ZJX controllers reported an altitude excursion from one aircraft resulting in an airborne conflict.

Attributes
ACN	1672112
Time
Date	201908
Local Time Of Day	1201-1800
Place
Locale Reference	ZJX.ARTCC
State Reference	FL
Aircraft 1
Make Model Name	Medium Transport Low Wing 2 Turbojet Eng
Operating Under FAR Part	Part 91
Flight Phase	Other Holding Pattern
Flight Plan	IFR
Aircraft 2
Make Model Name	Large Transport Low Wing 2 Turbojet Eng
Operating Under FAR Part	Part 121
Flight Phase	Other Holding Pattern
Flight Plan	IFR
Person 1
Function	Enroute Handoff / Assist
Qualification	Air Traffic Control Fully Certified
Experience	Air Traffic Control Time Certified In Pos 1 (yrs) 13
Person 2
Function	Approach
Qualification	Air Traffic Control Fully Certified
Experience	Air Traffic Control Time Certified In Pos 1 (yrs) 18.5
Events
Anomaly	Conflict Airborne Conflict Deviation - Altitude Excursion From Assigned Altitude Deviation - Procedural Clearance

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.

Original NASA ASRS Text

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

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.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.