An IMC air carrier aft at 7000 ft near sigbe intersection on the iad hyper two arrival had a near midair collision with an IMC VFR aircraft at 7300 ft. The crew responded to an RA.

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Attributes
ACN	794588
Time
Date	200807
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : iad.airport
State Reference	VA
Altitude	msl single value : 7300
Environment
Flight Conditions	IMC
Aircraft 1
Controlling Facilities	tracon : pct.tracon
Operator	common carrier : air carrier
Make Model Name	B777-200
Operating Under FAR Part	Part 121
Flight Phase	descent : approach
Route In Use	arrival star : hyper
Flight Plan	IFR
Aircraft 2
Controlling Facilities	tracon : pct.tracon
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Flight Phase	climbout : takeoff
Flight Plan	VFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Experience	flight time last 90 days : 180 flight time total : 11000 flight time type : 5500
ASRS Report	794588
Person 2
Affiliation	company : air carrier
Function	flight crew : relief pilot
Experience	flight time last 90 days : 120 flight time total : 12000 flight time type : 3000
ASRS Report	794592
Events
Anomaly	altitude deviation : excursion from assigned altitude conflict : airborne critical inflight encounter : weather inflight encounter : vfr in imc non adherence : far
Independent Detector	other flight crewa other flight crewb
Resolutory Action	flight crew : took evasive action
Supplementary
Problem Areas	Flight Crew Human Performance Weather ATC Human Performance
Primary Problem	Flight Crew Human Performance

Narrative:

An air carrier aircraft was cleared for hyper 2 arrival and descent to 7000 ft. Approximately 3 NM north from sigbe ATC announced traffic 12 O'clock position; 3 mi 6700 ft VFR. Traffic popped up on TCAS at less than 3 mi climbing. Received yellow caution then RA 'descend; descend now' as VFR aircraft climbed to 7200 ft MSL in our direct flight path. My aircraft was IMC at 7300 ft in descent; less than 1 NM; and followed TCAS descent and deployed speed brakes to assist. We descended to 6700 ft when target passed overhead at 7200 ft. We were IMC during entire maneuver and never acquired VFR aircraft. ATC was advised of unable to maintain 7000 ft due to RA. ATC immediately cleared further descent to 4000 ft. We entered VMC conditions descending through 6300 ft. TCAS can be credited with saving a hull loss and 254 lives over frederick; md; this day. It needs to be determined why this aircraft was operating at VFR altitudes; sort of; in IMC conditions in the dulles (iad) arrival pattern.

Original NASA ASRS Text

Title: AN IMC ACR AFT AT 7000 FT NEAR SIGBE INTXN ON THE IAD HYPER TWO ARR HAD A NMAC WITH AN IMC VFR ACFT AT 7300 FT. THE CREW RESPONDED TO AN RA.

Narrative: AN ACR ACFT WAS CLRED FOR HYPER 2 ARR AND DSCNT TO 7000 FT. APPROX 3 NM N FROM SIGBE ATC ANNOUNCED TFC 12 O'CLOCK POS; 3 MI 6700 FT VFR. TFC POPPED UP ON TCAS AT LESS THAN 3 MI CLBING. RECEIVED YELLOW CAUTION THEN RA 'DSND; DSND NOW' AS VFR ACFT CLBED TO 7200 FT MSL IN OUR DIRECT FLT PATH. MY ACFT WAS IMC AT 7300 FT IN DSCNT; LESS THAN 1 NM; AND FOLLOWED TCAS DSCNT AND DEPLOYED SPD BRAKES TO ASSIST. WE DSNDED TO 6700 FT WHEN TARGET PASSED OVERHEAD AT 7200 FT. WE WERE IMC DURING ENTIRE MANEUVER AND NEVER ACQUIRED VFR ACFT. ATC WAS ADVISED OF UNABLE TO MAINTAIN 7000 FT DUE TO RA. ATC IMMEDIATELY CLRED FURTHER DSCNT TO 4000 FT. WE ENTERED VMC CONDITIONS DSNDING THROUGH 6300 FT. TCAS CAN BE CREDITED WITH SAVING A HULL LOSS AND 254 LIVES OVER FREDERICK; MD; THIS DAY. IT NEEDS TO BE DETERMINED WHY THIS ACFT WAS OPERATING AT VFR ALTS; SORT OF; IN IMC CONDITIONS IN THE DULLES (IAD) ARR PATTERN.

Data retrieved from NASA's ASRS site as of May 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.