Date: 202001
Local time of day: 0001-0600
Location: ZNY.ARTCC, NY
Flight conditions: VMC
Aircraft: B737-900
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class A ZNY
ATC / advisory: Center ZNY
Second aircraft: B747-400
Reporter organization: Government
Reporter function: Enroute
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Communication Breakdown; Human-Machine Interface; Situational Awareness
Anomaly: ATC Issue All Types; Conflict Airborne Conflict; Deviation - Altitude Excursion From Assigned Altitude; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Weather / Turbulence
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued New Clearance
Contributing factors: Human Factors; Airspace Structure; ATC Equipment / Nav Facility / Buildings; Procedure
Primary problem: Human Factors

Narrative: I had just sat down to work sectors with heavy weather by the boundary of two sectors. Almost every aircraft in the sector airspace was deviating for weather. As I was sending clearances to other aircraft and getting multiple messages about moderate turbulence I received I got a message in my sector queue. Upon reviewing the message it alerted me to the fact that Aircraft X was level at 35;000 ft. instead of 34;000 ft. as shown in the coordination window between the sectors. The Controller advised he had also just sat down at the sector.When I called the other Controller to confirm altitude on Aircraft X; the Controller said that he was showing a radar hit of them level at 35;000 ft. When I alerted him that Aircraft X was supposed to be at 34;000 ft.; the controller tried to reach Aircraft X on frequency; but Aircraft X was no longer on frequency. As soon as he said that I sent a CPDLC message to Aircraft X to descend to and maintain 34;000 ft. Aircraft X had an immediate conflict with Aircraft Y that started as soon as Aircraft X progressed the boundary fix with an approximate crossing conflict of 4 minutes. While waiting for the Aircraft X pilot to acknowledge their descent clearance via CPDLC; I called ARINC and requested a phone patch with Aircraft X. As I waited to be connected I probed Aircraft Y to descend or climb; but was unable to move Aircraft Y due to the fact they were deviating left of course; making the conflict with Aircraft X worse; and also crossing traffic at both 34;000 ft. and 36;000 ft. While I waited to be connected via phone patch; Aircraft X finally acknowledged their descent clearance. Upon connection of the patch Aircraft X reported level at 34;000 ft. Aircraft X also had two other pieces of traffic at 35;000 ft. eastbound that were more than 30 minutes out.Upon review of the coordination between the previous controllers at the sectors it was confirmed that Aircraft X was approved by the sector controller at 34;000 ft. and the controller had forgotten to descend him causing an immediate lack of separation. Ensure that all relief briefings are accurate and the post brief period of 5 minutes is ensured so that aircraft are at the correct altitude entering oceanic airspace.




Synopsis: Center Controller reported a traffic conflict between two aircraft when one aircraft had not been given clearance to descend.