Date: 201709
Local time of day: 1201-1800
Location: ZTL.ARTCC, GA
Aircraft: B737-800
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Climb
Airspace: Class A ZTL
ATC / advisory: Center ZTL
Second aircraft: PC-12
Reporter organization: Government
Reporter function: Enroute
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Human-Machine Interface; Communication Breakdown; Situational Awareness
Anomaly: ATC Issue All Types; Conflict Airborne Conflict; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance
Detector: Automation Aircraft TA; Automation Aircraft RA; Person Air Traffic Control
Result: Air Traffic Control Issued New Clearance; Air Traffic Control Issued Advisory / Alert; Flight Crew Requested ATC Assistance / Clarification; Flight Crew FLC complied w / Automation / Advisory; Flight Crew Took Evasive Action
Contributing factors: Human Factors; Procedure
Primary problem: Procedure

Narrative: I had just taken the Radar Control position. One of the first things I did was call Aircraft Y to Aircraft X since Aircraft X was level at FL180 and Aircraft Y was at FL190 on a converging course. After Aircraft X acknowledged the traffic call I noticed that Aircraft Y was showing FL187 in his altitude readout. Before calling the traffic to him I asked him to verify that he was level at FL190. He responded by saying that he thought his altitude was FL180 and he was still descending. The aircraft were about 5 or 6 miles apart at this point. I made it a priority to attempt to separate the aircraft before issuing the traffic alert. I told Aircraft Y to climb to FL190 and then told Aircraft X to descend to 170. I got a read back from the Aircraft Y to climb to 190; but the next few transmissions by both pilots were either blocked by each other or me issuing the traffic alerts. Immediately after giving the descent and climb I issued traffic alerts to both aircraft. I observed via Mode C that Aircraft X was responding to an RA by both climbing and descending while Aircraft Y dropped to as low as FL183 I believe before climbing back to his assigned altitude of FL190. As the replay will show; the targets nearly merged and there was little altitude separation. Once I observed the aircraft doing a TCAS maneuver and the targets passing each other I discontinued advisories.Once the aircraft were clear I descended Aircraft Y and told him to call the front desk when he landed. Aircraft X wanted to verify before the frequency change that they did nothing wrong and made sure that I knew they were responding to TCAS. I told him that I did not hear the transmission because it was blocked; but saw that he was doing a TCAS maneuver based on his Mode C and that he was at his assigned altitude and not at fault in any way.There is nothing I would recommend to prevent this event. Since I did not give the initial descent clearance to Aircraft Y; I was initially unsure if a read back was missed; but after listening to the replay the pilot clearly stated FL190 as his assigned altitude and simply descended to an altitude he wasn't assigned. The situation was scary and unfortunate; but I don't think the incident was preventable from an ATC standpoint.




Synopsis: ZTL ARTCC Controller reported an aircraft descended below its assigned and acknowledged altitude assignment head on in to opposite direction traffic.