Narrative:

On RMBLN3 RNAV departure from atl runway 8R; at approximately 1500 ft MSL; both pilots' navigation display shifted what I estimate about 2 NM to the north; showing the aircraft south of the depicted course. The first officer (PF) made a correction to the north to recapture the depicted ground track. At about the time the aircraft recaptured the FMC generated ground track; the maps of both pilots shifted back to the south. Atl departure control queried us at that time if we were on the RNAV departure and stated we missed the hrshl intersection by about 1.5 NM. We received radar vectors for the remainder of the departure. FMC's showed normal updating and no other anomalies were noted during the rest of the flight. No loss of separation occurred.

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Original NASA ASRS Text

Title: B737-300 FLT CREW EXPERIENCES TWO MAP SHIFTS DURING INITIAL STAGES OF RMBLN RNAV SID FROM ATL. CREW RECEIVED RADAR VECTORS TO COMPLETE PROC.

Narrative: ON RMBLN3 RNAV DEP FROM ATL RWY 8R; AT APPROX 1500 FT MSL; BOTH PLTS' NAV DISPLAY SHIFTED WHAT I ESTIMATE ABOUT 2 NM TO THE N; SHOWING THE ACFT S OF THE DEPICTED COURSE. THE FO (PF) MADE A CORRECTION TO THE N TO RECAPTURE THE DEPICTED GND TRACK. AT ABOUT THE TIME THE ACFT RECAPTURED THE FMC GENERATED GND TRACK; THE MAPS OF BOTH PLTS SHIFTED BACK TO THE S. ATL DEP CTL QUERIED US AT THAT TIME IF WE WERE ON THE RNAV DEP AND STATED WE MISSED THE HRSHL INTXN BY ABOUT 1.5 NM. WE RECEIVED RADAR VECTORS FOR THE REMAINDER OF THE DEP. FMC'S SHOWED NORMAL UPDATING AND NO OTHER ANOMALIES WERE NOTED DURING THE REST OF THE FLT. NO LOSS OF SEPARATION OCCURRED.

Data retrieved from NASA's ASRS site as of January 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.