AN MD11 PILOT REPORTS AN FMS ANOMALY PREVENTING THE ACFT'S ND AND FMC FROM DISPLAYING THE PROPER SOLDO2 DEP TRACK.

Narrative

DEPARTED OFF OF RWY 17R FLYING THE SOLDO 2. NAV SYS WAS ARMED AND CORRECT SID LOADED IN THE FMC AND VERIFIED BY BOTH CREW MEMBERS. THE ACFT WAS FLYING THE DEP NORMALLY (OR APPEARED TO BE) BUT AS IT CYCLED THROUGH THE FIRST FIX (TREXX) THE 'TO' POINT DIDN'T ADVANCE ON THE TOP OF THE NAV DISPLAY. THE FMS ALSO SHOWED THE PREVIOUS POINT AS THE 'TO' POINT. THE NAV DISPLAY HOWEVER WAS HEADING THE ACFT TO THE NEXT WAYPOINT (L TURN) AND THE FLT DIRECTOR WAS COMMANDING THE ACFT TO THIS FIX AS WELL. THERE WAS THEN A BREAK IN THE MAGENTA LINE BTWN THE SECOND AND THIRD FIX ON THE NAV DISPLAY BUT NO 'DISCON' ON THE FLT PLAN. WHILE THE ACFT DID SHOW TO BE ON THE DEP AS RELATED TO THE TRACK LINE; THE CTLR QUERIED US AND ASKED IF WE WERE ON THE RNAV DEP. WE SAID YES AND TOLD HIM THE FMS FAILED TO CYCLE THROUGH THE PREVIOUS WAYPOINT. HE SAID NO PROB AND GAVE US DIRECT TO SOLDO WHICH IS WELL E OF THE ARPT. AT THE END OF THE FLT (1:24) THE #3 IRU HAD A DRIFT RATE OF 5.4 WHICH IS OBVIOUSLY EXCESSIVE BUT I DOUBT WAS RELATED. THIS WAS A UNIQUE SITUATION IN THAT I HAVE NEVER SEEN THE ACFT CONTINUE ON TO A FIX WHILE THE FMS AND 'TO' POINT ON THE NAV DISPLAY SHOW THE PREVIOUS WAYPOINT. I ONLY WROTE UP THE IRU IN THE LOGBOOK BECAUSE TRYING TO EXPLAIN THIS DISCREPANCY TO MAINT WOULD HAVE BEEN A WASTE OF TIME. WHEN ON AN RNAV DEP AS THIS; THERE IS NO WAY TO BACK UP THE RTE WITH NAVAIDS AS THE FIRST IDENTIFIABLE FIX IS SOLDO. OTHER THAN THE GLITCH DESCRIBED ABOVE; THE ACFT APPEARED TO BE TRACKING THE PROPER COURSE. WE WONDERED IF THIS WAS A ONE TIME EVENT BUT ON NOV/XX/07; WE FLEW THE SAME FLT IN THE SAME ACFT AND HAD AN IDENTICAL PROB WITH THE ACFT AND WAS IMMEDIATELY REPORTED TO ATC. THE DATABASE AND ACFT WAS WRITTEN UP AND WE ARE WAITING FOR AN ANSWER FROM THE COMPANY TO WHY THIS EVENT HAPPENED.