Date: 201908
Local time of day: 1201-1800
Location: ZJX.ARTCC, FL
Aircraft: Medium Transport; Low Wing; 2 Turbojet Eng
Operating under: Part 91
Flight plan: IFR
Flight phase: Other Holding Pattern
Airspace: Class A ZJX
ATC / advisory: Center ZJX
Second aircraft: Large Transport; Low Wing; 2 Turbojet Eng
Reporter organization: Government
Reporter function: Enroute; Handoff / Assist
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Situational Awareness; Communication Breakdown
Second reporter function: Approach
Anomaly: Conflict Airborne Conflict; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance
Detector: Automation Aircraft RA; Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Air Traffic Control Issued Advisory / Alert; Flight Crew Returned To Clearance
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: Holding pattern was being established for MCO near OMN VOR. Aircraft Y was established at FL 200; with aircraft below from FL190 to 16;000. Aircraft X was cleared to enter the hold descending to FL210. As aircraft entered the hold; nearly on top of Aircraft Y. the data block indicated FL208 and conflict alert activated. Aircraft Y was observed leaving FL200 as the R-side corrected Aircraft X and called the traffic. Aircraft Y reported the TCAS descent and then all aircraft returned to their assigned altitudes.When the tapes were pulled; it was determined Aircraft X was issued and read back FL210; and had a pilot deviation entering the hold.Not much to do when aircraft are holding so close with minimal vertical separation and a pilot deviates from altitude.


Second narrative: Weather was impacting the sector and MCO. I was holding several aircraft; approximately 9. Aircraft checking on all requested to deviate except for the MCO arrivals. I was working radar and issuing holding instructions for the aircraft to hold at the OMN. I descended aircraft into the holding stack as lower became available. Aircraft Z was holding at FL190 and established in the hold. Aircraft Y was established in the hold at FL200. Aircraft X was at FL220 and not established in the hold but almost there. I issued FL210 and Aircraft X read back FL210 (I listened to the tape to make sure). My D side also confirmed the altitude. I was accommodating deviation request and checking on aircraft when the conflict alert went off. The data blocks were clustered on the holding aircraft and as I separated the data blocks; it was apparent that Aircraft X was descending through FL207. I told Aircraft X to climb back to FL210 and he sounded surprised. I believed I issued traffic but not sure; because the Aircraft Y responded to an RA and descended quickly to FL190; becoming a factor for Aircraft Z. I remember issuing that traffic to Aircraft Y and then told Aircraft Y to climb to FL190 (Aircraft X was no longer a factor) and to fly heading 180. I issued the brasher warning to Aircraft X to call ZJX ARTCC.It was pilot error.


Synopsis: ZJX Controllers reported an altitude excursion from one aircraft resulting in an airborne conflict.