Aircraft was descended to an occupied altitude resulting in less than standard separation.

Attributes
ACN	135243
Time
Date	199001
Day	Fri
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : cha
State Reference	TN
Altitude	msl bound lower : 21000 msl bound upper : 21600
Environment
Flight Conditions	VMC
Light	Dusk
Aircraft 1
Controlling Facilities	artcc : ztl
Operator	general aviation : corporate
Make Model Name	Small Transport, Low Wing, 2 Turboprop Eng
Flight Phase	cruise other
Flight Plan	IFR
Aircraft 2
Operator	general aviation : corporate
Make Model Name	Light Transport, Low Wing, 2 Turboprop Eng
Flight Phase	descent other
Flight Plan	IFR
Person 1
Affiliation	government : faa
Function	controller : radar
Qualification	controller : radar
ASRS Report	135243
Person 2
Affiliation	Other
Function	flight crew : captain oversight : pic
Qualification	pilot : commercial pilot : instrument
Events
Anomaly	conflict : airborne less severe non adherence : published procedure non adherence : required legal separation
Independent Detector	atc equipment other atc equipment : unspecified other controllera
Resolutory Action	controller : issued new clearance
Consequence	Other
Miss Distance	horizontal : 15000 vertical : 0
Supplementary
Primary Problem	ATC Human Performance
Air Traffic Incident	Operational Error

Narrative:

Cga X sebnd just east of cha overflying atl at FL210. Cga Y checked on the frequency at FL240 over atl landing cha. Cga Y was descended to FL220 for cga X and turned 10 degree right to intrail behind an light transport. When I saw the intrail was good and I was going to miss another aircraft, I descended cga Y to 15,000. The 2 aircraft were 3 1/2 mi head-on at this time (cga X and Y). When I realized what I had done, I turned cga Y right heading 090 degree and issued traffic. I went to cga X, issued traffic and turned him to 270 heading. No reply. Later I learned he wasn't on my frequency. The conflict went off when cga Y was at FL216 ddg. I realized what I had done just before this. Up until this time I didn't think I needed a tracker, I mean I was busy but not sunk. About 1/2 minute before it occurred, I could have used one. I did have a trainee on the D side but I believe this error is all my fault. I feel the traffic volume was in the moderate to upper moderate range and very complex. I just forgot cga X and will have to be more aware in the future.

Original NASA ASRS Text

Title: ACFT WAS DESCENDED TO AN OCCUPIED ALT RESULTING IN LESS THAN STANDARD SEPARATION.

Narrative: CGA X SEBND JUST E OF CHA OVERFLYING ATL AT FL210. CGA Y CHECKED ON THE FREQ AT FL240 OVER ATL LNDG CHA. CGA Y WAS DESCENDED TO FL220 FOR CGA X AND TURNED 10 DEG RIGHT TO INTRAIL BEHIND AN LTT. WHEN I SAW THE INTRAIL WAS GOOD AND I WAS GOING TO MISS ANOTHER ACFT, I DESCENDED CGA Y TO 15,000. THE 2 ACFT WERE 3 1/2 MI HEAD-ON AT THIS TIME (CGA X AND Y). WHEN I REALIZED WHAT I HAD DONE, I TURNED CGA Y RIGHT HDG 090 DEG AND ISSUED TFC. I WENT TO CGA X, ISSUED TFC AND TURNED HIM TO 270 HDG. NO REPLY. LATER I LEARNED HE WASN'T ON MY FREQ. THE CONFLICT WENT OFF WHEN CGA Y WAS AT FL216 DDG. I REALIZED WHAT I HAD DONE JUST BEFORE THIS. UP UNTIL THIS TIME I DIDN'T THINK I NEEDED A TRACKER, I MEAN I WAS BUSY BUT NOT SUNK. ABOUT 1/2 MINUTE BEFORE IT OCCURRED, I COULD HAVE USED ONE. I DID HAVE A TRAINEE ON THE D SIDE BUT I BELIEVE THIS ERROR IS ALL MY FAULT. I FEEL THE TFC VOLUME WAS IN THE MODERATE TO UPPER MODERATE RANGE AND VERY COMPLEX. I JUST FORGOT CGA X AND WILL HAVE TO BE MORE AWARE IN THE FUTURE.

Data retrieved from NASA's ASRS site as of July 2007 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.