Narrative:

Air carrier X on radar vectors for a visual approach to runway 33. Following the controller's instructions, we were on a southerly heading at 4000 ft. The controller then issued a clearance to turn to a southwest heading and descend to 2500, to which the copilot responded immediately to by utilizing an autoplt descent. As we were going through 3700, the controller then transmitted 'on second thought, maintain 4000, traffic at (to the best of my memory) 11 O'clock, 3 mi, at 3500'. No traffic was showing on the TCASII which was operating in TA/RA mode and 10 mi scale. I responded to her, 'roger, we'll climb back to 4000' as the copilot disconnected the autoplt and began a manual climb back to 4000 ft. At that moment, I noticed pop-up traffic on the TCASII, slightly left of the nose, close in (perhaps 2 mi) and showing a '-1' readout. Just then, the TCASII issued the following commands: 'traffic, traffic', followed immediately by 'descend, descend'. The vertical speed lights showed the need for a descent in excess of 2000 FPM. I immediately took the aircraft from the copilot, pulled back the power and pushed the nose over. Within approximately 2 seconds of the descend command, the TCASII then commanded 'climb, climb' and displayed a required climb rate in excess of 2000 FPM. I immediately reversed direction by pushing up the power and pulling back on the wheel. About that time I caught sight of an small aircraft Y passing slightly to our left and approximately 200-300 ft below us. In my opinion, if I had continued to follow the first TCASII command rather than immediately changing direction with the second command, we would have hit the traffic. We then entered the VFR pattern and landed uneventfully at bwi. Discussing this event after landing with the bwi supervisor via telephone, the supervisor told me that the controller admitted that it had been completely her error, that she had 'forgotten about' the VFR traffic when she issued our descent clearance from 4000. Supplemental information from acn 223101. Air carrier X was issued a climb to maintain 4000 ft. Small aircraft Y was issued a descent from 3500 ft to 2500 ft. Air carrier X reported a TCASII alert.

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Original NASA ASRS Text

Title: ACR X HAD NMAC. VFR SMA Y IN TCA. ACR X TCASII RA WRONG INSTRUCTIONS DSND FOLLOWED BY CLB. EVASIVE ACTION TAKEN.

Narrative: ACR X ON RADAR VECTORS FOR A VISUAL APCH TO RWY 33. FOLLOWING THE CTLR'S INSTRUCTIONS, WE WERE ON A SOUTHERLY HDG AT 4000 FT. THE CTLR THEN ISSUED A CLRNC TO TURN TO A SW HDG AND DSND TO 2500, TO WHICH THE COPLT RESPONDED IMMEDIATELY TO BY UTILIZING AN AUTOPLT DSCNT. AS WE WERE GOING THROUGH 3700, THE CTLR THEN XMITTED 'ON SECOND THOUGHT, MAINTAIN 4000, TFC AT (TO THE BEST OF MY MEMORY) 11 O'CLOCK, 3 MI, AT 3500'. NO TFC WAS SHOWING ON THE TCASII WHICH WAS OPERATING IN TA/RA MODE AND 10 MI SCALE. I RESPONDED TO HER, 'ROGER, WE'LL CLB BACK TO 4000' AS THE COPLT DISCONNECTED THE AUTOPLT AND BEGAN A MANUAL CLB BACK TO 4000 FT. AT THAT MOMENT, I NOTICED POP-UP TFC ON THE TCASII, SLIGHTLY L OF THE NOSE, CLOSE IN (PERHAPS 2 MI) AND SHOWING A '-1' READOUT. JUST THEN, THE TCASII ISSUED THE FOLLOWING COMMANDS: 'TFC, TFC', FOLLOWED IMMEDIATELY BY 'DSND, DSND'. THE VERT SPD LIGHTS SHOWED THE NEED FOR A DSCNT IN EXCESS OF 2000 FPM. I IMMEDIATELY TOOK THE ACFT FROM THE COPLT, PULLED BACK THE PWR AND PUSHED THE NOSE OVER. WITHIN APPROX 2 SECONDS OF THE DSND COMMAND, THE TCASII THEN COMMANDED 'CLB, CLB' AND DISPLAYED A REQUIRED CLB RATE IN EXCESS OF 2000 FPM. I IMMEDIATELY REVERSED DIRECTION BY PUSHING UP THE PWR AND PULLING BACK ON THE WHEEL. ABOUT THAT TIME I CAUGHT SIGHT OF AN SMA Y PASSING SLIGHTLY TO OUR L AND APPROX 200-300 FT BELOW US. IN MY OPINION, IF I HAD CONTINUED TO FOLLOW THE FIRST TCASII COMMAND RATHER THAN IMMEDIATELY CHANGING DIRECTION WITH THE SECOND COMMAND, WE WOULD HAVE HIT THE TFC. WE THEN ENTERED THE VFR PATTERN AND LANDED UNEVENTFULLY AT BWI. DISCUSSING THIS EVENT AFTER LNDG WITH THE BWI SUPVR VIA TELEPHONE, THE SUPVR TOLD ME THAT THE CTLR ADMITTED THAT IT HAD BEEN COMPLETELY HER ERROR, THAT SHE HAD 'FORGOTTEN ABOUT' THE VFR TFC WHEN SHE ISSUED OUR DSCNT CLRNC FROM 4000. SUPPLEMENTAL INFO FROM ACN 223101. ACR X WAS ISSUED A CLB TO MAINTAIN 4000 FT. SMA Y WAS ISSUED A DSCNT FROM 3500 FT TO 2500 FT. ACR X RPTED A TCASII ALERT.

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.