Enroute controller described a loss of separation event during exiting holding procedures; the repor...

Narrative:

I was working an inbound rush into ZZZ airport at a low sector over abc. We went into metering and the airplanes needed to be put into a holding pattern. Military airspace was also active so I had to keep the holding stack at FL190 and above. I took as many airplanes as I could from high altitude which had saturated all my altitudes. I had started to clear airplanes from the stack on course. I had 2 going in at the time and started stepping airplanes down to receive more airplanes from the high sector. I saw one airplane leave fl 205 for FL200 so I looked for the next plane to take down. I saw aircraft X at FL230 so I descended them to FL210. I didn't see aircraft Y at FL220. I noticed it very soon after I gave the decent and told aircraft X to climb back up to FL230. I lost separation before aircraft X could regain FL230. I will in the future pay even more attention to altitudes before I issue one to an aircraft; especially in a holding stack. I could have also asked for a d-side to help me scan the various altitudes. I could have taken less airplanes from high to allow myself more altitudes to work with.

Original NASA ASRS Text

Title: Enroute Controller described a loss of separation event during exiting holding procedures; the reporter listing workload; failure to request a D-Side and attentiveness to altitude assignments as causal factors.

Narrative: I was working an inbound rush into ZZZ Airport at a low sector over ABC. We went into metering and the airplanes needed to be put into a holding pattern. Military Airspace was also active so I had to keep the holding stack at FL190 and above. I took as many airplanes as I could from high altitude which had saturated all my altitudes. I had started to clear airplanes from the stack on course. I had 2 going in at the time and started stepping airplanes down to receive more airplanes from the high sector. I saw one airplane leave FL 205 for FL200 so I looked for the next plane to take down. I saw Aircraft X at FL230 so I descended them to FL210. I didn't see Aircraft Y at FL220. I noticed it very soon after I gave the decent and told Aircraft X to climb back up to FL230. I lost separation before Aircraft X could regain FL230. I will in the future pay even more attention to altitudes before I issue one to an aircraft; especially in a holding stack. I could have also asked for a D-Side to help me scan the various altitudes. I could have taken less airplanes from high to allow myself more altitudes to work with.

Data retrieved from NASA's ASRS site as of April 2012 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.

Attributes
ACN	979511
Time
Date	201111
Local Time Of Day	1801-2400
Place
Locale Reference	ZZZ.ARTCC
State Reference	US
Aircraft 1
Make Model Name	B717 (Formerly MD-95)
Flight Phase	Cruise
Flight Plan	IFR
Person 1
Function	Enroute
Qualification	Air Traffic Control Developmental
Events
Anomaly	ATC Issue All Types Deviation - Procedural Published Material / Policy

Enroute controller described a loss of separation event during exiting holding procedures; the reporter listing workload; failure to request a d-side and attentiveness to altitude assignments as causal factors.