossing; apparently not having heard me amend the altitude to 13;000 feet.It's probably a pipe dream to think TRACON could be bothered with an overflight;  but I think Aircraft X needed to be vectored east a bit and handed off to TRACON. ARTCC Sector 1 coordination with TRACON did not include arrivals TRACON did not yet see but somewhere among the two TRACONs and ARTCC Sector 1 the traffic that caused ARTCC Sector 1 agitation was missed. Frankly; when I said 'point out approved reference ...' I should've made it clear; Aircraft Y had his clearance to 11;000 feet (though that WAS in the data block) and that I was expecting Aircraft X to be vectored to miss (they were head-on-ish.)


Synopsis: 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.