Commercial Fixed Wing
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 14.0
Confusion; Situational Awareness; Communication Breakdown
ATC Issue All Types; Conflict Airborne Conflict; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Issued New Clearance; Air Traffic Control Separated Traffic
Company Policy; Human Factors
I climbed Air Carrier X Flight ABBB to 36;000 feet. I believe Air Carrier X Flight AABB took the clearance and I did not realize it. I also believe that they both took the clearance but I'm not sure. Moments later I switched Flight AABB frequency via CPDLC. While Flight ABBB was climbing into my sector I noticed that Flight AABB started descending. I was not talking to Flight AABB. I verbally coordinated with the sector which is located next to me that I'm stopping Flight ABBB at 35;000 feet and turning south and then I issued the instruction to the pilot. Then the sector I gave Flight AABB to called and said that Flight AABB was returning to 38;000 feet. When traffic was no longer a factor I climbed Flight ABBB to 36;000 feet and turned them on course. I never realized how close the call signs were until it was too late. Be more aware of similar call signs.
ZID ARTCC Controller reported the wrong aircraft acknowledged for a new altitude assignment and the Controller did not detect it due to similar sounding call signs.