Narrative:

Cleared on an intercept heading for 27R at egll in IFR conditions; the PF selected approach and the FMC intercepted the localizer. We reported established with the final controller and were cleared to descend via the glide-slope and handed off to tower. About that time; the FMC flight director commanded a turn to the south to intercept the localizer; which was centered on the raw data portion of the eadi and ehsi. The aircraft; on autopilot; started to deviate south of course. I instructed the pm to reselect LNAV to recapture the localizer; then to reselect approach mode to re-intercept the final approach course. He did so and the aircraft recaptured the localizer. It then proceeded to turn to the north and once again deviated about a dot width (on the expanded scale) to the north. At this point I instructed the PF to disconnect the autopilot and fly the raw data manually. Egll tower asked us if we were established on the localizer and I replied that we were experiencing some navigational anomalies. He replied in the affirmative and we continued. We finally were centered on the localizer at approximately 1300 feet above field elevation concurrently with breaking out of the undercast. We confirmed that we were lined up on the localizer and 27R and landed uneventfully. The weather was not what was advertised on the ATIS and this particular aircraft; while under the ops bulletin regarding nav anomalies; was not suitable for IMC flying into egll due to its known difficulties with the FMC and mag var tables.

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Original NASA ASRS Text

Title: B767 captain reported experiencing navigation and course control deviations while flying the ILS approach to EGLL.

Narrative: Cleared on an intercept heading for 27R at EGLL in IFR conditions; the PF selected Approach and the FMC intercepted the localizer. We reported established with the final controller and were cleared to descend via the glide-slope and handed off to tower. About that time; the FMC flight director commanded a turn to the south to intercept the localizer; which was centered on the raw data portion of the EADI and EHSI. The aircraft; on autopilot; started to deviate south of course. I instructed the PM to reselect LNAV to recapture the localizer; then to reselect Approach mode to re-intercept the final approach course. He did so and the aircraft recaptured the localizer. It then proceeded to turn to the North and once again deviated about a dot width (on the expanded scale) to the north. At this point I instructed the PF to disconnect the Autopilot and fly the raw data manually. EGLL tower asked us if we were established on the localizer and I replied that we were experiencing some navigational anomalies. He replied in the affirmative and we continued. We finally were centered on the localizer at approximately 1300 feet above field elevation concurrently with breaking out of the undercast. We confirmed that we were lined up on the localizer AND 27R and landed uneventfully. The weather WAS NOT what was advertised on the ATIS and this particular aircraft; while under the Ops Bulletin regarding Nav anomalies; was not suitable for IMC flying into EGLL due to its known difficulties with the FMC and Mag Var Tables.

Data retrieved from NASA's ASRS site 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.