Narrative:

VFR aircraft did not maintain adequate separation from a descending IFR aircraft on a published arrival in IMC conditions. Descending on the eagul one RNAV arrival approximately 12-15 mi prior to homrr intersection; center called VFR traffic to us twice while we were in IMC conditions. Both times we replied that we were IMC. After our second reply; center requested we level off. As I was leveling the aircraft; we received an RA warning and I initiated a climb. After 4-8 seconds; we received a clear of conflict notification and I resumed my descent. We were IMC the entire time and I have trouble understanding how the other aircraft was maintaining VFR conditions. Passenger and flight attendants were unaware of the event. Once we received the 'clear of conflict' message I reestablished our descent to 12000 ft and reported the RA to ATC. I can't say why the other aircraft was flying VFR in marginal WX conditions on a busy arrival route to a major airport. I feel ATC tried to provide vertical separation as soon as they became aware that a possible conflict was present. We were IMC until well after we reestablished our descent on the arrival. TCAS probably saved the day.

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Original NASA ASRS Text

Title: MD80 IN IMC RESPONDED TO A TCAS RA AND CLIMBED TO AVOID TFC. TFC WAS OPERATING UNDER VFR.

Narrative: VFR ACFT DID NOT MAINTAIN ADEQUATE SEPARATION FROM A DSNDING IFR ACFT ON A PUBLISHED ARR IN IMC CONDITIONS. DSNDING ON THE EAGUL ONE RNAV ARR APPROX 12-15 MI PRIOR TO HOMRR INTXN; CTR CALLED VFR TFC TO US TWICE WHILE WE WERE IN IMC CONDITIONS. BOTH TIMES WE REPLIED THAT WE WERE IMC. AFTER OUR SECOND REPLY; CTR REQUESTED WE LEVEL OFF. AS I WAS LEVELING THE ACFT; WE RECEIVED AN RA WARNING AND I INITIATED A CLB. AFTER 4-8 SECONDS; WE RECEIVED A CLR OF CONFLICT NOTIFICATION AND I RESUMED MY DSCNT. WE WERE IMC THE ENTIRE TIME AND I HAVE TROUBLE UNDERSTANDING HOW THE OTHER ACFT WAS MAINTAINING VFR CONDITIONS. PAX AND FLT ATTENDANTS WERE UNAWARE OF THE EVENT. ONCE WE RECEIVED THE 'CLR OF CONFLICT' MESSAGE I REESTABLISHED OUR DSCNT TO 12000 FT AND REPORTED THE RA TO ATC. I CAN'T SAY WHY THE OTHER ACFT WAS FLYING VFR IN MARGINAL WX CONDITIONS ON A BUSY ARR RTE TO A MAJOR ARPT. I FEEL ATC TRIED TO PROVIDE VERT SEPARATION AS SOON AS THEY BECAME AWARE THAT A POSSIBLE CONFLICT WAS PRESENT. WE WERE IMC UNTIL WELL AFTER WE REESTABLISHED OUR DSCNT ON THE ARR. TCAS PROBABLY SAVED THE DAY.

Data retrieved from NASA's ASRS site as of January 2009 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.