Date: 201707
Local time of day: 1201-1800
Location: ZDV.ARTCC, CO
Weather elements / visibility: Thunderstorm
Light: Daylight
Aircraft: Large Transport; Low Wing; 2 Turbojet Eng
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class A ZDV
ATC / advisory: Center ZDV
Second aircraft: Widebody; Low Wing; 2 Turbojet Eng
Reporter organization: Government
Reporter function: Enroute
Reporter qualification: Air Traffic Control Developmental
Human factors: Workload; Distraction
Second reporter function: Enroute
Anomaly: ATC Issue All Types; Conflict Airborne Conflict; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance
Detector: Automation Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Air Traffic Control Separated Traffic
Contributing factors: Procedure; Weather; Human Factors; ATC Equipment / Nav Facility / Buildings
Primary problem: Procedure

Narrative: Two aircraft entering hold at same altitude. Both R-side and D-side missed it. Sector was incredibly busy because Denver had just got shut off due to weather; Area 4 arrivals had been rerouted to 28; on top of normal arrivals; one route; then shut off. No relief was given to sector 28 until the traffic levels got too high with all the holding. Multiple enroute aircraft pointed towards holding pattern; as the D-side I was busy taking hand offs and making sure no enroutes were pointed at the holding stack; as well as trying to move data blocks to see the aircraft. After event; a tracker was added to the position.Have TMU; or Supervisors; help more on spreading out the traffic and/or arrivals so it doesn't all congregate in one location. Later in that same day; Sector 30 had over 40 aircraft in their sector. It was a recurring issue all day.


Second narrative: I was working sectors 28 and 39 when D01 stopped accepting arrivals due to weather; so all sectors working DEN bound aircraft began holding. As traffic volume built; the D-side and I agreed that we could only handle a few more DEN landers in our holding pattern and we informed the supervisor. However; the adjacent sector was also extremely busy and we took at least three additional DEN landers to try to reduce congestion at sector 30.At the same time; overflight traffic volume increased and the D-side and I had to issue numerous control instructions to vector overflights away from the holding stack. At the time of the loss; we had control of 21 data blocks and I believe all 21 were on frequency. Eight were in the holding pattern involved; with two others entering another holding pattern. Neither the D-side nor I noticed the conflict until conflict alert activated due to numerous overlapping data blocks. I issued a descent to Aircraft Y and a 90-degree turn to Aircraft X; but at least one and I believe both of my first attempts at the clearances were blocked due to frequency congestion. Since I had to descend Aircraft Y to an altitude already in use by a third aircraft in the holding pattern; I issued several more clearances to various aircraft to ensure vertical separation in the stack and did not have time to issue traffic or a safety alert before I observed Aircraft X turning behind Aircraft Y. At the time; I was not sure if separation was maintained; but upon reviewing the Falcon; there were two hits with less than standard separation: 4.8 nm and 700 feet; and 3.8 nm and 900 feet. The volume was just too high to work that many overflights while trying to separate a holding stack. We tried to limit the hand offs we accepted but didn't have much choice since the adjacent sector was too busy to work them as well. We probably should have de-combined sectors 28 and 39 when holding began but I' m not sure if we had enough staffing. We should have stopped accepting DEN landers from adjacent centers as well; but I'm not sure if that was attempted.


Synopsis: ZDV Center controllers reported a loss of separation during high traffic when two aircraft were in holding at the same altitude.