Air Traffic Control Developmental
Situational Awareness; Training / Qualification
ATC Issue All Types; Conflict Airborne Conflict; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Issued New Clearance
Procedure; Human Factors
I was working the Radar position two sectors combined. I had just worked a departure push out of as well as aircraft inbound to TXK; an aircraft on the VOR/DME B approach at SLR and a jump plane. I took a handoff on Aircraft X descending to 24000 feet. I had traffic; Aircraft Y; at 20000 feet southwest bound across the sector that was traffic for Aircraft X landing SHV. Shortly after taking the handoff on Aircraft X I put a halo on Aircraft Y and gave them a 30 degree turn to the right to go behind Aircraft X and give myself room to descend.Upon check on I gave Aircraft X a descent to FL210 to monitor the rate of descent compared to the lateral spacing with Aircraft Y. As Aircraft X approached 21500 feet there was approximately 10 miles between Aircraft X and Aircraft Y. I displayed the vector lines out and Aircraft X looked to stay in front of Aircraft Y and the rate of descent had been good to 21000 feet. I gave Aircraft X a clearance to descend to 13000 feet and expedite through 19000 feet for Aircraft Y traffic. As I took out the interim altitude the data blocks began to flash. I immediately came back and instructed Aircraft X to maintain 21000 feet. The pilot replied that they were already through 21000 feet and would have to go back to it. I turned Aircraft Y to a heading of 280 to go further behind Aircraft X and turned Aircraft X 20 degrees to the right. Aircraft X returned to 21000 feet and on the other side of the halo of Aircraft Y I issued a descent to 13000 feet. Afterward I cleared Aircraft X to SHV and Aircraft Y to CRP back on their courses. There are several things I can do in the future to prevent a re-occurrence of this event. The first thing would be to maintain vertical separation until the aircraft that needs to descend is clear on the other side of the traffic and issue a further descent from there. To maintain lateral separation I could have turned the first aircraft sooner to go behind the descending aircraft; I also could have issued a turn for the descending aircraft sooner as well. Additionally; once the descending aircraft reported already through the previously assigned altitude I could have kept the descent going to maintain vertical separation since the aircraft is already in a descent configuration.
A Trainee Controller vectored an aircraft away from another aircraft that was descending through its altitude. The vector was insufficient to maintain required separation.