Narrative:

En route from atl-tlh at FL210 we saw a contact at 12 O'clock closing 1000 ft above and apparently level. As the contact came within 5 mi we suddenly saw it drop to plus 800 ft and go from white to yellow to red on the TCASII with an RA to descend. The red arc extended to nearly 3000 FPM down. We had no TA prior and no time to query ATC. As we left FL210 using vertical speed mode on the autoplt we advised ATC, who responded that they believed the other aircraft to be level at FL220. We got to FL204 before we got a 'clear of conflict.' at this point we immediately started back up to FL210. Due to our speed increase on the descent and the inability of the unpowered elevator to keep up, we overshot to about FL212-FL213 before the autoplt was able to catch up. I took the autoplt off and descended to FL210. We were advised by ATC that a possible conflict had indicated, but the descent indication they had was in error by about 300 ft. I'm not sure whether it was a transponder glitch or an old TCASII that caused the RA, but ATC showed the other aircraft level. An RA of this magnitude is difficult with flight attendants up with a cart in the aisle, especially without a TA to alert and, to make matters worse, probably false. Callback conversation with reporter revealed the following information: the reporting first officer believes that the TCASII was faulty in this incident. ATC insisted that the other aircraft was level, but there is the possibility that it descended in its turn or that its transponder was faulty. The FAA threatened an investigation, but has since sent a letter to the first officer stating that they are dropping all investigation. Supplemental information from acn 236622: it is my view that either the RA was erroneous or the target aircraft descended as he turned toward us from roughly our 4 O'clock.

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Original NASA ASRS Text

Title: AN ACR MLG DSNDED IN RESPONSE TO A TCASII COMMAND.

Narrative: ENRTE FROM ATL-TLH AT FL210 WE SAW A CONTACT AT 12 O'CLOCK CLOSING 1000 FT ABOVE AND APPARENTLY LEVEL. AS THE CONTACT CAME WITHIN 5 MI WE SUDDENLY SAW IT DROP TO PLUS 800 FT AND GO FROM WHITE TO YELLOW TO RED ON THE TCASII WITH AN RA TO DSND. THE RED ARC EXTENDED TO NEARLY 3000 FPM DOWN. WE HAD NO TA PRIOR AND NO TIME TO QUERY ATC. AS WE LEFT FL210 USING VERT SPD MODE ON THE AUTOPLT WE ADVISED ATC, WHO RESPONDED THAT THEY BELIEVED THE OTHER ACFT TO BE LEVEL AT FL220. WE GOT TO FL204 BEFORE WE GOT A 'CLR OF CONFLICT.' AT THIS POINT WE IMMEDIATELY STARTED BACK UP TO FL210. DUE TO OUR SPD INCREASE ON THE DSCNT AND THE INABILITY OF THE UNPOWERED ELEVATOR TO KEEP UP, WE OVERSHOT TO ABOUT FL212-FL213 BEFORE THE AUTOPLT WAS ABLE TO CATCH UP. I TOOK THE AUTOPLT OFF AND DSNDED TO FL210. WE WERE ADVISED BY ATC THAT A POSSIBLE CONFLICT HAD INDICATED, BUT THE DSCNT INDICATION THEY HAD WAS IN ERROR BY ABOUT 300 FT. I'M NOT SURE WHETHER IT WAS A XPONDER GLITCH OR AN OLD TCASII THAT CAUSED THE RA, BUT ATC SHOWED THE OTHER ACFT LEVEL. AN RA OF THIS MAGNITUDE IS DIFFICULT WITH FLT ATTENDANTS UP WITH A CART IN THE AISLE, ESPECIALLY WITHOUT A TA TO ALERT AND, TO MAKE MATTERS WORSE, PROBABLY FALSE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTING FO BELIEVES THAT THE TCASII WAS FAULTY IN THIS INCIDENT. ATC INSISTED THAT THE OTHER ACFT WAS LEVEL, BUT THERE IS THE POSSIBILITY THAT IT DSNDED IN ITS TURN OR THAT ITS XPONDER WAS FAULTY. THE FAA THREATENED AN INVESTIGATION, BUT HAS SINCE SENT A LETTER TO THE FO STATING THAT THEY ARE DROPPING ALL INVESTIGATION. SUPPLEMENTAL INFO FROM ACN 236622: IT IS MY VIEW THAT EITHER THE RA WAS ERRONEOUS OR THE TARGET ACFT DSNDED AS HE TURNED TOWARD US FROM ROUGHLY OUR 4 O'CLOCK.

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.