Date: 201511
Local time of day: 1201-1800
Location: ZDV.ARTCC, CO
Flight conditions: VMC
Aircraft: Medium Large Transport
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
Airspace: Class A ZDV
ATC / advisory: Center ZDV
Second aircraft: Large Transport
Reporter organization: Government
Reporter function: Enroute
Reporter qualification: Air Traffic Control Developmental
Human factors: Situational Awareness; Training / Qualification; Confusion
Second reporter function: Enroute
Anomaly: ATC Issue All Types; Conflict Airborne Conflict; Deviation / Discrepancy - Procedural Clearance
Detector: Automation Aircraft RA; Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance; Flight Crew FLC complied w / Automation / Advisory; Flight Crew Took Evasive Action; Flight Crew Returned To Clearance
Contributing factors: Procedure; Human Factors
Primary problem: Human Factors

Narrative: Aircraft X was given descent for arrival and to ensure that other departing aircraft could adequately climb above. Aircraft Y was opposite direction at 33;000 feet; and became traffic for aircraft X. Aircraft Y was missed in the traffic scan when analyzing climb performance of departures and the decision to start aircraft X down early. Vectors of 20 degrees right were issued to both aircraft; but both pilots received TCAS RAs and took the recommended action over the vectors issued by the controller. It was a case of solving a situation involving factors A and B; but forgetting about factor C. Controller vectors and TCAS saved the situation from becoming more dangerous.


Second narrative: I was conducting training on the Radar Assist position. Our Radar Controller had an aircraft climbing to 35;000 feet.  As soon as the aircraft was climbed I mentioned the traffic at 34;000 feet and the Radar Controller said he saw it; and if need be he would descend the aircraft X to 32;000 feet as he was an arrival and would eventually descend anyway. We did have aircraft Y at 33;000 feet between the two aircraft; but if done in a timely matter aircraft Y would have been no factor. Several minutes later we stepped off line to call a sector for coordination. While we were off line our Radar Controller issued a decent to aircraft X (a landing aircraft) without seeing the 33;000 foot traffic aircraft Y head on and one minute out. When we noticed the descent in the data block it instantly started to conflict alert flash and the Radar Controller noticed as well. Our Radar Controller issued a 20 degree right turn to aircraft X and then a 20 degree right turn to aircraft Y. We called the sector to coordinate the turning and descending for traffic. Both aircraft then received RA alerts and acted on them. We had 2.65 miles and 800 feet at the tightest point. It was just a case of human error.


Synopsis: Air carrier aircraft was descended form 34;000 feet to 32;000 feet. The aircraft had opposite direction traffic at 33;000 feet that the Controller did not notice or remember. The Assist Controller pointed out the potential confliction to the Radar Controller when the descent clearance was issued. At the time the Radar Controller issued instructions to separate the aircraft; the aircraft were responding to their TCAS/RA.