Air carrier was descended to altitude occupied by another aircraft.

Attributes
ACN	110189
Time
Date	198904
Day	Fri
Local Time Of Day	1801 To 2400
Place
Locale Reference	atc facility : ard
State Reference	PA
Altitude	msl bound lower : 11000 msl bound upper : 11300
Environment
Flight Conditions	VMC
Light	Dusk
Aircraft 1
Controlling Facilities	artcc : zdc
Operator	common carrier : air carrier
Make Model Name	Large Transport, Low Wing, 3 Turbojet Eng
Flight Phase	descent other
Route In Use	arrival other
Flight Plan	IFR
Aircraft 2
Operator	general aviation : corporate
Make Model Name	Light Transport, Low Wing, 2 Turbojet Eng
Flight Phase	descent other
Route In Use	arrival other
Flight Plan	IFR
Person 1
Affiliation	government : faa
Function	controller : radar
Qualification	controller : radar
Experience	controller radar : 4
ASRS Report	110189
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	atc equipment other atc equipment : unspecified other controllera
Resolutory Action	controller : issued new clearance
Consequence	faa : investigated
Miss Distance	horizontal : 3000 vertical : 300
Supplementary
Primary Problem	ATC Human Performance
Air Traffic Incident	Operational Error

Narrative:

I was working dupont lo sector by myself (R18). I was laddering air carrier X down reference cga Y. I told air carrier X to expect 10 south of ard at 13000'. When I had cga Y clear, I gave air carrier X his restriction. I thought I had given air carrier X 10 south at 13000', but I really gave him 10 south ard at 11000'. The data block and strip said 13000'. Mode C was lost on air carrier X at 13200'. It came back up when it showed 11600'. The next hit on the radar showed 11300', which I thought was a mode C jump to the cga Y. I then asked air carrier X his altitude. He said 11000'. I told cga Y to turn right heading 090 degrees. The aircraft said he didn't want to because there was a plane out there. I then climbed air carrier X to 12000'.

Original NASA ASRS Text

Title: ACR WAS DESCENDED TO ALT OCCUPIED BY ANOTHER ACFT.

Narrative: I WAS WORKING DUPONT LO SECTOR BY MYSELF (R18). I WAS LADDERING ACR X DOWN REF CGA Y. I TOLD ACR X TO EXPECT 10 S OF ARD AT 13000'. WHEN I HAD CGA Y CLR, I GAVE ACR X HIS RESTRICTION. I THOUGHT I HAD GIVEN ACR X 10 S AT 13000', BUT I REALLY GAVE HIM 10 S ARD AT 11000'. THE DATA BLOCK AND STRIP SAID 13000'. MODE C WAS LOST ON ACR X AT 13200'. IT CAME BACK UP WHEN IT SHOWED 11600'. THE NEXT HIT ON THE RADAR SHOWED 11300', WHICH I THOUGHT WAS A MODE C JUMP TO THE CGA Y. I THEN ASKED ACR X HIS ALT. HE SAID 11000'. I TOLD CGA Y TO TURN RIGHT HDG 090 DEGS. THE ACFT SAID HE DIDN'T WANT TO BECAUSE THERE WAS A PLANE OUT THERE. I THEN CLBED ACR X TO 12000'.

Data retrieved from NASA's ASRS site as of August 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.