Atx Y descent to occupied altitude had less than standard separation from atx X. System error.

Attributes
ACN	227572
Time
Date	199212
Day	Wed
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : ord
State Reference	IL
Altitude	msl bound lower : 4000 msl bound upper : 4000
Environment
Flight Conditions	IMC
Light	Night
Aircraft 1
Controlling Facilities	tracon : ord tracon : hln
Operator	common carrier : air taxi
Make Model Name	Small Aircraft, Low Wing, 1 Eng, Retractable Gear
Flight Phase	cruise other
Route In Use	enroute : direct
Flight Plan	IFR
Aircraft 2
Operator	common carrier : air taxi
Make Model Name	Small Aircraft, Low Wing, 1 Eng, Retractable Gear
Flight Phase	descent other
Route In Use	enroute : on vectors
Flight Plan	IFR
Person 1
Affiliation	government : faa
Function	controller : approach
Qualification	controller : radar
ASRS Report	227572
Person 2
Affiliation	company : air taxi
Function	flight crew : single pilot
Qualification	pilot : commercial pilot : instrument
Events
Anomaly	conflict : airborne less severe non adherence : required legal separation non adherence : published procedure
Independent Detector	other controllera
Resolutory Action	controller : issued new clearance other
Consequence	faa : investigated
Miss Distance	horizontal : 6000
Supplementary
Primary Problem	ATC Human Performance
Air Traffic Incident	Operational Error

Narrative:

I was working arrival to runway 27R at ord. Simultaneous runway 27L/right approachs in use. Atx X was given to me by the feeder controller on downwind. I descended him to 4000. Atx Y was on base leg, 20 NM northeast ord, at 7000, in a group of 3 aircraft. I descended the first aircraft, atx Y, to 4000 ft, but meant to descend atx Y to 5000. I said 4000. Read back 4000. I didn't catch it. A severe case of cerebral flatulence in the heat of battle. When the other arrival controller pointed out the conflict, I climbed atx Y back to 5000 and told him that's what his assigned altitude was. My awareness is at fault -- I mis-spoke.

Original NASA ASRS Text

Title: ATX Y DSCNT TO OCCUPIED ALT HAD LTSS FROM ATX X. SYS ERROR.

Narrative: I WAS WORKING ARR TO RWY 27R AT ORD. SIMULTANEOUS RWY 27L/R APCHS IN USE. ATX X WAS GIVEN TO ME BY THE FEEDER CTLR ON DOWNWIND. I DSNDED HIM TO 4000. ATX Y WAS ON BASE LEG, 20 NM NE ORD, AT 7000, IN A GROUP OF 3 ACFT. I DSNDED THE FIRST ACFT, ATX Y, TO 4000 FT, BUT MEANT TO DSND ATX Y TO 5000. I SAID 4000. READ BACK 4000. I DIDN'T CATCH IT. A SEVERE CASE OF CEREBRAL FLATULENCE IN THE HEAT OF BATTLE. WHEN THE OTHER ARR CTLR POINTED OUT THE CONFLICT, I CLBED ATX Y BACK TO 5000 AND TOLD HIM THAT'S WHAT HIS ASSIGNED ALT WAS. MY AWARENESS IS AT FAULT -- I MIS-SPOKE.

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.