Narrative:

We were climbing out of pittsburgh, cleared to 14000. The controller modified our clearance altitude to 8000 due to traffic as we were passing approximately 4000. We soon noticed our traffic on TCASII and then spotted him visually at 1 O'clock high. We confirmed that this was our traffic with departure. As we passed 7000 I realized that TCAS reported the relative altitude with the traffic as 1000 ft versus the 2000 that we expected. I asked departure to verify the target's altitude. Departure verified that our traffic was at 9000 and I stated that we showed him at 8000. We immediately slowed our climb rate for the traffic but also since we were now 1000 ft from our clearance altitude. As we passed 7400 ft, the TCASII gave us a traffic warning. We had the traffic visually but it was very difficult to correctly identify the vertical separation. At my query, the controller stated that the traffic says he is at 9000. Meanwhile the TCASII warning changed to an RA telling us to 'monitor climb' and advising us to climb at no greater than 600 FPM. Our climb rate was approximately 100 FPM at that time, and the TCASII relative altitude indicated approximately 500 ft difference. We were getting different conflicting information from the controller and the TCASII but due to the situation we were able to continue complying with our ATC clearance while obeying the TCASII commands. As the traffic passed behind us the TCAS announced 'clear of conflict' and the relative altitude changed immediately to '15' now indicating that the traffic was at 9000. We continued our climb to 8000 and proceeded uneventfully to our destination. I wrote up the 'TCAS malfunction' in the maintenance logbook and filed a TCASII advisory message with my company. We have no choice but to obey the TCASII commands unless we can positively confirm visually that there is no conflict. Even though the controller was verifying that the traffic was in fact at 9000, we couldn't be sure visually and elected to comply with TCASII's commands especially since we could do so without deviating from our ATC clearance. In hindsight, I think that it might have been more prudent to request a level off at 7000 to keep from even getting into a potential conflict in case for some reason the initial information that TCASII was presenting was correct and the other aircraft was in fact at 8000. Callback conversation with reporter revealed the following information: reporter stated that this was his first experience with any kind of traffic situation using TCASII and since this report has come to regard it quite highly. Reporter has not followed up with the air carrier as to the possible cause of the false readout but will endeavor to find out what he can, as a report was sent to company.

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Original NASA ASRS Text

Title: ACR MLG SLOWS ITS RATE OF CLB IN RESPONSE TO AN APPARENTLY ERRONEOUS SIGNAL, A POSSIBLE FALSE WARNING, ON TCASII IN MLG RECEIVING AN INAPPROPRIATE ALT ON THE MODE S INTERROGATION OF OPPOSITE DIRECTION TFC.

Narrative: WE WERE CLBING OUT OF PITTSBURGH, CLRED TO 14000. THE CTLR MODIFIED OUR CLRNC ALT TO 8000 DUE TO TFC AS WE WERE PASSING APPROX 4000. WE SOON NOTICED OUR TFC ON TCASII AND THEN SPOTTED HIM VISUALLY AT 1 O'CLOCK HIGH. WE CONFIRMED THAT THIS WAS OUR TFC WITH DEP. AS WE PASSED 7000 I REALIZED THAT TCAS RPTED THE RELATIVE ALT WITH THE TFC AS 1000 FT VERSUS THE 2000 THAT WE EXPECTED. I ASKED DEP TO VERIFY THE TARGET'S ALT. DEP VERIFIED THAT OUR TFC WAS AT 9000 AND I STATED THAT WE SHOWED HIM AT 8000. WE IMMEDIATELY SLOWED OUR CLB RATE FOR THE TFC BUT ALSO SINCE WE WERE NOW 1000 FT FROM OUR CLRNC ALT. AS WE PASSED 7400 FT, THE TCASII GAVE US A TFC WARNING. WE HAD THE TFC VISUALLY BUT IT WAS VERY DIFFICULT TO CORRECTLY IDENT THE VERT SEPARATION. AT MY QUERY, THE CTLR STATED THAT THE TFC SAYS HE IS AT 9000. MEANWHILE THE TCASII WARNING CHANGED TO AN RA TELLING US TO 'MONITOR CLB' AND ADVISING US TO CLB AT NO GREATER THAN 600 FPM. OUR CLB RATE WAS APPROX 100 FPM AT THAT TIME, AND THE TCASII RELATIVE ALT INDICATED APPROX 500 FT DIFFERENCE. WE WERE GETTING DIFFERENT CONFLICTING INFO FROM THE CTLR AND THE TCASII BUT DUE TO THE SITUATION WE WERE ABLE TO CONTINUE COMPLYING WITH OUR ATC CLRNC WHILE OBEYING THE TCASII COMMANDS. AS THE TFC PASSED BEHIND US THE TCAS ANNOUNCED 'CLR OF CONFLICT' AND THE RELATIVE ALT CHANGED IMMEDIATELY TO '15' NOW INDICATING THAT THE TFC WAS AT 9000. WE CONTINUED OUR CLB TO 8000 AND PROCEEDED UNEVENTFULLY TO OUR DEST. I WROTE UP THE 'TCAS MALFUNCTION' IN THE MAINT LOGBOOK AND FILED A TCASII ADVISORY MESSAGE WITH MY COMPANY. WE HAVE NO CHOICE BUT TO OBEY THE TCASII COMMANDS UNLESS WE CAN POSITIVELY CONFIRM VISUALLY THAT THERE IS NO CONFLICT. EVEN THOUGH THE CTLR WAS VERIFYING THAT THE TFC WAS IN FACT AT 9000, WE COULDN'T BE SURE VISUALLY AND ELECTED TO COMPLY WITH TCASII'S COMMANDS ESPECIALLY SINCE WE COULD DO SO WITHOUT DEVIATING FROM OUR ATC CLRNC. IN HINDSIGHT, I THINK THAT IT MIGHT HAVE BEEN MORE PRUDENT TO REQUEST A LEVEL OFF AT 7000 TO KEEP FROM EVEN GETTING INTO A POTENTIAL CONFLICT IN CASE FOR SOME REASON THE INITIAL INFO THAT TCASII WAS PRESENTING WAS CORRECT AND THE OTHER ACFT WAS IN FACT AT 8000. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THAT THIS WAS HIS FIRST EXPERIENCE WITH ANY KIND OF TFC SITUATION USING TCASII AND SINCE THIS RPT HAS COME TO REGARD IT QUITE HIGHLY. RPTR HAS NOT FOLLOWED UP WITH THE ACR AS TO THE POSSIBLE CAUSE OF THE FALSE READOUT BUT WILL ENDEAVOR TO FIND OUT WHAT HE CAN, AS A RPT WAS SENT TO COMPANY.

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.