Narrative:

I was the PF. We were level at 4000 ft on radar vectors for the localizer 10 approach at bwi. ATC gave us a turn and said to make it tight due to mdt traffic in front of us at 4000 ft. I manually banked the aircraft into a 30 degree bank and began the turn when we got a yellow TA at 12 O'clock and 5 mi at 4000 ft. We went to 5 mi range on TCASII and saw we were rapidly approaching target at same altitude. Captain was looking for traffic as I flew the aircraft. We then got an RA to descend. I asked the captain if he had the aircraft and he said no and said to comply, which I immediately did. At approximately 3500 ft, the RA was terminated. I estimate we got to within 2 mi based on the TCASII display. We never saw the traffic. I believe there was a real potential for a collision if we hadn't taken evasive action. When we told the controller we had departed our assigned altitude for lower to comply with an RA, he immediately cleared us to 2500 ft. No further mention was made of the RA or altitude deviation.

Related reports:

Google
 

Original NASA ASRS Text

Title: FLC OF AN MLG ACR ACFT DSNDED BELOW ASSIGNED APCH ALT IN RESPONSE TO A TCASII RA.

Narrative: I WAS THE PF. WE WERE LEVEL AT 4000 FT ON RADAR VECTORS FOR THE LOC 10 APCH AT BWI. ATC GAVE US A TURN AND SAID TO MAKE IT TIGHT DUE TO MDT TFC IN FRONT OF US AT 4000 FT. I MANUALLY BANKED THE ACFT INTO A 30 DEG BANK AND BEGAN THE TURN WHEN WE GOT A YELLOW TA AT 12 O'CLOCK AND 5 MI AT 4000 FT. WE WENT TO 5 MI RANGE ON TCASII AND SAW WE WERE RAPIDLY APCHING TARGET AT SAME ALT. CAPT WAS LOOKING FOR TFC AS I FLEW THE ACFT. WE THEN GOT AN RA TO DSND. I ASKED THE CAPT IF HE HAD THE ACFT AND HE SAID NO AND SAID TO COMPLY, WHICH I IMMEDIATELY DID. AT APPROX 3500 FT, THE RA WAS TERMINATED. I ESTIMATE WE GOT TO WITHIN 2 MI BASED ON THE TCASII DISPLAY. WE NEVER SAW THE TFC. I BELIEVE THERE WAS A REAL POTENTIAL FOR A COLLISION IF WE HADN'T TAKEN EVASIVE ACTION. WHEN WE TOLD THE CTLR WE HAD DEPARTED OUR ASSIGNED ALT FOR LOWER TO COMPLY WITH AN RA, HE IMMEDIATELY CLRED US TO 2500 FT. NO FURTHER MENTION WAS MADE OF THE RA OR ALTDEV.

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.