Pct controller misjudged descent rate and clearing aircraft to occupied altitude.

Attributes
ACN	697364
Time
Date	200605
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : pct.tracon
State Reference	VA
Altitude	msl bound lower : 5300 msl bound upper : 6000
Environment
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : pct.tracon tower : tpa.tower
Operator	common carrier : air carrier
Make Model Name	B737 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	descent : intermediate altitude descent : vacating altitude
Route In Use	arrival : on vectors
Flight Plan	IFR
Aircraft 2
Controlling Facilities	tracon : pct.tracon
Operator	common carrier : air carrier
Make Model Name	Dash 8 Series Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	descent : vacating altitude descent : intermediate altitude
Route In Use	arrival : on vectors
Flight Plan	IFR
Person 1
Affiliation	government : faa
Function	controller : approach controller : radar
Qualification	controller : radar
Experience	controller limited radar : 5 controller radar : 10
ASRS Report	697364
Person 2
Affiliation	company : air carrier
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	controller : issued new clearance controller : issued alert flight crew : took evasive action
Consequence	faa : investigated
Miss Distance	horizontal : 10500 vertical : 900
Supplementary
Problem Areas	ATC Human Performance
Primary Problem	ATC Human Performance

Narrative:

I turned aircraft X to the left to follow traffic flow of traffic landing runway 33L; when I noticed aircraft Y was not descending at the rate expected. I stopped aircraft X descent and turned to avoid aircraft Y. I then asked aircraft Y to expedite descent; that transmission was blocked. I restated descent to aircraft Y. I notified supervisor of incident.

Original NASA ASRS Text

Title: PCT CTLR MISJUDGED DSCNT RATE AND CLRING ACFT TO OCCUPIED ALT.

Narrative: I TURNED ACFT X TO THE L TO FOLLOW TFC FLOW OF TFC LNDG RWY 33L; WHEN I NOTICED ACFT Y WAS NOT DSNDING AT THE RATE EXPECTED. I STOPPED ACFT X DSCNT AND TURNED TO AVOID ACFT Y. I THEN ASKED ACFT Y TO EXPEDITE DSCNT; THAT XMISSION WAS BLOCKED. I RESTATED DSCNT TO ACFT Y. I NOTIFIED SUPVR OF INCIDENT.

Data retrieved from NASA's ASRS site as of January 2009 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.