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

Attributes
ACN	154569
Time
Date	199008
Day	Mon
Local Time Of Day	1801 To 2400
Place
Locale Reference	atc facility : gsh
State Reference	IN
Altitude	msl bound lower : 9000 msl bound upper : 9000
Environment
Flight Conditions	VMC
Light	Dusk
Aircraft 1
Controlling Facilities	artcc : zau
Operator	common carrier : air carrier
Make Model Name	Light Transport, Low Wing, 2 Turboprop Eng
Flight Phase	descent other
Route In Use	enroute : direct
Flight Plan	IFR
Aircraft 2
Operator	common carrier : air carrier
Make Model Name	Light Transport, Low Wing, 2 Turboprop Eng
Flight Phase	cruise other
Route In Use	enroute : direct
Flight Plan	IFR
Person 1
Affiliation	government : faa
Function	controller : radar
Qualification	controller : radar controller : developmental
Experience	controller military : 6 controller non radar : 2 controller radar : 1
ASRS Report	154569
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	conflict : airborne less severe non adherence : published procedure non adherence : required legal separation
Independent Detector	other controllera
Resolutory Action	none taken : anomaly accepted
Consequence	faa : investigated
Miss Distance	horizontal : 24000 vertical : 500
Supplementary
Primary Problem	ATC Human Performance
Air Traffic Incident	Operational Error

Narrative:

Air carrier Y at 9000' sebnd and ar X westbound descending to 10000'. Air carrier X westbound was required by SOP to be descended to 9000' (propeller landing ord). I watched the 2 aircraft pass each other. I then took care of some other business (i.e., frequency changes, sequencing for mdw) and felt that a considerable amount of time had passed by the time I returned to these 2 aircraft. I descended air carrier X to 9000', there being no doubt in my mind that there was 5 mi between them. However, I was still moving the data blocks apart, and after doing so I realized my mistake after it was too late to stop air carrier X descent. There was less than 5 mi. My perception of how much had passed since observing the targets pass, combined with the fact both aircraft were propellers (light transport), and poor controller technique (descending the aircraft while separating data blocks) combined to cause this error.

Original NASA ASRS Text

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

Narrative: ACR Y AT 9000' SEBND AND AR X WBND DSNDING TO 10000'. ACR X WBND WAS REQUIRED BY SOP TO BE DSNDED TO 9000' (PROP LNDG ORD). I WATCHED THE 2 ACFT PASS EACH OTHER. I THEN TOOK CARE OF SOME OTHER BUSINESS (I.E., FREQ CHANGES, SEQUENCING FOR MDW) AND FELT THAT A CONSIDERABLE AMOUNT OF TIME HAD PASSED BY THE TIME I RETURNED TO THESE 2 ACFT. I DSNDED ACR X TO 9000', THERE BEING NO DOUBT IN MY MIND THAT THERE WAS 5 MI BTWN THEM. HOWEVER, I WAS STILL MOVING THE DATA BLOCKS APART, AND AFTER DOING SO I REALIZED MY MISTAKE AFTER IT WAS TOO LATE TO STOP ACR X DSNT. THERE WAS LESS THAN 5 MI. MY PERCEPTION OF HOW MUCH HAD PASSED SINCE OBSERVING THE TARGETS PASS, COMBINED WITH THE FACT BOTH ACFT WERE PROPS (LTT), AND POOR CTLR TECHNIQUE (DSNDING THE ACFT WHILE SEPARATING DATA BLOCKS) COMBINED TO CAUSE THIS ERROR.

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.