Narrative:

Location approximately 37 NM west of dayton airport while descending to 10000 ft, heading roughly ene towards the dayton airport, and in radar and radio contact being vectored by day approach control, in very hazy conditions. We received a sudden TCASII RA. The first officer, PF, immediately punched off the autoplt and smoothly followed the TCASII RA commands to climb. We climbed up to about 11100 ft, and then got a clear of conflict -- returning recommencing our descent to 10000 ft. I tried to spot the offending aircraft, but never got a visual on him, and he was apparently a VFR light aircraft flying south westerly at 10000 ft level at an IFR altitude. He never changed his course nor altitude, nor was in radio contact, but apparently had a good TCASII capable transponder. His closest point of approach was 500 ft below us according to the TCASII display. As he was flying at an IFR altitude, an attempt to find this pilot should be made for corrective action. Also, the approach controller gave us no warning of this aircraft that should have been clearly displayed on his scope. This was a genuine TCASII save, and I am glad that we have them. I would like to know the outcome of the search for this aircraft and pilot that does not follow the appropriate rules for VFR flight cruising altitude. This pilot presents a serious hazard to other airspace users.

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Original NASA ASRS Text

Title: F100 FLT CREW HAS TCASII RA DURING INITIAL DSCNT INTO DAY.

Narrative: LOCATION APPROX 37 NM W OF DAYTON ARPT WHILE DSNDING TO 10000 FT, HEADING ROUGHLY ENE TOWARDS THE DAYTON ARPT, AND IN RADAR AND RADIO CONTACT BEING VECTORED BY DAY APCH CTL, IN VERY HAZY CONDITIONS. WE RECEIVED A SUDDEN TCASII RA. THE FO, PF, IMMEDIATELY PUNCHED OFF THE AUTOPLT AND SMOOTHLY FOLLOWED THE TCASII RA COMMANDS TO CLB. WE CLBED UP TO ABOUT 11100 FT, AND THEN GOT A CLR OF CONFLICT -- RETURNING RECOMMENCING OUR DSCNT TO 10000 FT. I TRIED TO SPOT THE OFFENDING ACFT, BUT NEVER GOT A VISUAL ON HIM, AND HE WAS APPARENTLY A VFR LIGHT ACFT FLYING S WESTERLY AT 10000 FT LEVEL AT AN IFR ALT. HE NEVER CHANGED HIS COURSE NOR ALT, NOR WAS IN RADIO CONTACT, BUT APPARENTLY HAD A GOOD TCASII CAPABLE XPONDER. HIS CLOSEST POINT OF APCH WAS 500 FT BELOW US ACCORDING TO THE TCASII DISPLAY. AS HE WAS FLYING AT AN IFR ALT, AN ATTEMPT TO FIND THIS PLT SHOULD BE MADE FOR CORRECTIVE ACTION. ALSO, THE APCH CTLR GAVE US NO WARNING OF THIS ACFT THAT SHOULD HAVE BEEN CLRLY DISPLAYED ON HIS SCOPE. THIS WAS A GENUINE TCASII SAVE, AND I AM GLAD THAT WE HAVE THEM. I WOULD LIKE TO KNOW THE OUTCOME OF THE SEARCH FOR THIS ACFT AND PLT THAT DOES NOT FOLLOW THE APPROPRIATE RULES FOR VFR FLT CRUISING ALT. THIS PLT PRESENTS A SERIOUS HAZARD TO OTHER AIRSPACE USERS.

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.