B757-200 crew attempted an LNAV approach when map shift was apparent at sfo.

Attributes
ACN	515775
Time
Date	200106
Day	Mon
Local Time Of Day	1801 To 2400
Place
Locale Reference	airport : sfo.airport
State Reference	CA
Altitude	msl single value : 1400
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Controlling Facilities	tracon : o90.tracon tower : yip.tower
Operator	common carrier : air carrier
Make Model Name	B757 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Navigation In Use	ils localizer only : 28r other
Flight Phase	descent : approach
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 210 flight time total : 13800 flight time type : 800
ASRS Report	515775
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Events
Anomaly	aircraft equipment problem : less severe non adherence : published procedure non adherence : company policies non adherence : clearance other anomaly other other spatial deviation
Independent Detector	other flight crewa other flight crewb
Resolutory Action	none taken : detected after the fact
Supplementary
Problem Areas	Flight Crew Human Performance Aircraft
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

Stin 1 arrival to sfo programmed in the FMC. After sfo, given radar vector of 140 degrees for visual to runway 28R. We requested the FMC bridge visual approach to runway 28R. With runway 28R in sight, we were cleared for a visual or FMS bridge visual to runway 28R (our choice). LNAV was selected and the autoplt intercepted the bridge visual. At samul intersection, the autoplt started a slight left turn to fly to fioid intersection as programmed and I noticed a map shift to the left and the aircraft made a steeper turn to the left and crossed the runway 28R centerline. I disengaged the autoplt and flew the aircraft back to the runway 28R centerline. There was no traffic on approach to runway 28L and the tower did not make any comment. This is the first time that I have seen a map shift on approach. I have flown this FMC approach 4 or 5 times without a problem. I let the automation take the aircraft too far and should have disconnected the autoplt immediately when I saw the map shift.

Original NASA ASRS Text

Title: B757-200 CREW ATTEMPTED AN LNAV APCH WHEN MAP SHIFT WAS APPARENT AT SFO.

Narrative: STIN 1 ARR TO SFO PROGRAMMED IN THE FMC. AFTER SFO, GIVEN RADAR VECTOR OF 140 DEGS FOR VISUAL TO RWY 28R. WE REQUESTED THE FMC BRIDGE VISUAL APCH TO RWY 28R. WITH RWY 28R IN SIGHT, WE WERE CLRED FOR A VISUAL OR FMS BRIDGE VISUAL TO RWY 28R (OUR CHOICE). LNAV WAS SELECTED AND THE AUTOPLT INTERCEPTED THE BRIDGE VISUAL. AT SAMUL INTXN, THE AUTOPLT STARTED A SLIGHT L TURN TO FLY TO FIOID INTXN AS PROGRAMMED AND I NOTICED A MAP SHIFT TO THE L AND THE ACFT MADE A STEEPER TURN TO THE L AND CROSSED THE RWY 28R CTRLINE. I DISENGAGED THE AUTOPLT AND FLEW THE ACFT BACK TO THE RWY 28R CTRLINE. THERE WAS NO TFC ON APCH TO RWY 28L AND THE TWR DID NOT MAKE ANY COMMENT. THIS IS THE FIRST TIME THAT I HAVE SEEN A MAP SHIFT ON APCH. I HAVE FLOWN THIS FMC APCH 4 OR 5 TIMES WITHOUT A PROB. I LET THE AUTOMATION TAKE THE ACFT TOO FAR AND SHOULD HAVE DISCONNECTED THE AUTOPLT IMMEDIATELY WHEN I SAW THE MAP SHIFT.

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.