ZDC controller experienced operational error at FL240, assumed one aircraft was at FL250.

Attributes
ACN	626726
Time
Date	200407
Day	Fri
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : zdc.artcc
State Reference	DC
Altitude	msl bound lower : 24000 msl bound upper : 25.000
Environment
Flight Conditions	IMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zdc.artcc tower : zzz.tower
Operator	general aviation : corporate
Make Model Name	Super King Air 300
Operating Under FAR Part	Part 91
Flight Phase	descent : intermediate altitude
Flight Plan	IFR
Aircraft 2
Controlling Facilities	artcc : zdc.artcc
Operator	common carrier : air carrier
Make Model Name	Regional Jet CL65, Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	descent : intermediate altitude
Flight Plan	IFR
Person 1
Affiliation	government : faa
Function	controller : radar
Qualification	controller : radar
ASRS Report	626726
Person 2
Affiliation	company : corporate
Function	flight crew : captain oversight : pic
Events
Anomaly	conflict : airborne less severe non adherence : required legal separation
Independent Detector	atc equipment : conflict alert other controllera
Resolutory Action	flight crew : took evasive action
Consequence	faa : investigated
Miss Distance	horizontal : 18000 vertical : 0
Supplementary
Problem Areas	ATC Human Performance
Primary Problem	ATC Human Performance
Air Traffic Incident	Operational Error

Narrative:

Thought rear aircraft was at FL250. Descended lead aircraft to FL240. Both aircraft leveled at FL240.

Original NASA ASRS Text

Title: ZDC CTLR EXPERIENCED OPERROR AT FL240, ASSUMED ONE ACFT WAS AT FL250.

Narrative: THOUGHT REAR ACFT WAS AT FL250. DSNDED LEAD ACFT TO FL240. BOTH ACFT LEVELED AT FL240.

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.