An air carrier B727 flight crew reports an encounter with an intruder aircraft in class B airspace.

Attributes
ACN	399531
Time
Date	199804
Day	Sun
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : dca
State Reference	DC
Altitude	msl bound lower : 2500 msl bound upper : 2700
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : dca
Operator	common carrier : air carrier
Make Model Name	B727 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	descent : approach
Route In Use	approach : visual enroute : on vectors
Flight Plan	IFR
Aircraft 2
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Operating Under FAR Part	other : unknown
Flight Phase	climbout : intermediate altitude
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp
Experience	flight time last 90 days : 70 flight time total : 4300 flight time type : 190
ASRS Report	399531
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 : far other anomaly other
Independent Detector	aircraft equipment other aircraft equipment : unspecified
Resolutory Action	flight crew : took evasive action
Consequence	Other
Miss Distance	horizontal : 1125 vertical : 200
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	other

Narrative:

After being cleared by approach control to descend to 2500 ft, a TCASII gave us traffic resolution. We applied go around power and climbed. I notified approach we had an RA and were climbing, at which time approach said we had traffic at 2500 ft. I guess it was popup traffic squawking 1200 and not talking to approach in class B airspace.

Original NASA ASRS Text

Title: AN ACR B727 FLC RPTS AN ENCOUNTER WITH AN INTRUDER ACFT IN CLASS B AIRSPACE.

Narrative: AFTER BEING CLRED BY APCH CTL TO DSND TO 2500 FT, A TCASII GAVE US TFC RESOLUTION. WE APPLIED GAR PWR AND CLBED. I NOTIFIED APCH WE HAD AN RA AND WERE CLBING, AT WHICH TIME APCH SAID WE HAD TFC AT 2500 FT. I GUESS IT WAS POPUP TFC SQUAWKING 1200 AND NOT TALKING TO APCH IN CLASS B AIRSPACE.

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.