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.