Narrative:

We were south of stapleton on an assigned heading 080 degree at an assigned altitude of 11000 ft. Denver approach issued us VFR traffic 12 O'clock 11500 ft. I believe denver said the range was 6 mi. TCASII displayed a target in that position at 11500 and closing. A moment later displayed (TCASII) altitude for the target indicated 11400 ft, then 11300 ft. With the 11300 ft indication appeared a down arrow. The down arrow meant to me the target was now descending at least 500 FPM. I estimate range then at about 2-3 NM. With the down arrow TCASII issued a descend RA. The copilot, responding to the TCASII RA and my commands, initiated a descent. I notified denver that the traffic was leaving his altitude and descending into us. TCASII then issued a 'monitor vertical speed' RA. TCASII now showed the traffic climbing again. The lowest altitude I saw for the traffic was 11100 ft. Denver replied 'the traffic is VFR at 11500 ft'. I informed denver that TCASII showed the traffic had left his altitude and descended into us. I told denver we had left 11000 ft in response and asked if he wanted us to return to 11000. Denver replied, 'no stay at 10000 ft now'. The entire event took place very quickly. From my perspective the VFR traffic made a very rapid altitude excursion from 11500 to at least 11100 ft and back. This was a target verified by denver radar. In my mind it was a very real target. A target that TCASII said was closing rapidly and descending into my altitude. I had no time to second guess. For me to have ignored a TCASII RA on a target that had been verified by denver radar would have been grossly irresponsible and stupid. In this instance I believe TCASII prevented a midair collision. That I am now the subject of a pilot deviation report by denver ATC distresses me.

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Original NASA ASRS Text

Title: ACR MLG ALT DEV EXCURSION FROM CLRNC ALT IN RESPONSE TO TCASII RA.

Narrative: WE WERE S OF STAPLETON ON AN ASSIGNED HDG 080 DEG AT AN ASSIGNED ALT OF 11000 FT. DENVER APCH ISSUED US VFR TFC 12 O'CLOCK 11500 FT. I BELIEVE DENVER SAID THE RANGE WAS 6 MI. TCASII DISPLAYED A TARGET IN THAT POS AT 11500 AND CLOSING. A MOMENT LATER DISPLAYED (TCASII) ALT FOR THE TARGET INDICATED 11400 FT, THEN 11300 FT. WITH THE 11300 FT INDICATION APPEARED A DOWN ARROW. THE DOWN ARROW MEANT TO ME THE TARGET WAS NOW DSNDING AT LEAST 500 FPM. I ESTIMATE RANGE THEN AT ABOUT 2-3 NM. WITH THE DOWN ARROW TCASII ISSUED A DSND RA. THE COPLT, RESPONDING TO THE TCASII RA AND MY COMMANDS, INITIATED A DSCNT. I NOTIFIED DENVER THAT THE TFC WAS LEAVING HIS ALT AND DSNDING INTO US. TCASII THEN ISSUED A 'MONITOR VERT SPD' RA. TCASII NOW SHOWED THE TFC CLBING AGAIN. THE LOWEST ALT I SAW FOR THE TFC WAS 11100 FT. DENVER REPLIED 'THE TFC IS VFR AT 11500 FT'. I INFORMED DENVER THAT TCASII SHOWED THE TFC HAD LEFT HIS ALT AND DSNDED INTO US. I TOLD DENVER WE HAD LEFT 11000 FT IN RESPONSE AND ASKED IF HE WANTED US TO RETURN TO 11000. DENVER REPLIED, 'NO STAY AT 10000 FT NOW'. THE ENTIRE EVENT TOOK PLACE VERY QUICKLY. FROM MY PERSPECTIVE THE VFR TFC MADE A VERY RAPID ALT EXCURSION FROM 11500 TO AT LEAST 11100 FT AND BACK. THIS WAS A TARGET VERIFIED BY DENVER RADAR. IN MY MIND IT WAS A VERY REAL TARGET. A TARGET THAT TCASII SAID WAS CLOSING RAPIDLY AND DSNDING INTO MY ALT. I HAD NO TIME TO SECOND GUESS. FOR ME TO HAVE IGNORED A TCASII RA ON A TARGET THAT HAD BEEN VERIFIED BY DENVER RADAR WOULD HAVE BEEN GROSSLY IRRESPONSIBLE AND STUPID. IN THIS INSTANCE I BELIEVE TCASII PREVENTED A MIDAIR COLLISION. THAT I AM NOW THE SUBJECT OF A PLTDEV RPT BY DENVER ATC DISTRESSES ME.

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.