MD80 in IMC responded to a TCAS RA and climbed to avoid traffic. Traffic was operating under VFR.

Attributes
ACN	748076
Time
Date	200708
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : phx.airport
State Reference	NM
Altitude	msl single value : 15600
Environment
Flight Conditions	IMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zab.artcc
Operator	common carrier : air carrier
Make Model Name	MD-80 Series (DC-9-80) Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	descent : vacating altitude
Route In Use	arrival star : eagul
Flight Plan	IFR
Aircraft 2
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Flight Phase	descent : approach
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
ASRS Report	748076
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp
ASRS Report	748077
Events
Anomaly	conflict : airborne critical
Independent Detector	aircraft equipment : tcas other flight crewa other flight crewb
Resolutory Action	controller : issued advisory flight crew : took evasive action
Supplementary
Problem Areas	Weather Flight Crew Human Performance
Primary Problem	Flight Crew Human Performance

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.

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.