Narrative:

Our flight air carrier X at FL260 en route okk VOR, 110 mi south on 170 degree radial. We observed a TCASII advisory of traffic at 12 O'clock, 1000 ft above at about 15 mi on an opposing heading. Shortly after we observed target on TCASII display, descend from 1000 ft above to 500 ft above. TCASII commanded a descent of at least 2000 FPM to avoid traffic. We queried ZID of traffic. They told us we had an air carrier Y jet 1000 ft below us on a converging heading. At about the same time we visually acquired air carrier Y about 500 to 1000 ft below our altitude. Center also confirmed he was assigned FL250. We observed no traffic above nor did center have any traffic above us. Our TCASII continued to command a descent and continued to show our traffic conflict 500 ft above us. Center advised us that air carrier Y's mode C did momentarily show '26500' and then returned to 250 on their scope. We had altered course slightly to the right to offset conflict but did not follow TCASII RA. If we had followed TCASII 'RA' we, in my opinion, would have impacted the opposing aircraft. Callback conversation with reporter revealed the following information: that the reporter had some ambivalent feelings about TCASII, and feels that there is a long way to go for system maturity. Supplemental information from acn 210600: we were being step climbed to our cruise altitude and presently climbing to level at 25000 ft. It was the captain's leg so we were operating on #1 ATC transponder #1 altitude reporting (mode C). ZID called traffic at 12 O'clock about 12 mi at 26000 ft. I, first officer, spotted traffic about 7-8 mi out. Then we heard the air carrier X jet report a TCASII advisory saying the guy was showing 500 ft above them. Our aircraft was not TCASII equipped. I continued to observe our traffic as he passed over us and in a slight r-hand bank. He appeared closer than the standard 1000 ft separation. ATC then told us our mode C showed 26500. The captain and I both verified 29.92 and 25000 on our altimeters. The whole situation became very clear. Air carrier X got a TCASII RA showing us at 26500 ft telling him to descend from 26000 -- while we were actually at 25000 with a faulty mode C which pilots have no way of monitoring. (It's interesting though, that for being step climbed for 25000 ft, not a single ATC controller informed us or noticed our incorrect altitude reporting mode C.) we informed ATC and switched to our #2 transponder and #2 altitude reporting (mode C). Then ATC advised us our mode C showed the correct altitude of 25000 ft. Later with ZTL we asked him to verify both #1 and #2 mode C's -- he said #1 showed 1500 to 1600 ft high, but #2 was right on our assigned altitude. Note that this problem had been written up in the maintenance logbook twice in the past week. This could have been a potentially fatal situation had it not been visual conditions.

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Original NASA ASRS Text

Title: FAULTY MODE C ON ACR Y LEADS TO ERRONEOUS TCASII RA FOR ACR X.

Narrative: OUR FLT ACR X AT FL260 ENRTE OKK VOR, 110 MI S ON 170 DEG RADIAL. WE OBSERVED A TCASII ADVISORY OF TFC AT 12 O'CLOCK, 1000 FT ABOVE AT ABOUT 15 MI ON AN OPPOSING HDG. SHORTLY AFTER WE OBSERVED TARGET ON TCASII DISPLAY, DSND FROM 1000 FT ABOVE TO 500 FT ABOVE. TCASII COMMANDED A DSCNT OF AT LEAST 2000 FPM TO AVOID TFC. WE QUERIED ZID OF TFC. THEY TOLD US WE HAD AN ACR Y JET 1000 FT BELOW US ON A CONVERGING HDG. AT ABOUT THE SAME TIME WE VISUALLY ACQUIRED ACR Y ABOUT 500 TO 1000 FT BELOW OUR ALT. CTR ALSO CONFIRMED HE WAS ASSIGNED FL250. WE OBSERVED NO TFC ABOVE NOR DID CTR HAVE ANY TFC ABOVE US. OUR TCASII CONTINUED TO COMMAND A DSCNT AND CONTINUED TO SHOW OUR TFC CONFLICT 500 FT ABOVE US. CTR ADVISED US THAT ACR Y'S MODE C DID MOMENTARILY SHOW '26500' AND THEN RETURNED TO 250 ON THEIR SCOPE. WE HAD ALTERED COURSE SLIGHTLY TO THE R TO OFFSET CONFLICT BUT DID NOT FOLLOW TCASII RA. IF WE HAD FOLLOWED TCASII 'RA' WE, IN MY OPINION, WOULD HAVE IMPACTED THE OPPOSING ACFT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THAT THE RPTR HAD SOME AMBIVALENT FEELINGS ABOUT TCASII, AND FEELS THAT THERE IS A LONG WAY TO GO FOR SYS MATURITY. SUPPLEMENTAL INFO FROM ACN 210600: WE WERE BEING STEP CLBED TO OUR CRUISE ALT AND PRESENTLY CLBING TO LEVEL AT 25000 FT. IT WAS THE CAPT'S LEG SO WE WERE OPERATING ON #1 ATC TRANSPONDER #1 ALT RPTING (MODE C). ZID CALLED TFC AT 12 O'CLOCK ABOUT 12 MI AT 26000 FT. I, FO, SPOTTED TFC ABOUT 7-8 MI OUT. THEN WE HEARD THE ACR X JET RPT A TCASII ADVISORY SAYING THE GUY WAS SHOWING 500 FT ABOVE THEM. OUR ACFT WAS NOT TCASII EQUIPPED. I CONTINUED TO OBSERVE OUR TFC AS HE PASSED OVER US AND IN A SLIGHT R-HAND BANK. HE APPEARED CLOSER THAN THE STANDARD 1000 FT SEPARATION. ATC THEN TOLD US OUR MODE C SHOWED 26500. THE CAPT AND I BOTH VERIFIED 29.92 AND 25000 ON OUR ALTIMETERS. THE WHOLE SITUATION BECAME VERY CLR. ACR X GOT A TCASII RA SHOWING US AT 26500 FT TELLING HIM TO DSND FROM 26000 -- WHILE WE WERE ACTUALLY AT 25000 WITH A FAULTY MODE C WHICH PLTS HAVE NO WAY OF MONITORING. (IT'S INTERESTING THOUGH, THAT FOR BEING STEP CLBED FOR 25000 FT, NOT A SINGLE ATC CTLR INFORMED US OR NOTICED OUR INCORRECT ALT RPTING MODE C.) WE INFORMED ATC AND SWITCHED TO OUR #2 TRANSPONDER AND #2 ALT RPTING (MODE C). THEN ATC ADVISED US OUR MODE C SHOWED THE CORRECT ALT OF 25000 FT. LATER WITH ZTL WE ASKED HIM TO VERIFY BOTH #1 AND #2 MODE C'S -- HE SAID #1 SHOWED 1500 TO 1600 FT HIGH, BUT #2 WAS RIGHT ON OUR ASSIGNED ALT. NOTE THAT THIS PROBLEM HAD BEEN WRITTEN UP IN THE MAINT LOGBOOK TWICE IN THE PAST WK. THIS COULD HAVE BEEN A POTENTIALLY FATAL SITUATION HAD IT NOT BEEN VISUAL CONDITIONS.

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.