Aircraft under control of autoplt and FMS. Controller advised off course and gave two correcting vectors. Flight crew assumed they were now on a vector approach and did not follow arrival procedure at aubrn to turn north 340 degrees.

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Attributes
ACN	153355
Time
Date	199008
Day	Mon
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : sea
State Reference	WA
Altitude	msl bound lower : 15000 msl bound upper : 15000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : sea
Operator	common carrier : air carrier
Make Model Name	Widebody, Low Wing, 3 Turbojet Eng
Flight Phase	descent : approach
Route In Use	arrival other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : commercial pilot : flight engineer pilot : instrument pilot : atp
Experience	flight time last 90 days : 100 flight time total : 15000 flight time type : 1000
ASRS Report	153355
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp
Experience	flight time last 90 days : 90 flight time total : 15000 flight time type : 3000
ASRS Report	153372
Events
Anomaly	other spatial deviation
Independent Detector	other controllera
Resolutory Action	controller : issued new clearance
Consequence	Other
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

First officer was flying the chins one arrival into seatac using the FMS for primary navigation. The FMS display was selected and showed us to be on course. Both the B autoplt with FMS selected in the navigation mode and autothrottles were selected and engaged. Approximately 40 NM +/- southeast of sea VOR, approach control told us that his radar showed us to be +/- 3 mi north of course. Approach control gave us a left turn vector to return to course. A check of the sea VOR right showed that even though FMS showed centered on course it was in error. Approach control did not request that we intercept the arrival and the xed through the arrival right (sea 101 degree). Approach control then gave us a right turn vector and we believed that we were then not on the published arrival. In fact the first officer remarked that we must be in an approach control vector for a west downwind for a south landing at seatac. Therefore, we did not make the published turn to 340 degree. Subsequent comments from approach suggested that this was not uncommon indicating problems with this arrival.

Original NASA ASRS Text

Title: ACFT UNDER CTL OF AUTOPLT AND FMS. CTLR ADVISED OFF COURSE AND GAVE TWO CORRECTING VECTORS. FLT CREW ASSUMED THEY WERE NOW ON A VECTOR APCH AND DID NOT FOLLOW ARR PROC AT AUBRN TO TURN NORTH 340 DEGREES.

Narrative: F/O WAS FLYING THE CHINS ONE ARR INTO SEATAC USING THE FMS FOR PRIMARY NAV. THE FMS DISPLAY WAS SELECTED AND SHOWED US TO BE ON COURSE. BOTH THE B AUTOPLT WITH FMS SELECTED IN THE NAV MODE AND AUTOTHROTTLES WERE SELECTED AND ENGAGED. APPROX 40 NM +/- SE OF SEA VOR, APCH CTL TOLD US THAT HIS RADAR SHOWED US TO BE +/- 3 MI N OF COURSE. APCH CTL GAVE US A L TURN VECTOR TO RETURN TO COURSE. A CHK OF THE SEA VOR R SHOWED THAT EVEN THOUGH FMS SHOWED CTRED ON COURSE IT WAS IN ERROR. APCH CTL DID NOT REQUEST THAT WE INTERCEPT THE ARR AND THE XED THROUGH THE ARR R (SEA 101 DEG). APCH CTL THEN GAVE US A R TURN VECTOR AND WE BELIEVED THAT WE WERE THEN NOT ON THE PUBLISHED ARR. IN FACT THE F/O REMARKED THAT WE MUST BE IN AN APCH CTL VECTOR FOR A W DOWNWIND FOR A S LNDG AT SEATAC. THEREFORE, WE DID NOT MAKE THE PUBLISHED TURN TO 340 DEG. SUBSEQUENT COMMENTS FROM APCH SUGGESTED THAT THIS WAS NOT UNCOMMON INDICATING PROBS WITH THIS ARR.

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.