An aircraft was issued a descent clearance in error which resulted in less than standard separation.

Attributes
ACN	150165
Time
Date	199006
Day	Sat
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : jot airport : zau
State Reference	IL
Altitude	msl bound lower : 8000 msl bound upper : 8000
Environment
Flight Conditions	IMC
Light	Daylight
Aircraft 1
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Flight Phase	cruise other
Route In Use	arrival other enroute : on vectors
Flight Plan	IFR
Aircraft 2
Operator	common carrier : air carrier
Make Model Name	Small Transport, Low Wing, 2 Turboprop Eng
Flight Phase	cruise other
Route In Use	arrival other enroute : on vectors
Flight Plan	IFR
Person 1
Affiliation	government : faa
Function	controller : radar
Qualification	controller : radar
ASRS Report	150165
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 : detected after the fact
Consequence	Other
Miss Distance	horizontal : 15600 vertical : 700
Supplementary
Primary Problem	ATC Human Performance
Air Traffic Incident	Operational Error

Narrative:

Aircraft #2 was direct ord about a 40 degree heading and maintaining 8000'. Aircraft #1 checked on my frequency descending to 10000', direct to jot VOR about a 60 degree heading and 10 mi behind aircraft #2. Aircraft #1 was cleared to 9000' about the time aircraft #2 was passing #1, I turned #1 to an 020 degree heading in order to establish latitude sep. Shortly after they passed I turned aircraft #2 to 090 degree heading and inadvertently issued 8000'. The closest the aircraft got was 2.57 mi and 700'. I immediately questioned the pilot of aircraft #2 why he had left 9000', thinking that was his assigned altitude. No evasive action was necessary seeing they had already passed each other. There was excessive noise in the area which prevented me from hearing the readback; this may have alerted me to my mistake.

Original NASA ASRS Text

Title: AN ACFT WAS ISSUED A DESCENT CLRNC IN ERROR WHICH RESULTED IN LESS THAN STANDARD SEPARATION.

Narrative: ACFT #2 WAS DIRECT ORD ABOUT A 40 DEG HDG AND MAINTAINING 8000'. ACFT #1 CHKED ON MY FREQ DSNDING TO 10000', DIRECT TO JOT VOR ABOUT A 60 DEG HDG AND 10 MI BEHIND ACFT #2. ACFT #1 WAS CLRED TO 9000' ABOUT THE TIME ACFT #2 WAS PASSING #1, I TURNED #1 TO AN 020 DEG HDG IN ORDER TO ESTABLISH LAT SEP. SHORTLY AFTER THEY PASSED I TURNED ACFT #2 TO 090 DEG HDG AND INADVERTENTLY ISSUED 8000'. THE CLOSEST THE ACFT GOT WAS 2.57 MI AND 700'. I IMMEDIATELY QUESTIONED THE PLT OF ACFT #2 WHY HE HAD LEFT 9000', THINKING THAT WAS HIS ASSIGNED ALT. NO EVASIVE ACTION WAS NECESSARY SEEING THEY HAD ALREADY PASSED EACH OTHER. THERE WAS EXCESSIVE NOISE IN THE AREA WHICH PREVENTED ME FROM HEARING THE READBACK; THIS MAY HAVE ALERTED ME TO MY MISTAKE.

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.