A C90 controller and one of the two pilots involved reported an airborne conflict in C90 airspace.

Attributes
ACN	1430051
Time
Date	201703
Local Time Of Day	1201-1800
Place
Locale Reference	C90.TRACON
State Reference	IL
Environment
Flight Conditions	VMC
Aircraft 1
Make Model Name	Citationjet (C525/C526) - CJ I / II / III / IV
Flight Phase	Climb
Flight Plan	IFR
Aircraft 2
Make Model Name	Cessna Stationair/Turbo Stationair 6
Flight Phase	Descent
Flight Plan	VFR
Person 1
Function	Departure Instructor
Qualification	Air Traffic Control Fully Certified
Experience	Air Traffic Control Time Certified In Pos 1 (mon) 6 Air Traffic Control Time Certified In Pos 1 (yrs) 3
Person 2
Function	Single Pilot Pilot Flying
Qualification	Flight Crew Commercial
Experience	Flight Crew Last 90 Days 120 Flight Crew Total 5200 Flight Crew Type 4000
Events
Anomaly	ATC Issue All Types Conflict Airborne Conflict Inflight Event / Encounter Weather / Turbulence
Miss Distance	Horizontal 3000 Vertical 300

Narrative:

I was the on-the-job training instructor (ojti) during this session. A C206 was southwest of ZZZ operating VFR at 8;500 and outside of the ord class B airspace. They wanted to descend because they were fighting nearly 90 knots (reported winds 298 at 89 at 6;000) of headwind. They were advised that they were outside class B and could descend and navigate at their discretion.a C525 was a ZZZ IFR departure. I mentioned to the 1S controller that the C206 was descending and to watch out. The C525 was handed off to us climbing to 6;000 as coordinated. My trainee continued the climb to 10;000 and issued traffic with the C206. There was no reply on the traffic call; and then my trainee issued a left turn to 180 to go behind the C206.the C206 descended far faster than either of us had anticipated with very little forward movement; approximately 40 knots ground speed. By the time the C525 replied; the pilot stated 'that was way too close for comfort;' and the C206 even keyed up and agreed. I asked both pilots; but neither said they got a TCAS RA.I had prompted my trainee to allow the C206 to navigate at their discretion since they were well outside of the class B airspace. However; we didn't issue any control instructions to avoid this situation. I don't believe any legal separation was lost; but I feel it was poor performance on my own part.

Original NASA ASRS Text

Title: A C90 Controller and one of the two pilots involved reported an airborne conflict in C90 airspace.

Narrative: I was the On-the-Job Training Instructor (OJTI) during this session. A C206 was southwest of ZZZ operating VFR at 8;500 and outside of the ORD Class B airspace. They wanted to descend because they were fighting nearly 90 knots (reported winds 298 at 89 at 6;000) of headwind. They were advised that they were outside Class B and could descend and navigate at their discretion.A C525 was a ZZZ IFR departure. I mentioned to the 1S Controller that the C206 was descending and to watch out. The C525 was handed off to us climbing to 6;000 as coordinated. My trainee continued the climb to 10;000 and issued traffic with the C206. There was no reply on the traffic call; and then my trainee issued a left turn to 180 to go behind the C206.The C206 descended far faster than either of us had anticipated with very little forward movement; approximately 40 knots ground speed. By the time the C525 replied; the pilot stated 'That was way too close for comfort;' and the C206 even keyed up and agreed. I asked both pilots; but neither said they got a TCAS RA.I had prompted my trainee to allow the C206 to navigate at their discretion since they were well outside of the Class B airspace. However; we didn't issue any control instructions to avoid this situation. I don't believe any legal separation was lost; but I feel it was poor performance on my own part.

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.