Narrative:

At 4000 ft on downwind for the approach into knoxville, we were sitting on the top of the overcast that was reported to us as 3000 ft overcast, 7 mi visibility. There was only one other aircraft in the area, a piper tomahawk. We were told he was at 2500 ft and had canceled IFR and was not talking to anyone. Our TCASII showed him on the nose about 10 mi from us. We were then cleared to descend to 3000 ft. During the descent, we got a TCASII advisory (TA) on the tomahawk. I was concentrating on the TCASII display as the captain continued the descent since we believed the piper to be at 2500 ft. The TA turned into an RA and the captain disengaged the autoplt and began a left hand climbing turn (to heading 330 degrees and back up to 4000 ft). Our TCASII showed minus 400 ft separation on the piper. There is no reason why the controller should have had us on a conflicting flight path since there were only 2 of us out there -- one above the deck and one below. In addition, the tomahawk was not at 2500 ft. We were at about 3500 ft when our TCASII was showing minus 400 separation during our recovery. If we had not initiated our recovery based on TCASII alone, we would almost certainly have hit the tomahawk. I believe it saved our lives and those of our passenger. The entire conflict occurred in IMC and we never saw the other aircraft.

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Original NASA ASRS Text

Title: NMAC.

Narrative: AT 4000 FT ON DOWNWIND FOR THE APCH INTO KNOXVILLE, WE WERE SITTING ON THE TOP OF THE OVCST THAT WAS RPTED TO US AS 3000 FT OVCST, 7 MI VISIBILITY. THERE WAS ONLY ONE OTHER ACFT IN THE AREA, A PIPER TOMAHAWK. WE WERE TOLD HE WAS AT 2500 FT AND HAD CANCELED IFR AND WAS NOT TALKING TO ANYONE. OUR TCASII SHOWED HIM ON THE NOSE ABOUT 10 MI FROM US. WE WERE THEN CLRED TO DSND TO 3000 FT. DURING THE DSCNT, WE GOT A TCASII ADVISORY (TA) ON THE TOMAHAWK. I WAS CONCENTRATING ON THE TCASII DISPLAY AS THE CAPT CONTINUED THE DSCNT SINCE WE BELIEVED THE PIPER TO BE AT 2500 FT. THE TA TURNED INTO AN RA AND THE CAPT DISENGAGED THE AUTOPLT AND BEGAN A L HAND CLBING TURN (TO HDG 330 DEGS AND BACK UP TO 4000 FT). OUR TCASII SHOWED MINUS 400 FT SEPARATION ON THE PIPER. THERE IS NO REASON WHY THE CTLR SHOULD HAVE HAD US ON A CONFLICTING FLT PATH SINCE THERE WERE ONLY 2 OF US OUT THERE -- ONE ABOVE THE DECK AND ONE BELOW. IN ADDITION, THE TOMAHAWK WAS NOT AT 2500 FT. WE WERE AT ABOUT 3500 FT WHEN OUR TCASII WAS SHOWING MINUS 400 SEPARATION DURING OUR RECOVERY. IF WE HAD NOT INITIATED OUR RECOVERY BASED ON TCASII ALONE, WE WOULD ALMOST CERTAINLY HAVE HIT THE TOMAHAWK. I BELIEVE IT SAVED OUR LIVES AND THOSE OF OUR PAX. THE ENTIRE CONFLICT OCCURRED IN IMC AND WE NEVER SAW THE OTHER 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.