Air carrier X had less than standard separation from air carrier Y. System error.

Attributes
ACN	141263
Time
Date	199004
Day	Sun
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : psx
State Reference	TX
Altitude	msl bound lower : 22000 msl bound upper : 22400
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zhu tower : bwi
Operator	common carrier : air carrier
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Flight Phase	cruise other
Route In Use	enroute : direct
Flight Plan	IFR
Aircraft 2
Operator	common carrier : air carrier
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Flight Phase	descent other
Route In Use	enroute : direct
Flight Plan	IFR
Person 1
Affiliation	government : faa
Function	controller : radar
Qualification	controller : radar
Experience	controller radar : 20
ASRS Report	141263
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	conflict : airborne less severe non adherence : required legal separation non adherence : published procedure
Independent Detector	atc equipment other atc equipment : unspecified other controllera
Resolutory Action	controller : issued new clearance other
Consequence	faa : investigated
Miss Distance	horizontal : 25200 vertical : 300
Supplementary
Primary Problem	ATC Human Performance
Air Traffic Incident	Operational Error

Narrative:

Air carrier X swbnd at FL220 and air carrier Y northeast bound descending to FL230. Air carrier Y requested lower and was given one zero thousand. I immediately realized the two aircraft would conflict and told air carrier Y to maintain FL230. He replied climbing back to FL230. Aircraft were laterally sep at this time. I noticed mode 'C' readout of FL228 and assumed he was climbing back to FL230. Approximately 30 seconds later I observed mode 'C' readout of FL224. I asked air carrier Y if he was at FL230 and he said we are getting there.

Original NASA ASRS Text

Title: ACR X HAD LESS THAN STANDARD SEPARATION FROM ACR Y. SYSTEM ERROR.

Narrative: ACR X SWBND AT FL220 AND ACR Y NE BOUND DSNDING TO FL230. ACR Y REQUESTED LOWER AND WAS GIVEN ONE ZERO THOUSAND. I IMMEDIATELY REALIZED THE TWO ACFT WOULD CONFLICT AND TOLD ACR Y TO MAINTAIN FL230. HE REPLIED CLBING BACK TO FL230. ACFT WERE LATERALLY SEP AT THIS TIME. I NOTICED MODE 'C' READOUT OF FL228 AND ASSUMED HE WAS CLBING BACK TO FL230. APPROX 30 SECS LATER I OBSERVED MODE 'C' READOUT OF FL224. I ASKED ACR Y IF HE WAS AT FL230 AND HE SAID WE ARE GETTING THERE.

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.