Narrative:

Aircraft a was RNAV arrival. Aircraft B was being handed off at 11000 feet from ZID as well. There was coordination taking place between ZID and the supervisor on duty as to how these two aircraft would be separated. The supervisor told me and my trainee that ZID was stopping the aircraft a at 12000 feet reference the aircraft B at 11000 feet. When aircraft a checked in I completely was under the impression that he would stop at 12000 feet as per the coordination. He did not. He crossed babne at 12000 feet; then descended on the RNAV arrival. At that point; aircraft B was right beneath him at 11000 feet southeast-bound. Aircraft a said he climbed back up because of traffic below him. After a review of the tapes; we found out that the actual coordination was to have aircraft a cross babne at 12000 feet; descend via the snakepit arrival.these two aircraft were both handed off from the same center sector. This was poor coordination without positive control; especially since ind has control for 30 degree turns; which we utilized during this scenario. ZID should have given aircraft a 'descend via the snakepit; except maintain 120'. This would have prevented aircraft a from continuing his descent and would have ensured separation. Additionally; the supervisor who relayed the coordination simply said that the aircraft a was stopping at 12000 feet and nothing else. More thorough and exact coordination needed to be done here. We definitely should have reiterated to aircraft a upon initial check-in to 'maintain 12000'. We failed to do this.

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Original NASA ASRS Text

Title: IND TRACON Controller reported an airborne conflict resulting from an air carrier flight descending through their assigned altitude.

Narrative: Aircraft A was RNAV arrival. Aircraft B was being handed off at 11000 feet from ZID as well. There was coordination taking place between ZID and the Supervisor on duty as to how these two aircraft would be separated. The Supervisor told me and my trainee that ZID was stopping the Aircraft A at 12000 feet reference the Aircraft B at 11000 feet. When Aircraft A checked in I completely was under the impression that he would stop at 12000 feet as per the coordination. He did not. He crossed BABNE at 12000 feet; then descended on the RNAV arrival. At that point; Aircraft B was right beneath him at 11000 feet SE-bound. Aircraft A said he climbed back up because of traffic below him. After a review of the tapes; we found out that the actual coordination was to have Aircraft A cross BABNE at 12000 feet; descend via the Snakepit arrival.These two aircraft were both handed off from the same Center sector. This was poor coordination without positive control; especially since IND has control for 30 degree turns; which we utilized during this scenario. ZID should have given Aircraft A 'descend via the snakepit; EXCEPT MAINTAIN 120'. This would have prevented Aircraft A from continuing his descent and would have ensured separation. Additionally; the Supervisor who relayed the coordination simply said that the Aircraft A was stopping at 12000 feet and nothing else. More thorough and exact coordination needed to be done here. We definitely should have reiterated to Aircraft A upon initial check-in to 'maintain 12000'. We failed to do this.

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.