Date: 201512
Local time of day: 0001-0600
Location: ZZZ.ARTCC, US
Light: Dusk
Aircraft: Light Transport; Low Wing; 2 Turbojet Eng
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Reporter organization: Government
Reporter function: Enroute
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Confusion; Situational Awareness
Second reporter function: Supervisor / CIC; Enroute
Anomaly: ATC Issue All Types; Conflict Airborne Conflict; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Automation Aircraft TA; Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance; Flight Crew FLC complied w / Automation / Advisory
Contributing factors: Airspace Structure; Human Factors
Primary problem: Human Factors

Narrative: Working an arrival sequence. Waiting on Aircraft X to check on to descend from 34000 feet to 31000 feet. Aircraft Y checks on at 33000 feet. My attention was on Aircraft X to descend to get down under traffic. I inadvertently issued descent clearance to Aircraft Y on his initial check on to 31000 feet thinking it was Aircraft X. Aircraft Z was westbound at 32000 feet. Aircraft Y reported a TCAS Alert and that he was going to stop his descent; I advised him to please stop his descent and maintain 33000 feet. At the time I believed it was a pilot deviation on Aircraft Y until Aircraft X finally checks on at 34000 feet.Needed to exercise more patience to issue a clearance; and make sure it is to the correct aircraft.


Second narrative: I was the supervisor in charge of the area. Two sectors were combined. Combining these sectors at this time of night is not unusual during this time of the year. Traffic was moderate and fairly complex. Predicting traffic complexity in these sectors is difficult. In order to predict traffic volume for these sectors I look at the En Route Decision Support Tool (EDST) to see what mix of overflight and arrival traffic is coming and look at the departure list in our low altitude sectors to try to predict the volume of departure traffic. Traffic was departing slowly from ZZZ that night. The monitor alert function does not work for these sectors and hasn't since I returned to the center two years ago. It is a known problem. The radar controller had over 25 years experience and is a Controller-In-Charge and Instructor. There was training on the Radar Assist position. The Radar Assist position Instructor also has over 25 years experience. I check on these sectors regularly if I have them combined on evening shifts and did not feel the volume or complexity was an issue. I heard the radar controller question a flight crew. The controller sounded pretty upset. He asked for the pilot deviation phraseology card we keep at the supervisors' desk. He had two same company flights on frequency and said that one of them took the other's clearance even though the flight numbers were not close. He told me Aircraft X got a TCAS RA from a flight beneath him. The Radar Assist position printed out strips on the Aircraft X and Aircraft Y flights. I took the strips to the watch desk and advised the operations manager of the situation. The operations manager enters reports and takes calls from the pilots. As I returned to the area I was advised by a supervisor from the area handling the Aircraft Y flight that they had responded to an RA as well. I relieved the radar controller with the next person back from break so he could fill out a TCAS questionnaire and [safety report] prior to leaving for the night. I learned from the operations manager that reviewed the FALCON and audio shortly afterwards that the controller issued the descent clearance to the wrong flight. The controller was over 9 hours into a 10 hour shift on his second evening shift of the week at the time of the incident. A review of the Radar Assist position audio indicated they were on a landline coordinating when the clearance was issued to the wrong aircraft. I'm not sure what I can do differently to prevent something like this from re-occurring. I will encourage controllers I work with to issue traffic more often even if it is not required by merging target procedures. In this case the targets did not merge. Their closest proximity was around 2 miles. I feel that had Aircraft X been issued traffic they would have questioned the subsequent descent clearance. The next time I take a familiarization trip I will encourage the flight crews to take a quick glance at their TCAS when issued an altitude change en-route. Splitting the sectors would have meant that the radar controller would have been on position even longer at the end of his day. I have already been questioned about my staffing of the area. That is the first thing the facility looks at whenever there is an incident. Specifically the facility looks at how many controllers were on break that could have been staffing more radar positions or Radar Assist positions. I feel that staffing the area appropriately means finding a good balance between having enough positions open that the controllers are not too busy and not having so many open that they are not engaged. I also think that over-staffing the area and leaving controllers on position longer as a result has risks. That being said I will have to staff the area more conservatively for a while as I am anticipating getting a performance discussion as a result of this incident. I stay in the area a lot and ensure I am in the area during peak traffic periods. I feel this is my responsibility as a supervisor but also means I am more at risk for being counseled on how I staff the area.


Synopsis: While waiting for an aircraft to check in on frequency; another aircraft of the same company checked on to the frequency at 33000 feet. The Controller inadvertently descended this aircraft to 31000 feet resulting in a TCAS alert for conflicting traffic. The Controller then advised the descending aircraft to maintain 33000 feet.