Date: 201205
Local time of day: 1801-2400
Location: ZZZ.ARTCC, US
Aircraft: Regional Jet 700 ER/LR (CRJ700)
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Second aircraft: A319
Reporter organization: Government
Reporter function: Enroute
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Situational Awareness; Confusion
Second reporter function: Enroute
Anomaly: ATC Issue All Types; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Air Traffic Control
When detected: In-flight
Result: General None Reported / Taken
Contributing factors: Human Factors; Procedure
Primary problem: Human Factors

Narrative: I was relieving RADAR Controller at the Sector during a very busy complex time for traffic. After I logged into the position; the RADAR Controller I relieved stayed behind me to make sure I had the traffic. During this period the RADAR Controller I relieved; trying to help; changed my range to a closer range so we could see the traffic better. My range went from a 100 range to a 65 range. This did help at the time; but I am used to running on a 90-100 range. Therefore I saw the traffic between a CRJ7 and an A319 and believed I had more than 5 miles lateral when I descended the A319 to FL300 in order to start this aircraft down for their destination. Since I was on a 65 range this was not the case. Conflict Alert did not go off until I lost separation. I believe this happened because the RADAR Controller I relieved was trying to help; but by doing so; I was put into a situation that I was not used to; i.e. running traffic on a 65 mile range.


Second narrative: I was the D-Side position for at least 30 minutes prior to the event until 30 minutes after the event. The Radar Controller was working complex traffic at a high volume. The incoming controller monitored the position for about 2 minutes prior to accepting the position; during the exchange of Radar Controller; the outgoing controller stood behind the position for an overlap. At this time the controller doing the overlap recognized that between the complexity and volume of traffic and that there were a lot of Data Blocks overlapping making it hard to recognize what needed to be done and what actions were a priority. The controller that was overlapping decided to help the Radar position out by telling the controller sitting at the Radar position what to do and trying to separate Data Blocks; the overlapping controller changed the range of the RADAR scope. It was adjusted to a 90 mile range which made the Data Blocks easier to see; but somehow; or for some reason; the scope was then adjusted to a 65 mile range without (in my opinion) the Radar position knowing. I was not aware that we were working on a 65 mile range at the time and when both aircraft passed each other; laterally it looked like we had more than 5 miles between targets. It happened quickly; by the time we knew what happened the two aircraft had lost separation. I recommended to the controller taking position that he starts adjusting his range from 100 to 90 at this sector; because when we are busy 100 mile range is too large and creates a lot of Data Block overlap. I also would recommend the exiting controller to never adjust a Radar side range while someone else is working.


Synopsis: Enroute Controller being relieved by another controller adjusted the display range to assist the on-coming controller with Data Block overlaps; the effort resulting in a loss of separation event.