A B757-200 on a visual approach to bwi runway 15R experienced a TCAS RA with unidented traffic.

Attributes
ACN	720576
Time
Date	200612
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : bwi.airport
State Reference	MD
Altitude	msl bound lower : 4000 msl bound upper : 4300
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : pct.tracon
Operator	common carrier : air carrier
Make Model Name	B757-200
Operating Under FAR Part	Part 121
Flight Phase	descent : approach
Route In Use	approach : visual
Flight Plan	IFR
Aircraft 2
Controlling Facilities	tracon : bwi.tracon
Make Model Name	B737 Undifferentiated or Other Model
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
ASRS Report	720576
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Events
Anomaly	conflict : airborne critical
Independent Detector	aircraft equipment : tcas other controllera other flight crewa other flight crewb
Resolutory Action	flight crew : took evasive action
Miss Distance	horizontal : 5000 vertical : 400
Supplementary
Problem Areas	Airspace Structure ATC Human Performance
Primary Problem	Airspace Structure

Narrative:

First officer was flying. We were on a base to final for runway 15R at 4000 ft in VMC conditions. We called traffic to follow a B737 in sight. A few seconds later we received a 'traffic' alert from TCAS. It showed an aircraft at about 2 O'clock position; and 2 mi; 500 ft below us and climbing. We looked but no contact. ATC then informed us of the traffic which was now 400 ft below us and 1 mi. At that time we received an RA to climb. First officer clicked off the autoplt and began an immediate climb to follow the RA. At the same time I informed ATC that we were following an RA and climbing. Almost immediately we got the 'clear of conflict' alert and descended back to 4000 ft from 4300 ft. ATC acknowledged and we continued the approach and landing without incident.

Original NASA ASRS Text

Title: A B757-200 ON A VISUAL APCH TO BWI RWY 15R EXPERIENCED A TCAS RA WITH UNIDENTED TFC.

Narrative: FO WAS FLYING. WE WERE ON A BASE TO FINAL FOR RWY 15R AT 4000 FT IN VMC CONDITIONS. WE CALLED TFC TO FOLLOW A B737 IN SIGHT. A FEW SECONDS LATER WE RECEIVED A 'TFC' ALERT FROM TCAS. IT SHOWED AN ACFT AT ABOUT 2 O'CLOCK POS; AND 2 MI; 500 FT BELOW US AND CLBING. WE LOOKED BUT NO CONTACT. ATC THEN INFORMED US OF THE TFC WHICH WAS NOW 400 FT BELOW US AND 1 MI. AT THAT TIME WE RECEIVED AN RA TO CLB. FO CLICKED OFF THE AUTOPLT AND BEGAN AN IMMEDIATE CLB TO FOLLOW THE RA. AT THE SAME TIME I INFORMED ATC THAT WE WERE FOLLOWING AN RA AND CLBING. ALMOST IMMEDIATELY WE GOT THE 'CLR OF CONFLICT' ALERT AND DSNDED BACK TO 4000 FT FROM 4300 FT. ATC ACKNOWLEDGED AND WE CONTINUED THE APCH AND LNDG WITHOUT 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.