Narrative:

While on a radar vector 'downwind' for runway 10 in IMC conditions, heading was approximately 270 degrees. We were abeam the field, flaps 5 degrees, slowed to 180 KTS. Then, potomac approach gave us heading 350 degrees. We slowed to flaps 10 degrees, about 170-160 KTS. We were then issued heading of 020 degrees. The first officer and I noticed the controller getting a company flight number and another carrier flight confused. The first officer made a comment that we were going to be vectored through the localizer. I was watching multiple traffic on the TCASII, but one of concern was just on the north side of the localizer coming straight for us at our altitude. I kept watching this traffic at our 12 O'clock position. I could not understand why our TCASII was not giving us an RA. ATC was very busy and we could not get an inquiry to ATC about the traffic. I could not wait any longer. I replied, 'we're out OF4 here.' I disconnected the autoplt and shoved the throttles forward initiating a climb and started a right turn (traffic on the nose and just slightly left). The other carrier flight inquired about traffic at their 12 O'clock position. I climbed from 3000 ft to 3300 ft abruptly and turned 30-50 degrees right. I was afraid to climb any higher because there were other aircraft in close proximity. A brief second went by wondering if we were going to hit the traffic. At this time, I did not think the TCASII performed the way I thought it should. The traffic went from white to yellow at our altitude, but no aural warning to respond. I just could not situation there and keep watching the traffic getting closer and closer. While climbing from 3000 ft to 3300 ft, I was watching the traffic getting 300 ft vertical separation. First officer thought he saw traffic pass very close! About that time, ATC (new voice) said, 'company flight number, turn left heading 335 degrees.' first officer read back flight number, turning left to heading 335 degrees. I was hesitant at first to turn left because traffic was slightly to our left on TCASII and at this point, I did not have any trust in ATC. I slowly started a left turn to 335 degrees. We received further vectors (left traffic pattern) to join localizer for runway 10 and cleared for approach. When approach handed us over to bwi tower, first officer read back going to tower and added, 'that was close.' the approach controller replied, 'I know.' flight continued without any further incident. Throughout the whole approach, first officer was continually 'in the loop' in facilitating me with situational awareness and crew coordination.

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Original NASA ASRS Text

Title: B737 FLT CREW IS REQUIRED TO PERFORM AN EVASIVE MANEUVER DUE TO LACK OF ACFT SEPARATION DURING INITIAL APCH TO BWI.

Narrative: WHILE ON A RADAR VECTOR 'DOWNWIND' FOR RWY 10 IN IMC CONDITIONS, HEADING WAS APPROX 270 DEGS. WE WERE ABEAM THE FIELD, FLAPS 5 DEGS, SLOWED TO 180 KTS. THEN, POTOMAC APCH GAVE US HEADING 350 DEGS. WE SLOWED TO FLAPS 10 DEGS, ABOUT 170-160 KTS. WE WERE THEN ISSUED HEADING OF 020 DEGS. THE FO AND I NOTICED THE CTLR GETTING A COMPANY FLT NUMBER AND ANOTHER CARRIER FLT CONFUSED. THE FO MADE A COMMENT THAT WE WERE GOING TO BE VECTORED THROUGH THE LOC. I WAS WATCHING MULTIPLE TFC ON THE TCASII, BUT ONE OF CONCERN WAS JUST ON THE N SIDE OF THE LOC COMING STRAIGHT FOR US AT OUR ALT. I KEPT WATCHING THIS TFC AT OUR 12 O'CLOCK POS. I COULD NOT UNDERSTAND WHY OUR TCASII WAS NOT GIVING US AN RA. ATC WAS VERY BUSY AND WE COULD NOT GET AN INQUIRY TO ATC ABOUT THE TFC. I COULD NOT WAIT ANY LONGER. I REPLIED, 'WE'RE OUT OF4 HERE.' I DISCONNECTED THE AUTOPLT AND SHOVED THE THROTTLES FORWARD INITIATING A CLB AND STARTED A R TURN (TFC ON THE NOSE AND JUST SLIGHTLY L). THE OTHER CARRIER FLT INQUIRED ABOUT TFC AT THEIR 12 O'CLOCK POS. I CLBED FROM 3000 FT TO 3300 FT ABRUPTLY AND TURNED 30-50 DEGS R. I WAS AFRAID TO CLB ANY HIGHER BECAUSE THERE WERE OTHER ACFT IN CLOSE PROX. A BRIEF SECOND WENT BY WONDERING IF WE WERE GOING TO HIT THE TFC. AT THIS TIME, I DID NOT THINK THE TCASII PERFORMED THE WAY I THOUGHT IT SHOULD. THE TFC WENT FROM WHITE TO YELLOW AT OUR ALT, BUT NO AURAL WARNING TO RESPOND. I JUST COULD NOT SIT THERE AND KEEP WATCHING THE TFC GETTING CLOSER AND CLOSER. WHILE CLBING FROM 3000 FT TO 3300 FT, I WAS WATCHING THE TFC GETTING 300 FT VERT SEPARATION. FO THOUGHT HE SAW TFC PASS VERY CLOSE! ABOUT THAT TIME, ATC (NEW VOICE) SAID, 'COMPANY FLT NUMBER, TURN L HEADING 335 DEGS.' FO READ BACK FLT NUMBER, TURNING L TO HEADING 335 DEGS. I WAS HESITANT AT FIRST TO TURN L BECAUSE TFC WAS SLIGHTLY TO OUR L ON TCASII AND AT THIS POINT, I DID NOT HAVE ANY TRUST IN ATC. I SLOWLY STARTED A L TURN TO 335 DEGS. WE RECEIVED FURTHER VECTORS (L TFC PATTERN) TO JOIN LOC FOR RWY 10 AND CLRED FOR APCH. WHEN APCH HANDED US OVER TO BWI TWR, FO READ BACK GOING TO TWR AND ADDED, 'THAT WAS CLOSE.' THE APCH CTLR REPLIED, 'I KNOW.' FLT CONTINUED WITHOUT ANY FURTHER INCIDENT. THROUGHOUT THE WHOLE APCH, FO WAS CONTINUALLY 'IN THE LOOP' IN FACILITATING ME WITH SITUATIONAL AWARENESS AND CREW COORD.

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.