A320
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 8
Communication Breakdown; Situational Awareness
ATC Issue All Types; Conflict Airborne Conflict; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Issued New Clearance
Manuals; Human Factors; Company Policy; Airspace Structure; Procedure
Aircraft Y departed and was climbing in reference to Aircraft X. Our Center/TRACON LOA allows Center to increase speed reference known traffic. I saw Aircraft X out of 11200 ft while Aircraft Y was at 8800 ft. Since 11000 ft had been vacated and Center had the handoff on Aircraft Y I assumed Aircraft X would keep his climb rate and climbed Aircraft Y to 11000 ft. When I next checked Aircraft Y was leveling 11000 ft while Aircraft X was showing 11600 ft with less than 3 miles separation. The way I understand the LOA is that if Center has the handoff on both aircraft I can climb my aircraft to 11000 ft; since it has been vacated. However; if Center speeds Aircraft X up; changing the climb rate they should advise me since I am authorized to climb in reference. I believe Center should either not speed up aircraft when they have the handoff on the aircraft climbing in reference or should wait to take the handoff until after they sped up Aircraft X so I know that the climb rate has changed.
C90 TRACON Controller reported a loss of separation after clearing one aircraft to climb to an altitude that another aircraft had just left.