Date: 201301
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Daylight
Aircraft: B737-300
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
Airspace: Class E ZZZ
ATC / advisory: Center ZZZ
Aircraft component: FMS/FMC
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Not Flying
Human factors: Human-Machine Interface; Situational Awareness; Workload; Distraction
Second reporter function: Pilot Flying; Captain
Anomaly: Aircraft Equipment Problem Less Severe; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Air Traffic Control; Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Air Traffic Control Issued New Clearance; Flight Crew Requested ATC Assistance / Clarification; General Maintenance Action
Contributing factors: Aircraft
Primary problem: Aircraft

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.


Second narrative: As we tried to figure out what was the problem; we switched both NAV switches to normal and back and noticed the NAV receivers would not auto tune; and they stayed in manual regardless of NAV switch position. During all this; I disengaged autopilot as we neared the arrival waypoint; which I was attempting to navigate to using the FMC. Seems I turned as we went overhead the arrival waypoint; the best I could tell; when Center said we missed the waypoint and handed us off to Approach. At this time we told ATC we were unable our own NAV and then given headings to our destination. We had Maintenance come out to take a look at the problem. A very senior AMT investigated; and eventually said he had never seen anything quite like this before. I agreed. I called our ATC Specialist; and he called Center and said they did not have any problems with us.


Synopsis: 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.