Narrative:

During descent at approximately 15;000 ft MSL; we noticed the LNAV was taking the aircraft off course (one dot). I (pilot not flying) executed another 'direct' to waypoint on the CDU with no change in aircraft heading. The captain (pilot flying) disconnected the autopilot and steered toward the arrival waypoint using the FMC information. As we approached the waypoint at about 12;000 ft MSL; center said we appeared about five miles off course and then handed us off to approach. I estimate we passed within 1-2 NM of the waypoint according to the FMC. Upon check-in; approach stated that we appeared off the arrival. I told approach that we incurred a navigation system problem and requested vectors for a visual approach. The subsequent approach and landing were uneventful.first; LNAV worked normally during departure. Given a heading to intercept the procedure; LNAV captured normally; turned onto and maintained the initial course. Second; LNAV appeared to turn the aircraft correctly when direct the arrival waypoint was executed. When we noticed the course bar offset; I also noticed that both of our 'automatic' tune functions read 'manual' and we could not select 'automatic.' no other anomalies were found except as stated above. Third; although unrelated; VOR and ILS worked normally as well as all MCP functions. Fourth; as we approached the airfield; known waypoints correlated with the FMC. Finally; maintenance confirmed the LNAV problem and was able to fix by restarting the computers. I thought the event was handled well by all; including ATC (both center and approach). Even though we were VMC; I chose to request IFR vectors immediately rather than fly the arrival using VOR's and wind-corrected tracking--simple and safe. There were no failure flags; caution lights; or other indicators of LNAV failure; maintaining a good crosscheck of system performance is still important with automation.

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Original NASA ASRS Text

Title: A B737-300 FMC was unable to track directly toward and over an arrival waypoint even though it indicated that it was going to and over it. ATC stated they off the arrival by five miles at one point.

Narrative: During descent at approximately 15;000 FT MSL; we noticed the LNAV was taking the aircraft off course (one dot). I (pilot not flying) executed another 'direct' to waypoint on the CDU with no change in aircraft heading. The Captain (pilot flying) disconnected the autopilot and steered toward the arrival waypoint using the FMC information. As we approached the waypoint at about 12;000 FT MSL; Center said we appeared about five miles off course and then handed us off to Approach. I estimate we passed within 1-2 NM of the waypoint according to the FMC. Upon check-in; Approach stated that we appeared off the arrival. I told Approach that we incurred a navigation system problem and requested vectors for a visual approach. The subsequent approach and landing were uneventful.First; LNAV worked normally during departure. Given a heading to intercept the procedure; LNAV captured normally; turned onto and maintained the initial course. Second; LNAV appeared to turn the aircraft correctly when direct the arrival waypoint was executed. When we noticed the course bar offset; I also noticed that both of our 'AUTO' tune functions read 'MANUAL' and we could not select 'AUTO.' No other anomalies were found except as stated above. Third; although unrelated; VOR and ILS worked normally as well as all MCP functions. Fourth; as we approached the airfield; known waypoints correlated with the FMC. Finally; Maintenance confirmed the LNAV problem and was able to fix by restarting the computers. I thought the event was handled well by all; including ATC (both Center and Approach). Even though we were VMC; I chose to request IFR vectors immediately rather than fly the arrival using VOR's and wind-corrected tracking--simple and safe. There were no failure flags; caution lights; or other indicators of LNAV failure; maintaining a good crosscheck of system performance is still important with automation.

Data retrieved from NASA's ASRS site as of July 2013 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.