Descending light transport has near midair collision wioth small aircraft VFR in IMC.

Attributes
ACN	103499
Time
Date	198902
Day	Fri
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : sea
State Reference	WA
Altitude	msl bound lower : 10500 msl bound upper : 10500
Environment
Flight Conditions	IMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zse
Operator	common carrier : air carrier
Make Model Name	Light Transport, Low Wing, 2 Turboprop Eng
Flight Phase	descent other
Flight Plan	IFR
Aircraft 2
Make Model Name	Small Aircraft
Flight Phase	climbout : intermediate altitude
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 240 flight time total : 18000 flight time type : 240
ASRS Report	103499
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : instrument
Events
Anomaly	conflict : nmac inflight encounter : weather inflight encounter : vfr in imc non adherence : far
Independent Detector	other flight crewa
Resolutory Action	flight crew : took evasive action
Consequence	Other
Miss Distance	horizontal : 100 vertical : 0
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

Under control of ZSE and having been cleared to seattle tocoma airport by the finney three arrival we were descending through 10500' to maintain 10000'. At 40 DME south of the seattle VORTAC on the seattle VOR 158 degree radial, the first officer called an alert, took the controls and started a roll to the right. I then saw the aircraft which was southbound on a collision course. It passed to our left at an estimated 100' and at our altitude. This all took place in about 5 seconds. WX at the time was an obscured condition caused by ice crystals. The ground was not visible. The other aircraft was not in contact with ZSE and I believe he had got into a WX condition that was not expected and was trying to climb on top.

Original NASA ASRS Text

Title: DESCENDING LTT HAS NMAC WIOTH SMA VFR IN IMC.

Narrative: UNDER CTL OF ZSE AND HAVING BEEN CLRED TO SEATTLE TOCOMA ARPT BY THE FINNEY THREE ARR WE WERE DSNDING THROUGH 10500' TO MAINTAIN 10000'. AT 40 DME S OF THE SEATTLE VORTAC ON THE SEATTLE VOR 158 DEG RADIAL, THE F/O CALLED AN ALERT, TOOK THE CONTROLS AND STARTED A ROLL TO THE RIGHT. I THEN SAW THE ACFT WHICH WAS SBND ON A COLLISION COURSE. IT PASSED TO OUR L AT AN ESTIMATED 100' AND AT OUR ALT. THIS ALL TOOK PLACE IN ABOUT 5 SECONDS. WX AT THE TIME WAS AN OBSCURED CONDITION CAUSED BY ICE CRYSTALS. THE GND WAS NOT VISIBLE. THE OTHER ACFT WAS NOT IN CONTACT WITH ZSE AND I BELIEVE HE HAD GOT INTO A WX CONDITION THAT WAS NOT EXPECTED AND WAS TRYING TO CLB ON TOP.

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