Narrative:

Prior to departure on NOBLY2 lit; RNAV departure; route and legs were checked twice; once on preflight; and again on taxi-out after closeout received (this captain's personal standard procedure); all legs and sequencing verified correct and chart out and ready. Correct sequence: navye-jgirl-corts-nobly; displayed on navigation display. After takeoff aircraft flown manually; LNAV engaged. Passing jgirl; FMC dropped corts as we turned with no discontinuity or execute light. By the time we reloaded and referred to charts for correct sequence we had strayed 3 mi north of RNAV track. ATC also queried during this time as we retyped correct legs back into box. We corrected back to corts-nobly leg; as quickly as it is possible to type into legs. I have not seen the FMS do this before and it is impossible to catch in time at 250 KTS once the turn has started; as crew attention is outside clearing for traffic and we were given clearance to 17000 ft which both pilots were confirming. I took a photo immediately to show departure/arrival page and route page with correct loading still active; but no corts there anymore. 'ATC route' was still shown on legs; not 'mod route.' we apologized to ATC. On arrival into ZZZ on RNAV arrival; we were watching the aircraft carefully; when it did it again; and dropped the active waypoint with about 5 mi to run and sequenced to a point down-route; with 'resyncing' displayed in legs. We intervened and flew to correct point; wrote up the FMC on arrival. Maintenance found latest navigation database incorrectly loaded or corrupted; and re-loaded data before next flight; with no recurrence of anomaly.

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Original NASA ASRS Text

Title: A B737-800 FMS DATABASE CORRUPTION CAUSED THE DFW NOBLY 2 LIT TRANSITION CORTS WAYPOINT TO DROP OUT AFTER TKOF. THE CREW VERIFIED THE ROUTE ON PREFLT.

Narrative: PRIOR TO DEP ON NOBLY2 LIT; RNAV DEP; RTE AND LEGS WERE CHKED TWICE; ONCE ON PREFLT; AND AGAIN ON TAXI-OUT AFTER CLOSEOUT RECEIVED (THIS CAPT'S PERSONAL STANDARD PROC); ALL LEGS AND SEQUENCING VERIFIED CORRECT AND CHART OUT AND READY. CORRECT SEQUENCE: NAVYE-JGIRL-CORTS-NOBLY; DISPLAYED ON NAV DISPLAY. AFTER TKOF ACFT FLOWN MANUALLY; LNAV ENGAGED. PASSING JGIRL; FMC DROPPED CORTS AS WE TURNED WITH NO DISCONTINUITY OR EXECUTE LIGHT. BY THE TIME WE RELOADED AND REFERRED TO CHARTS FOR CORRECT SEQUENCE WE HAD STRAYED 3 MI N OF RNAV TRACK. ATC ALSO QUERIED DURING THIS TIME AS WE RETYPED CORRECT LEGS BACK INTO BOX. WE CORRECTED BACK TO CORTS-NOBLY LEG; AS QUICKLY AS IT IS POSSIBLE TO TYPE INTO LEGS. I HAVE NOT SEEN THE FMS DO THIS BEFORE AND IT IS IMPOSSIBLE TO CATCH IN TIME AT 250 KTS ONCE THE TURN HAS STARTED; AS CREW ATTN IS OUTSIDE CLRING FOR TFC AND WE WERE GIVEN CLRNC TO 17000 FT WHICH BOTH PLTS WERE CONFIRMING. I TOOK A PHOTO IMMEDIATELY TO SHOW DEP/ARR PAGE AND RTE PAGE WITH CORRECT LOADING STILL ACTIVE; BUT NO CORTS THERE ANYMORE. 'ATC RTE' WAS STILL SHOWN ON LEGS; NOT 'MOD RTE.' WE APOLOGIZED TO ATC. ON ARR INTO ZZZ ON RNAV ARR; WE WERE WATCHING THE ACFT CAREFULLY; WHEN IT DID IT AGAIN; AND DROPPED THE ACTIVE WAYPOINT WITH ABOUT 5 MI TO RUN AND SEQUENCED TO A POINT DOWN-RTE; WITH 'RESYNCING' DISPLAYED IN LEGS. WE INTERVENED AND FLEW TO CORRECT POINT; WROTE UP THE FMC ON ARR. MAINT FOUND LATEST NAV DATABASE INCORRECTLY LOADED OR CORRUPTED; AND RE-LOADED DATA BEFORE NEXT FLT; WITH NO RECURRENCE OF ANOMALY.

Data retrieved from NASA's ASRS site as of May 2009 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.