Narrative:

Flight cleared to FL190 by dca center. Descending out of 19500', TCAS ii TA was heard, followed shortly after by a climb command (RA by TCAS). Aircraft was then flown as dictated by TCAS ii which required a climb of about 2000 FPM. At about 20500' conflict was resolved and descent to FL190 was resumed. ATC was notified and indicated they were not aware of any conflict. They did speculate that an aircraft approximately 3000' below us may have triggered the alarm, due to his high rate of climb. There are some bugs with TCAS ii, but it works well. The aircraft below us was below FL180, and the local altimeter setting was 29.14. We were above FL180 and altimeter setting 29.92. He had a high rate of climb to 17000'. This probably triggered the conflict.

Google
 

Original NASA ASRS Text

Title: TCAS II RA ON DESCENT AT 19500' MANDATED A CLIMB. RAS WAS RESOLVED AT 20500'. FLT RETURNED TO ASSIGNED 19000'.

Narrative: FLT CLRED TO FL190 BY DCA CTR. DSNDING OUT OF 19500', TCAS II TA WAS HEARD, FOLLOWED SHORTLY AFTER BY A CLB COMMAND (RA BY TCAS). ACFT WAS THEN FLOWN AS DICTATED BY TCAS II WHICH REQUIRED A CLB OF ABOUT 2000 FPM. AT ABOUT 20500' CONFLICT WAS RESOLVED AND DSNT TO FL190 WAS RESUMED. ATC WAS NOTIFIED AND INDICATED THEY WERE NOT AWARE OF ANY CONFLICT. THEY DID SPECULATE THAT AN ACFT APPROX 3000' BELOW US MAY HAVE TRIGGERED THE ALARM, DUE TO HIS HIGH RATE OF CLB. THERE ARE SOME BUGS WITH TCAS II, BUT IT WORKS WELL. THE ACFT BELOW US WAS BELOW FL180, AND THE LCL ALTIMETER SETTING WAS 29.14. WE WERE ABOVE FL180 AND ALTIMETER SETTING 29.92. HE HAD A HIGH RATE OF CLB TO 17000'. THIS PROBABLY TRIGGERED THE CONFLICT.

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.