Narrative:

Per company bulletin (due to problems with RA mode), we were operating TCASII in TA mode, traffic southwest automatic. Thunderstorms south and east of dtw required change of plans from ILS 3R to 3L. We were air carrier X level at 11000 having just passed over dtw wnw bound. TCASII called 'traffic' and we observed target at 1-2 O'clock, converging, 400 ft above us descending. No resolution advisory (turned off) or visual contact. Resisting urge to begin descent I asked ATC 'what about 12 O'clock traffic for us?' ATC responded with clearance to descend to 7000. While first officer made aggressive descent (spdbrakes, hardover)(TCASII showed traffic inside 2 mi ring converging at plus 200 ft descending) ATC cleared the other aircraft Y to climb to 12000 immediately and turn left. We did see traffic at 1 O'clock high but could not judge proximity. After clearing, ATC asked us to expedite descent. After conflict was resolved, ATC thanked us for our help and I replied 'TCASII sure came in handy tonight.' I feel the traffic level and WX may have put the controller just beyond his capability. He sounded like he was working very hard but very competent. By his reaction though I think our situation caught him by surprise. Lack of TCASII resolution advisory caused us much initial confusion (for a couple seconds) about what to do. Fortunately, the frequency was available for a timely query and clearance or emergency authority would have been used to take evasive action (deviate from assigned altitude) based solely on a blip and data block on the ivsi. Supplemental information from acn 187470. We had been on vectors from dtw approach control. Flight was level at 11000 ft and on a northwest heading. TCASII was installed on the aircraft and functioning in the 'TA' mode due to a 'flight information file distributed to crew members. It reads as follows: 'it has been discovered that certain transponders installed on these airplane types may randomly provide incorrect altitude resolution data to the TCASII computer. Until further notice, TCASII should be operated in the TA mode on these airplane types.' the crew received a traffic advisory from TCASII (both voice and pictorially) that traffic was about 1 O'clock and at the 2 mi ring, plus 400 ft and descending. I wanted to immediately descend to avoid conflict. The captain said to hold off, and luckily was able to get through to dtw approach to ask 'what about the traffic at 12 O'clock.' the controller gave us a descent to 7000 ft and a turn to about 250 degree heading, followed by 'expedite descent'. I immediately went to flight idle, spdbrakes deployed, and initiated a descending turn. Target then showed on TCASII screen 'plus 100 descending' and I looked out the window and saw a set of navigation lights go over us. The captain later informed me that the aircraft had been given an expedite climb instruction from the controller, so it appeared to be greater than 100 ft when I saw the lights. We were given a handoff, the captain said 'TCASII come in real handy' and the controller said thanks a lot for the help. Were then handed off to approach on 124.07, I believe. I think a large contributing factor to this was that the WX was very bad in the south area of detroit. Numerous thunderstorms along the approach paths may have caused a bunching of traffic. The one thing I think I would do differently given the same situation, is to operate the TCASII in 'on' rather than in 'automatic' mode. We may have detected the conflict sooner as proximate traffic rather than as conflict traffic.

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Original NASA ASRS Text

Title: ACR X HAS LTSS FROM ACFT Y. SYS ERROR. TCASII TA EVASIVE ACTION TAKEN. ACFT EQUIP PROBLEM TCASII RA INOPERATIVE.

Narrative: PER COMPANY BULLETIN (DUE TO PROBLEMS WITH RA MODE), WE WERE OPERATING TCASII IN TA MODE, TFC SW AUTO. TSTMS S AND E OF DTW REQUIRED CHANGE OF PLANS FROM ILS 3R TO 3L. WE WERE ACR X LEVEL AT 11000 HAVING JUST PASSED OVER DTW WNW BOUND. TCASII CALLED 'TFC' AND WE OBSERVED TARGET AT 1-2 O'CLOCK, CONVERGING, 400 FT ABOVE US DSNDING. NO RESOLUTION ADVISORY (TURNED OFF) OR VISUAL CONTACT. RESISTING URGE TO BEGIN DSCNT I ASKED ATC 'WHAT ABOUT 12 O'CLOCK TFC FOR US?' ATC RESPONDED WITH CLRNC TO DSND TO 7000. WHILE FO MADE AGGRESSIVE DSCNT (SPDBRAKES, HARDOVER)(TCASII SHOWED TFC INSIDE 2 MI RING CONVERGING AT PLUS 200 FT DSNDING) ATC CLRED THE OTHER ACFT Y TO CLB TO 12000 IMMEDIATELY AND TURN L. WE DID SEE TFC AT 1 O'CLOCK HIGH BUT COULD NOT JUDGE PROX. AFTER CLRING, ATC ASKED US TO EXPEDITE DSCNT. AFTER CONFLICT WAS RESOLVED, ATC THANKED US FOR OUR HELP AND I REPLIED 'TCASII SURE CAME IN HANDY TONIGHT.' I FEEL THE TFC LEVEL AND WX MAY HAVE PUT THE CTLR JUST BEYOND HIS CAPABILITY. HE SOUNDED LIKE HE WAS WORKING VERY HARD BUT VERY COMPETENT. BY HIS REACTION THOUGH I THINK OUR SITUATION CAUGHT HIM BY SURPRISE. LACK OF TCASII RESOLUTION ADVISORY CAUSED US MUCH INITIAL CONFUSION (FOR A COUPLE SECONDS) ABOUT WHAT TO DO. FORTUNATELY, THE FREQ WAS AVAILABLE FOR A TIMELY QUERY AND CLRNC OR EMER AUTHORITY WOULD HAVE BEEN USED TO TAKE EVASIVE ACTION (DEVIATE FROM ASSIGNED ALT) BASED SOLELY ON A BLIP AND DATA BLOCK ON THE IVSI. SUPPLEMENTAL INFO FROM ACN 187470. WE HAD BEEN ON VECTORS FROM DTW APCH CTL. FLT WAS LEVEL AT 11000 FT AND ON A NW HDG. TCASII WAS INSTALLED ON THE ACFT AND FUNCTIONING IN THE 'TA' MODE DUE TO A 'FLT INFO FILE DISTRIBUTED TO CREW MEMBERS. IT READS AS FOLLOWS: 'IT HAS BEEN DISCOVERED THAT CERTAIN TRANSPONDERS INSTALLED ON THESE AIRPLANE TYPES MAY RANDOMLY PROVIDE INCORRECT ALT RESOLUTION DATA TO THE TCASII COMPUTER. UNTIL FURTHER NOTICE, TCASII SHOULD BE OPERATED IN THE TA MODE ON THESE AIRPLANE TYPES.' THE CREW RECEIVED A TFC ADVISORY FROM TCASII (BOTH VOICE AND PICTORIALLY) THAT TFC WAS ABOUT 1 O'CLOCK AND AT THE 2 MI RING, PLUS 400 FT AND DSNDING. I WANTED TO IMMEDIATELY DSND TO AVOID CONFLICT. THE CAPT SAID TO HOLD OFF, AND LUCKILY WAS ABLE TO GET THROUGH TO DTW APCH TO ASK 'WHAT ABOUT THE TFC AT 12 O'CLOCK.' THE CTLR GAVE US A DSCNT TO 7000 FT AND A TURN TO ABOUT 250 DEG HDG, FOLLOWED BY 'EXPEDITE DSCNT'. I IMMEDIATELY WENT TO FLT IDLE, SPDBRAKES DEPLOYED, AND INITIATED A DSNDING TURN. TARGET THEN SHOWED ON TCASII SCREEN 'PLUS 100 DSNDING' AND I LOOKED OUT THE WINDOW AND SAW A SET OF NAV LIGHTS GO OVER US. THE CAPT LATER INFORMED ME THAT THE ACFT HAD BEEN GIVEN AN EXPEDITE CLB INSTRUCTION FROM THE CTLR, SO IT APPEARED TO BE GREATER THAN 100 FT WHEN I SAW THE LIGHTS. WE WERE GIVEN A HDOF, THE CAPT SAID 'TCASII COME IN REAL HANDY' AND THE CTLR SAID THANKS A LOT FOR THE HELP. WERE THEN HANDED OFF TO APCH ON 124.07, I BELIEVE. I THINK A LARGE CONTRIBUTING FACTOR TO THIS WAS THAT THE WX WAS VERY BAD IN THE S AREA OF DETROIT. NUMEROUS TSTMS ALONG THE APCH PATHS MAY HAVE CAUSED A BUNCHING OF TFC. THE ONE THING I THINK I WOULD DO DIFFERENTLY GIVEN THE SAME SITUATION, IS TO OPERATE THE TCASII IN 'ON' RATHER THAN IN 'AUTO' MODE. WE MAY HAVE DETECTED THE CONFLICT SOONER AS PROXIMATE TFC RATHER THAN AS CONFLICT TFC.

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.