Narrative:

Air carrier X was under radar vectors by fay approach control for ASR 10 approach. The ILS 4 was unusable due to thunderstorms on the final approach course. We were on a southerly vector when all of a sudden the TCASII gave us an alert of traffic at 12 O'clock at 100 ft above us. Right after the alert went off, the controller called the traffic out to us. We were just below a cloud layer and therefore were unable to visually see the traffic. We advised the controller of this and just before he gave us a vector, the TCASII gave us a descent command. When we advised him of the TCASII mandated descent, he gave us a heading of 270. Our descent totaled to about 600 ft and after the conflict was over, he climbed us back up to 4000 ft. Afterwards, I talked with the radar supervisor and he told me that they had tracked the target for a while, which gave a constant altitude readout of 4100 ft and called the controling facility whose airspace he entered, to track him and if he ever talked to them to give fay a call by phone. The next facility, by the way, also got an altitude readout of 4100 ft. When the pilot called fay approach and was told what happened, he told the controller that he was at 9500 ft and level. Being told that 3 independent system tracked him at 4100 ft, he insisted he was at 9500 ft. It is like the controller told me, it's the pilot's word against ours and if he was indeed at 9500 ft he did not violate any airspace. I have heard over the yrs of altitude encoders being off by a lot, but 5400 ft is extreme. But this was a case where I was glad that TCASII has been developed. Supplemental information from acn 221495: I am an air traffic controller at fay ATCT, nc. While working radar conducting OJT, I being the instructor. I issued traffic to air carrier X on an unidented VFR small aircraft whose altitude indicated 4100 ft. Air carrier X was at 4000 on a converging course. Traffic issued was: traffic 12 O'clock, 3 mi, opposite direction, altitude indicates 4100 ft. Pilot response was 'roger we're descending.' my response was 'air carrier X, turn right heading 270 additional traffic 12 O'clock 3 mi small transport at 3000.' pilot response was: 'roger, turn right heading 270.' my response was, 'previously issued traffic is now 12 O'clock less than a mi altitude indicates 4000.' pilot response was, 'we're heading 270 and traffic not in sight.' at this time approximately 20 seconds after the initial traffic call, air carrier and the VFR tag crossed and air carrier X was climbed back to 4000, and given a heading of 220 upon reaching 4000. Prior to that last transmission the small transport who was IFR at 3000 heading 220 was turned left to 180. When the situation was over I asked air carrier X if he descended because of a TCASII alert. Pilot response was affirmative. I then asked if air carrier X ever saw the aircraft. Pilot response was 'negative and, if the aircraft was at 4000 he had to be in IMC conditions.' air carrier X and the unidented small aircraft passed less than a mi and 400 ft apart. Air carrier X and the small transport never lost separation.

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Original NASA ASRS Text

Title: VFR SMA ACFT EQUIP PROB ERRONEOUS MODE C ALT CAUSED ACR X TCASII RA DSND NON ADHERENCE TO ATC CLRNC.

Narrative: ACR X WAS UNDER RADAR VECTORS BY FAY APCH CTL FOR ASR 10 APCH. THE ILS 4 WAS UNUSABLE DUE TO TSTMS ON THE FINAL APCH COURSE. WE WERE ON A SOUTHERLY VECTOR WHEN ALL OF A SUDDEN THE TCASII GAVE US AN ALERT OF TFC AT 12 O'CLOCK AT 100 FT ABOVE US. RIGHT AFTER THE ALERT WENT OFF, THE CTLR CALLED THE TFC OUT TO US. WE WERE JUST BELOW A CLOUD LAYER AND THEREFORE WERE UNABLE TO VISUALLY SEE THE TFC. WE ADVISED THE CTLR OF THIS AND JUST BEFORE HE GAVE US A VECTOR, THE TCASII GAVE US A DSCNT COMMAND. WHEN WE ADVISED HIM OF THE TCASII MANDATED DSCNT, HE GAVE US A HDG OF 270. OUR DSCNT TOTALED TO ABOUT 600 FT AND AFTER THE CONFLICT WAS OVER, HE CLBED US BACK UP TO 4000 FT. AFTERWARDS, I TALKED WITH THE RADAR SUPVR AND HE TOLD ME THAT THEY HAD TRACKED THE TARGET FOR A WHILE, WHICH GAVE A CONSTANT ALT READOUT OF 4100 FT AND CALLED THE CTLING FACILITY WHOSE AIRSPACE HE ENTERED, TO TRACK HIM AND IF HE EVER TALKED TO THEM TO GIVE FAY A CALL BY PHONE. THE NEXT FACILITY, BY THE WAY, ALSO GOT AN ALT READOUT OF 4100 FT. WHEN THE PLT CALLED FAY APCH AND WAS TOLD WHAT HAPPENED, HE TOLD THE CTLR THAT HE WAS AT 9500 FT AND LEVEL. BEING TOLD THAT 3 INDEPENDENT SYS TRACKED HIM AT 4100 FT, HE INSISTED HE WAS AT 9500 FT. IT IS LIKE THE CTLR TOLD ME, IT'S THE PLT'S WORD AGAINST OURS AND IF HE WAS INDEED AT 9500 FT HE DID NOT VIOLATE ANY AIRSPACE. I HAVE HEARD OVER THE YRS OF ALT ENCODERS BEING OFF BY A LOT, BUT 5400 FT IS EXTREME. BUT THIS WAS A CASE WHERE I WAS GLAD THAT TCASII HAS BEEN DEVELOPED. SUPPLEMENTAL INFO FROM ACN 221495: I AM AN AIR TFC CTLR AT FAY ATCT, NC. WHILE WORKING RADAR CONDUCTING OJT, I BEING THE INSTRUCTOR. I ISSUED TFC TO ACR X ON AN UNIDENTED VFR SMA WHOSE ALT INDICATED 4100 FT. ACR X WAS AT 4000 ON A CONVERGING COURSE. TFC ISSUED WAS: TFC 12 O'CLOCK, 3 MI, OPPOSITE DIRECTION, ALT INDICATES 4100 FT. PLT RESPONSE WAS 'ROGER WE'RE DSNDING.' MY RESPONSE WAS 'ACR X, TURN R HDG 270 ADDITIONAL TFC 12 O'CLOCK 3 MI SMT AT 3000.' PLT RESPONSE WAS: 'ROGER, TURN R HDG 270.' MY RESPONSE WAS, 'PREVIOUSLY ISSUED TFC IS NOW 12 O'CLOCK LESS THAN A MI ALT INDICATES 4000.' PLT RESPONSE WAS, 'WE'RE HDG 270 AND TFC NOT IN SIGHT.' AT THIS TIME APPROX 20 SECONDS AFTER THE INITIAL TFC CALL, ACR AND THE VFR TAG CROSSED AND ACR X WAS CLBED BACK TO 4000, AND GIVEN A HDG OF 220 UPON REACHING 4000. PRIOR TO THAT LAST XMISSION THE SMT WHO WAS IFR AT 3000 HDG 220 WAS TURNED L TO 180. WHEN THE SIT WAS OVER I ASKED ACR X IF HE DSNDED BECAUSE OF A TCASII ALERT. PLT RESPONSE WAS AFFIRMATIVE. I THEN ASKED IF ACR X EVER SAW THE ACFT. PLT RESPONSE WAS 'NEGATIVE AND, IF THE ACFT WAS AT 4000 HE HAD TO BE IN IMC CONDITIONS.' ACR X AND THE UNIDENTED SMA PASSED LESS THAN A MI AND 400 FT APART. ACR X AND THE SMT NEVER LOST SEPARATION.

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.