Medium Transport; Low Wing; 2 Turbojet Eng
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 22
Confusion; Distraction; Situational Awareness
ATC Issue All Types; Conflict Airborne Conflict; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Provided Assistance; Flight Crew Returned To Clearance
Aircraft; Procedure
I was performing OJT (On the Job Training) instruction with my developmental (DEV) who is on his second set of radar sectors. He accepted the hand off on Aircraft X from HNN at FL360 inbound to LEX. Once the aircraft called on; my DEV descended him to FL350. My DEV then accepted a hand off on an aircraft; Aircraft Y; about 40 miles west of Aircraft X converging at FL360 landing IAD. There was a TMU message out requiring IAD arrivals at or below FL330 entering ZDC airspace. Aircraft X was level at FL350 at the time. My DEV then cleared Aircraft X to descend to FL240. Once he had observed Aircraft X leaving FL350; he issued descent clearance to Aircraft Y to FL350 in accordance of 7110.65 paragraph 5-5-5b. The two aircraft were still 25 miles apart converging when this descent clearance was issued. A minute or so later; Aircraft Y advised he was alerted to climb 300 feet by his TCAS. DEV then called the traffic to the Aircraft Y. Aircraft passed each other and Aircraft Y returned to FL350.A prescribed standard rate of descent amongst all turbojet aircraft would have prevented this occurrence. The Aircraft X; with performance characteristics very similar to Aircraft Y; descended at only 500 feet per minute whereas the Aircraft Y descended at about a 1500 feet per minute rate. This was the pilot's choice without advising ATC. Had the Aircraft X maintained a very reasonable 1000ft per minute rate; Aircraft Y would never had received an RA from TCAS and the event would not have occurred.
ZID instructor reports of his developmental descending aircraft using the vacated altitude rule; which led to a loss of separation.