Large transport air carrier has TCASII alert then vector when ATC advised of level off.

Attributes
ACN	274613
Time
Date	199405
Day	Fri
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : dca
State Reference	DC
Altitude	msl bound lower : 11000 msl bound upper : 11000
Environment
Flight Conditions	IMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : dca
Operator	common carrier : air carrier
Make Model Name	Commercial Fixed Wing
Operating Under FAR Part	Part 121
Flight Phase	climbout : intermediate altitude
Route In Use	enroute : on vectors
Flight Plan	IFR
Aircraft 2
Operator	common carrier : air carrier
Make Model Name	Commercial Fixed Wing
Operating Under FAR Part	Part 121
Flight Phase	climbout : intermediate altitude
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 150 flight time total : 12000 flight time type : 6000
ASRS Report	274613
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : instrument pilot : commercial pilot : atp
Events
Anomaly	conflict : airborne less severe non adherence : far
Independent Detector	aircraft equipment other aircraft equipment : unspecified
Resolutory Action	flight crew : took evasive action
Consequence	faa : reviewed incident with flight crew
Miss Distance	horizontal : 25000 vertical : 400
Supplementary
Primary Problem	ATC Human Performance
Air Traffic Incident	other

Narrative:

Flight departed iad. While being vectored east in the vicinity of washington national (dca) we were cleared to climb from 10000-12000 ft and given a southeast heading as a vector for 'daily' intersection. Approaching 11000 ft we received a TCASII warning and observed another aircraft on TCASII scope closing rapidly at approximately the same altitude. We quickly evaluated that this aircraft was climbing also and proceeding in our general direction but much more slowly. A decision was made to level at 11000 ft and advise ATC while letting other aircraft continue climb. ATC responded by giving us an immediate left turn 20 degrees and apologizing profusely about being temporarily distracted. It is my professional opinion that this incident could easily have resulted in a mid-air collision without the TCASII system. We were IFR throughout the incident and never saw the other aircraft.

Original NASA ASRS Text

Title: LGT ACR HAS TCASII ALERT THEN VECTOR WHEN ATC ADVISED OF LEVEL OFF.

Narrative: FLT DEPARTED IAD. WHILE BEING VECTORED E IN THE VICINITY OF WASHINGTON NATIONAL (DCA) WE WERE CLRED TO CLB FROM 10000-12000 FT AND GIVEN A SE HDG AS A VECTOR FOR 'DAILY' INTXN. APCHING 11000 FT WE RECEIVED A TCASII WARNING AND OBSERVED ANOTHER ACFT ON TCASII SCOPE CLOSING RAPIDLY AT APPROX THE SAME ALT. WE QUICKLY EVALUATED THAT THIS ACFT WAS CLBING ALSO AND PROCEEDING IN OUR GENERAL DIRECTION BUT MUCH MORE SLOWLY. A DECISION WAS MADE TO LEVEL AT 11000 FT AND ADVISE ATC WHILE LETTING OTHER ACFT CONTINUE CLB. ATC RESPONDED BY GIVING US AN IMMEDIATE L TURN 20 DEGS AND APOLOGIZING PROFUSELY ABOUT BEING TEMPORARILY DISTRACTED. IT IS MY PROFESSIONAL OPINION THAT THIS INCIDENT COULD EASILY HAVE RESULTED IN A MID-AIR COLLISION WITHOUT THE TCASII SYS. WE WERE IFR THROUGHOUT THE INCIDENT AND NEVER SAW THE OTHER ACFT.

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.