Narrative:

At 10000 ft MSL and 250 KTS, 3 NM west of pivot intersection on v- 84 to pappi intersection (PMM2 arrival to ord) we had a TCASII RA 'climb, climb, climb.' a climb was immediately initiated as TCASII VFR traffic was noted on TCASII at 9500 ft MSL and climbing slowly on a southeast heading. The VFR traffic was at 9800 ft MLS and we were at 10350 ft MSL when we received a 'monitor vertical speed' from TCASII and clear of conflict annunciation as traffic passed underneath us but never in visual contact. We descended smoothly back to 10000 ft MSL and then to 8000 ft MSL as directed by ord approach control for a normal landing at ord. Even though the VFR traffic was legal to be 9500 ft MSL sebound in that area, was he/she wise to be transiting a main terminal arrival route with highspd jet traffic at 10000 ft MSL down to 7000 ft MSL? Perhaps an FAA advisory safety letter should be published reference this problem.

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Original NASA ASRS Text

Title: ALTDEV ALT EXCURSION AFTER RECEIVING TCASII RA.

Narrative: AT 10000 FT MSL AND 250 KTS, 3 NM W OF PIVOT INTXN ON V- 84 TO PAPPI INTXN (PMM2 ARR TO ORD) WE HAD A TCASII RA 'CLB, CLB, CLB.' A CLB WAS IMMEDIATELY INITIATED AS TCASII VFR TFC WAS NOTED ON TCASII AT 9500 FT MSL AND CLBING SLOWLY ON A SE HDG. THE VFR TFC WAS AT 9800 FT MLS AND WE WERE AT 10350 FT MSL WHEN WE RECEIVED A 'MONITOR VERT SPD' FROM TCASII AND CLR OF CONFLICT ANNUNCIATION AS TFC PASSED UNDERNEATH US BUT NEVER IN VISUAL CONTACT. WE DSNDED SMOOTHLY BACK TO 10000 FT MSL AND THEN TO 8000 FT MSL AS DIRECTED BY ORD APCH CTL FOR A NORMAL LNDG AT ORD. EVEN THOUGH THE VFR TFC WAS LEGAL TO BE 9500 FT MSL SEBOUND IN THAT AREA, WAS HE/SHE WISE TO BE TRANSITING A MAIN TERMINAL ARR RTE WITH HIGHSPD JET TFC AT 10000 FT MSL DOWN TO 7000 FT MSL? PERHAPS AN FAA ADVISORY SAFETY LETTER SHOULD BE PUBLISHED REF THIS PROB.

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.