Narrative:

While in level flight at 16000 ft, ZLA advised us of small transport type traffic 12 O'clock and 10 mi operating VFR at 15500 ft. We immediately established visual contact with the traffic but our onboard TCASII indicated his altitude as 16400 ft rather than 15500 ft. Due to only 500 ft separation and 10 mi range, we were unsure of the traffic's actual altitude and whether it would pass us above or below. We then asked center to verify the traffic's altitude and informed center of our TCASII received altitude of the traffic. Due to frequency congestion, a reply was delayed from the small transport. We immediately received a 'descend' TCASII RA but fortunately were able to visually confirm that the traffic would pass below us. Unbeknownst to us, the small transport's transponder was intermittently transmitting false altitude data during most of his flight. It was fortunate that the small transport was in radio contact with center and the in-flight visibility was high otherwise there would have been a collision while complying with our erroneous TCASII advisory. To prevent similar incidents, center and approach controllers should immediately have suspect transponder aircraft to 'stop altitude squawk' and not be allowed to reactivate it for trouble-shooting purposes with TCASII equipped aircraft operations in the same area.

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Original NASA ASRS Text

Title: A FAULTY TCASII RA, IF FOLLOWED, WOULD HAVE CAUSED AN LTT TO DSND TOWARD A COLLISION RATHER THAN MAINTAIN ALT FOR PROPER SEPARATION.

Narrative: WHILE IN LEVEL FLT AT 16000 FT, ZLA ADVISED US OF SMT TYPE TFC 12 O'CLOCK AND 10 MI OPERATING VFR AT 15500 FT. WE IMMEDIATELY ESTABLISHED VISUAL CONTACT WITH THE TFC BUT OUR ONBOARD TCASII INDICATED HIS ALT AS 16400 FT RATHER THAN 15500 FT. DUE TO ONLY 500 FT SEPARATION AND 10 MI RANGE, WE WERE UNSURE OF THE TFC'S ACTUAL ALT AND WHETHER IT WOULD PASS US ABOVE OR BELOW. WE THEN ASKED CTR TO VERIFY THE TFC'S ALT AND INFORMED CTR OF OUR TCASII RECEIVED ALT OF THE TFC. DUE TO FREQ CONGESTION, A REPLY WAS DELAYED FROM THE SMT. WE IMMEDIATELY RECEIVED A 'DSND' TCASII RA BUT FORTUNATELY WERE ABLE TO VISUALLY CONFIRM THAT THE TFC WOULD PASS BELOW US. UNBEKNOWNST TO US, THE SMT'S XPONDER WAS INTERMITTENTLY XMITTING FALSE ALT DATA DURING MOST OF HIS FLT. IT WAS FORTUNATE THAT THE SMT WAS IN RADIO CONTACT WITH CTR AND THE INFLT VISIBILITY WAS HIGH OTHERWISE THERE WOULD HAVE BEEN A COLLISION WHILE COMPLYING WITH OUR ERRONEOUS TCASII ADVISORY. TO PREVENT SIMILAR INCIDENTS, CTR AND APCH CTLRS SHOULD IMMEDIATELY HAVE SUSPECT XPONDER ACFT TO 'STOP ALT SQUAWK' AND NOT BE ALLOWED TO REACTIVATE IT FOR TROUBLE-SHOOTING PURPOSES WITH TCASII EQUIPPED ACFT OPS IN THE SAME AREA.

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.