A B737-300 on civet arrival into lax experienced a map shift that led to track deviation.

Attributes
ACN	709598
Time
Date	200609
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : lax.airport
State Reference	CA
Altitude	msl single value : 10000
Environment
Flight Conditions	IMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : sct.tracon tower : lex.tower
Operator	common carrier : air carrier
Make Model Name	B737-300
Operating Under FAR Part	Part 121
Navigation In Use	other
Flight Phase	descent : intermediate altitude
Route In Use	arrival star : civet
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Experience	flight time last 90 days : 260 flight time total : 10000 flight time type : 4500
ASRS Report	709598
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Events
Anomaly	aircraft equipment problem : less severe non adherence : published procedure non adherence : clearance other spatial deviation
Supplementary
Problem Areas	Aircraft
Primary Problem	Aircraft

Narrative:

Originally set up for civet arrival; runway 25R. Both navigation radios in automatic. Assigned runway 24R during descent. First officer put runway 24R in computer; while captain continued to fly and ensure crossing restrs were met. Autoplt was engaged; navigation mode was LNAV. Both navigation radios still in automatic. While on proper LNAV course; approach control said we were 'a little south of course;' and gave us a slight right turn to intercept runway 24R localizer. This puzzled me because we were precisely on the correct LNAV course. We both double checked this. We took the heading; and intercepted the runway 24R localizer course. Both radios now in manual; set to localizer. After intercepting the localizer; with LNAV course a couple of mi to the south; there was a significant map shift north; essentially overlying current magnetic track. It appears there was a corrupt FMC position during the descent on civet arrival which corrected itself on ILS final using localizer updating. Don't know the cause. As I've said; both radios were in automatic with proper updating. We did not go to manual radios until on ILS final; runway 24R.

Original NASA ASRS Text

Title: A B737-300 ON CIVET ARR INTO LAX EXPERIENCED A MAP SHIFT THAT LED TO TRACK DEV.

Narrative: ORIGINALLY SET UP FOR CIVET ARR; RWY 25R. BOTH NAV RADIOS IN AUTO. ASSIGNED RWY 24R DURING DSCNT. FO PUT RWY 24R IN COMPUTER; WHILE CAPT CONTINUED TO FLY AND ENSURE XING RESTRS WERE MET. AUTOPLT WAS ENGAGED; NAV MODE WAS LNAV. BOTH NAV RADIOS STILL IN AUTO. WHILE ON PROPER LNAV COURSE; APCH CTL SAID WE WERE 'A LITTLE S OF COURSE;' AND GAVE US A SLIGHT R TURN TO INTERCEPT RWY 24R LOC. THIS PUZZLED ME BECAUSE WE WERE PRECISELY ON THE CORRECT LNAV COURSE. WE BOTH DOUBLE CHKED THIS. WE TOOK THE HDG; AND INTERCEPTED THE RWY 24R LOC COURSE. BOTH RADIOS NOW IN MANUAL; SET TO LOC. AFTER INTERCEPTING THE LOC; WITH LNAV COURSE A COUPLE OF MI TO THE S; THERE WAS A SIGNIFICANT MAP SHIFT N; ESSENTIALLY OVERLYING CURRENT MAG TRACK. IT APPEARS THERE WAS A CORRUPT FMC POS DURING THE DSCNT ON CIVET ARR WHICH CORRECTED ITSELF ON ILS FINAL USING LOC UPDATING. DON'T KNOW THE CAUSE. AS I'VE SAID; BOTH RADIOS WERE IN AUTO WITH PROPER UPDATING. WE DID NOT GO TO MANUAL RADIOS UNTIL ON ILS FINAL; RWY 24R.

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.