B737-300 flight crew has a track heading deviation during the tyssn 1 arrival to las.

Attributes
ACN	694856
Time
Date	200604
Local Time Of Day	0601 To 1200
Place
Locale Reference	intersection : kaddy
State Reference	NV
Altitude	msl single value : 12000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : l30.tracon
Operator	common carrier : air carrier
Make Model Name	B737-300
Operating Under FAR Part	Part 121
Navigation In Use	other
Flight Phase	descent : approach
Route In Use	arrival star : tyssn1
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp pilot : instrument pilot : multi engine pilot : commercial pilot : flight engineer
Experience	flight time last 90 days : 150 flight time total : 5300 flight time type : 950
ASRS Report	694856
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Events
Anomaly	aircraft equipment problem : less severe non adherence : published procedure other spatial deviation
Independent Detector	other controllera
Resolutory Action	controller : issued new clearance controller : issued advisory
Supplementary
Problem Areas	Flight Crew Human Performance Aircraft
Primary Problem	Aircraft

Narrative:

Upon arrival at the kaddy intersection on the tyssn 1 RNAV arrival into las; the autoplt made an approximately 60 degree right turn at the kaddy intersection. ATC called to ask us where we were going. Our response was that we were on the assigned RNAV arrival just past kaddy intersection. His reply stated that we had turned approximately 6 NM early on the arrival and now we were north of course. We then requested radar vectors to a visual approach. Upon further investigation; found out that the FMS was not automatic updating the position relative to available navaids along our route. We had been cleared direct to different fixes and navaids along our route; never actually reaching the NAVAID prior to being cleared further downline to other fixes. The first indication we were off course was the center call. Our FMS's showed the IRS's were within .2 NM of the course and position assigned. Coming down final the FMS showed that we were on the runway at the FAF. The captain manually updated the FMS and the map then showed the correct position. I believe this was a map shift that we didn't readily identify. Although I'm aware of the possibility of map shifts; I had never seen one and had no reason to believe that I was off course.

Original NASA ASRS Text

Title: B737-300 FLT CREW HAS A TRACK HDG DEV DURING THE TYSSN 1 ARR TO LAS.

Narrative: UPON ARR AT THE KADDY INTXN ON THE TYSSN 1 RNAV ARR INTO LAS; THE AUTOPLT MADE AN APPROX 60 DEG R TURN AT THE KADDY INTXN. ATC CALLED TO ASK US WHERE WE WERE GOING. OUR RESPONSE WAS THAT WE WERE ON THE ASSIGNED RNAV ARR JUST PAST KADDY INTXN. HIS REPLY STATED THAT WE HAD TURNED APPROX 6 NM EARLY ON THE ARR AND NOW WE WERE N OF COURSE. WE THEN REQUESTED RADAR VECTORS TO A VISUAL APCH. UPON FURTHER INVESTIGATION; FOUND OUT THAT THE FMS WAS NOT AUTO UPDATING THE POS RELATIVE TO AVAILABLE NAVAIDS ALONG OUR RTE. WE HAD BEEN CLRED DIRECT TO DIFFERENT FIXES AND NAVAIDS ALONG OUR RTE; NEVER ACTUALLY REACHING THE NAVAID PRIOR TO BEING CLRED FURTHER DOWNLINE TO OTHER FIXES. THE FIRST INDICATION WE WERE OFF COURSE WAS THE CTR CALL. OUR FMS'S SHOWED THE IRS'S WERE WITHIN .2 NM OF THE COURSE AND POS ASSIGNED. COMING DOWN FINAL THE FMS SHOWED THAT WE WERE ON THE RWY AT THE FAF. THE CAPT MANUALLY UPDATED THE FMS AND THE MAP THEN SHOWED THE CORRECT POS. I BELIEVE THIS WAS A MAP SHIFT THAT WE DIDN'T READILY IDENT. ALTHOUGH I'M AWARE OF THE POSSIBILITY OF MAP SHIFTS; I HAD NEVER SEEN ONE AND HAD NO REASON TO BELIEVE THAT I WAS OFF COURSE.

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.