A B757 flight crew received a TCASII TA.

Attributes
ACN	436530
Time
Date	199904
Day	Thu
Local Time Of Day	1801 To 2400
Place
Locale Reference	navaid : sea.vortac
State Reference	WA
Altitude	msl single value : 4000
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Controlling Facilities	tracon : s46.tracon
Operator	common carrier : air carrier
Make Model Name	B757 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Navigation In Use	other
Flight Phase	descent : approach
Route In Use	arrival : on vectors
Flight Plan	IFR
Aircraft 2
Controlling Facilities	tracon : s46.tracon
Operator	common carrier : air carrier
Make Model Name	B737 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Navigation In Use	other
Flight Phase	descent : approach
Route In Use	arrival : on vectors
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 100 flight time total : 12000 flight time type : 300
ASRS Report	436530
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : instrument pilot : commercial pilot : multi engine
Events
Anomaly	conflict : airborne less severe
Independent Detector	aircraft equipment : tcas
Resolutory Action	flight crew : took precautionary avoidance action
Miss Distance	horizontal : 5000 vertical : 1200
Supplementary
Problem Areas	FAA Flight Crew Human Performance
Primary Problem	Ambiguous

Narrative:

Descending on downwind seatac to 4000 ft as instructed. Traffic observed 10 O'clock at 4000 ft MSL (TCASII and visually). At 5000 ft on descent, TA illuminated. Captain applied power, stopped descent, and climbed to 5200 ft ! B737 aircraft instructed to make left turn, we to make a right turn to base. TCASII closed to 1 mi.

Original NASA ASRS Text

Title: A B757 FLC RECEIVED A TCASII TA.

Narrative: DSNDING ON DOWNWIND SEATAC TO 4000 FT AS INSTRUCTED. TFC OBSERVED 10 O'CLOCK AT 4000 FT MSL (TCASII AND VISUALLY). AT 5000 FT ON DSCNT, TA ILLUMINATED. CAPT APPLIED PWR, STOPPED DSCNT, AND CLBED TO 5200 FT ! B737 ACFT INSTRUCTED TO MAKE L TURN, WE TO MAKE A R TURN TO BASE. TCASII CLOSED TO 1 MI.

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.