Narrative:

While eastbound at night (VMC) at FL330 over ubon VOR on A1, we showed double image traffic on our TCASII of FL350 and FL329, but because they were so close, I figured the lower traffic at FL309 since the 2 aircraft were at the same position on our TCASII screen. The FL350 traffic showed landing lights which we acknowledged and at that time could not see the lower traffic. About that time we received a TA and the lower traffic showed his landing lights which we acknowledged and asked bangkok control of his altitude. At the same time we received an RA 'climb, climb.' since we could not detect relative motion, we climbed to FL336 until the lower traffic was clear and we received a 'clear of traffic' then descended back to FL330. Bangkok control asked air carrier X (lower traffic) of his altitude and was advised FL310 (we were all outside radar coverage). When air carrier X was in radar range, bkk also showed him at FL329 at which time air carrier X advised that leaving hong kong their transponders were in error. When we arrived in hong kong airspace, I questioned them about air carrier X and they advised he had a bad transponder and they had relayed the message to bangkok. Both hong kong and ourselves agreed that in the future such offending aircraft should turn off the altitude reporting function of their transponder. With the increased numbers of TCASII equipped aircraft, I believe that it should be made ATC policy to require any aircraft with unreliable altitude readouts to turn off their altitude reporting function of their transponder. If it is not done, you could end up with confused ATC readouts and also other TCASII equipped aircraft.

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Original NASA ASRS Text

Title: FAULTY MODE C CAUSES TCASII TO WARN OF AN RA.

Narrative: WHILE EBOUND AT NIGHT (VMC) AT FL330 OVER UBON VOR ON A1, WE SHOWED DOUBLE IMAGE TFC ON OUR TCASII OF FL350 AND FL329, BUT BECAUSE THEY WERE SO CLOSE, I FIGURED THE LOWER TFC AT FL309 SINCE THE 2 ACFT WERE AT THE SAME POS ON OUR TCASII SCREEN. THE FL350 TFC SHOWED LNDG LIGHTS WHICH WE ACKNOWLEDGED AND AT THAT TIME COULD NOT SEE THE LOWER TFC. ABOUT THAT TIME WE RECEIVED A TA AND THE LOWER TFC SHOWED HIS LNDG LIGHTS WHICH WE ACKNOWLEDGED AND ASKED BANGKOK CTL OF HIS ALT. AT THE SAME TIME WE RECEIVED AN RA 'CLB, CLB.' SINCE WE COULD NOT DETECT RELATIVE MOTION, WE CLBED TO FL336 UNTIL THE LOWER TFC WAS CLR AND WE RECEIVED A 'CLR OF TFC' THEN DSNDED BACK TO FL330. BANGKOK CTL ASKED ACR X (LOWER TFC) OF HIS ALT AND WAS ADVISED FL310 (WE WERE ALL OUTSIDE RADAR COVERAGE). WHEN ACR X WAS IN RADAR RANGE, BKK ALSO SHOWED HIM AT FL329 AT WHICH TIME ACR X ADVISED THAT LEAVING HONG KONG THEIR TRANSPONDERS WERE IN ERROR. WHEN WE ARRIVED IN HONG KONG AIRSPACE, I QUESTIONED THEM ABOUT ACR X AND THEY ADVISED HE HAD A BAD TRANSPONDER AND THEY HAD RELAYED THE MESSAGE TO BANGKOK. BOTH HONG KONG AND OURSELVES AGREED THAT IN THE FUTURE SUCH OFFENDING ACFT SHOULD TURN OFF THE ALT RPTING FUNCTION OF THEIR TRANSPONDER. WITH THE INCREASED NUMBERS OF TCASII EQUIPPED ACFT, I BELIEVE THAT IT SHOULD BE MADE ATC POLICY TO REQUIRE ANY ACFT WITH UNRELIABLE ALT READOUTS TO TURN OFF THEIR ALT RPTING FUNCTION OF THEIR TRANSPONDER. IF IT IS NOT DONE, YOU COULD END UP WITH CONFUSED ATC READOUTS AND ALSO OTHER TCASII EQUIPPED ACFT.

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.