Small Aircraft; Low Wing; 1 Eng; Fixed Gear
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (mon) 3
Situational Awareness; Distraction; Communication Breakdown; Training / Qualification
ATC Issue All Types; Conflict Airborne Conflict; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Issued New Clearance
Human Factors; Procedure
Aircraft X en route to LOU was heading west direct to LOU at 12;000 feet MSL. For airspace transition; traffic; and routing I had planned to descend Aircraft X to 10;000 and hand off to Lexington approach. Aircraft Y was en route to LUK at 11;000 feet MSL. When I first looked at the traffic scenario I knew that I needed to descend Aircraft X to 10;000 early enough to get him below Aircraft Y. To provide a service I was trying to let Aircraft X stay at 12;000 for a couple of minutes before beginning his descent. However I started to get a little more busy and a few minutes went by when I noticed I had not descended Aircraft X yet. My plan was to descend Aircraft Y to 8;000 which would have helped with my traffic situation with Aircraft X as well as traffic that was overtaking Aircraft Y. My D-side was training at the time and I had asked him to coordinate a descent on Aircraft Y with LEX Approach. Somehow there was a miscommunication and the altitude was not coordinated. I issued a descent for Aircraft X to 10;000 thinking I still had plenty of room between Aircraft X and Aircraft Y. After issuing the descent I realized I would need vectors to maintain separation. I turned both aircraft 20 [degrees] left for the descent. It still looked tight after the turns had started to stabilize. I asked my D-side at this time if 8;000 had been coordinated on Aircraft Y with LEX Approach. The D-side told me he had not requested lower with LEX Approach so I asked Aircraft X to expedite his descent and turn an additional 20L. I am sure I maintained separation and to double check I watched FALCON which showed that I had 5.35 Miles at the closest point however this was closer than I would have liked. I need to be more authoritative in giving instructions to D-sides especially in training. If there is something for separation that I need done I should do it myself to make sure the coordination is done timely to keep from having to use big vectors.
ZID Controller reported an airborne conflict he observed. Controller descended one aircraft through the altitude of the other. The aircraft were close to losing separation but did not. Controller reported he needed to be more authoritative to the D-Side in issuing instructions for coordination.