Narrative:

Advanced/automated glass cockpit aircraft, widebody transport. Fully coupled approach on localizer and GS with all indications for a fully automated approach annunciated. Below 1000 ft AGL, aircraft reverted to heading mode. 23 KT crosswind caused full scale localizer deflection to occur. Subsequent missed approach accomplished. Training and standards personnel contacted and only explanation is momentary loss of localizer signal which was not annunciated resulting in coupled approach mode reverting to heading and subsequent localizer deviation due to crosswind and narrowness of beam close to localizer antenna. Software supplier to be contacted to investigate anomaly of loss of localizer signal or signal not within flight management computer tolerances resulting in loss of localizer guidance without loss of localizer identify annunciation. Callback conversation with reporter revealed the following information: the reporting first officer states that technical representatives have told him that there was probably a momentary loss of localizer signal that caused the autoplt to revert to heading mode. The autoplt was then unable to recapture the localizer as it was then flying away from it. His air carrier's training department has heard of the same thing happening to the md-11 fleet of another air carrier. The signal loss was so brief that there were no warnings noticed.

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Original NASA ASRS Text

Title: AN ACR WDB FO RPTS THAT HIS ACFT REVERTED TO HDG MODE WHILE FLYING A COUPLED ILS.

Narrative: ADVANCED/AUTOMATED GLASS COCKPIT ACFT, WDB. FULLY COUPLED APCH ON LOC AND GS WITH ALL INDICATIONS FOR A FULLY AUTOMATED APCH ANNUNCIATED. BELOW 1000 FT AGL, ACFT REVERTED TO HDG MODE. 23 KT XWIND CAUSED FULL SCALE LOC DEFLECTION TO OCCUR. SUBSEQUENT MISSED APCH ACCOMPLISHED. TRAINING AND STANDARDS PERSONNEL CONTACTED AND ONLY EXPLANATION IS MOMENTARY LOSS OF LOC SIGNAL WHICH WAS NOT ANNUNCIATED RESULTING IN COUPLED APCH MODE REVERTING TO HDG AND SUBSEQUENT LOC DEV DUE TO XWIND AND NARROWNESS OF BEAM CLOSE TO LOC ANTENNA. SOFTWARE SUPPLIER TO BE CONTACTED TO INVESTIGATE ANOMALY OF LOSS OF LOC SIGNAL OR SIGNAL NOT WITHIN FLT MGMNT COMPUTER TOLERANCES RESULTING IN LOSS OF LOC GUIDANCE WITHOUT LOSS OF LOC IDENT ANNUNCIATION. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTING FO STATES THAT TECHNICAL REPRESENTATIVES HAVE TOLD HIM THAT THERE WAS PROBABLY A MOMENTARY LOSS OF LOC SIGNAL THAT CAUSED THE AUTOPLT TO REVERT TO HDG MODE. THE AUTOPLT WAS THEN UNABLE TO RECAPTURE THE LOC AS IT WAS THEN FLYING AWAY FROM IT. HIS ACR'S TRAINING DEPT HAS HEARD OF THE SAME THING HAPPENING TO THE MD-11 FLEET OF ANOTHER ACR. THE SIGNAL LOSS WAS SO BRIEF THAT THERE WERE NO WARNINGS NOTICED.

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.