AN MD80 CAPT RPTS NAV DATA ON THE FMS OUT-OF-DATE WITH NO ERROR MESSAGES DISPLAYED. SYS HAS A LONG HISTORY OF PROBS.

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ON MY ARR AT THE ACFT IN ZZZ; THE FLT PLAN WAS ALREADY LOADED INTO THE GFMS. WE ACCEPTED IT AND EXECUTED IT. I THEN CHKED IT AGAINST THE FLT PLAN WITH THE FO AND IT WAS CORRECT AND COMPLETE. WE PROCEEDED WITH OUR PREDEP CHKLISTS. I FOUND THE CAPT'S ALTIMETER WAS OUT OF LIMITS ON THE HIGH SIDE AND CALLED MAINT. WHILE THEY WERE CHKING THE #1 CADC; I NOTED THE GFMS INFO ON THE NAV DISPLAY HAD AN ERRATIC FLT PATH VECTOR SYMBOL. IT WAS RANDOMLY MOVING AROUND THE COMPASS ROSE. THIS WAS A REPEAT ITEM FROM 4 PREVIOUS FLTS OVER 10 DAYS. I INFORMED MAINT OF THE ADDITIONAL PROB. MAINT REPLACED THE #1 CADC AND THE CAPT'S NAV COMPUTER. BOTH PROBS SEEMED RESOLVED. THE FO AND I CHKED OUR RTE AGAIN ON THE GFMS SINCE THE COMPUTER HAD BEEN REPLACED. THE RTE WAS STILL ACTIVE; CORRECT; AND COMPLETE. ON ARR AT ZZZ1; I PULLED THE NEW FLT PLAN AND BEGAN TO LOAD IT IN THE GFMS. ON THE IDENT PAGE; I NOTED THE DATABASE WAS BAD. THE ACTUAL DATE WAS JUL/XB/05. THE AVAILABLE NAV DATABASE DATES WERE JUN/XA/05 - JUL/XA/05; AND AUG/XA/05 - SEP/XA/05. I ENTERED THIS INTO THE MAINT LOG. I HAVE NO IDEA HOW OR WHEN THE NAV DATA CHANGED FROM THE CURRENT CYCLE. I DO KNOW THE ACFT HAD FLOWN TWICE EARLIER IN THE DAY AND; THEREFORE; HAVE TO ASSUME IT WAS CORRECT THEN. ADDITIONALLY; THE NAV DATA OUT-OF-DATE MESSAGE NEVER APPEARED ON THE GFMS DISPLAY FOR OUR 1 LEG. SINCE THAT AMBER MESSAGE WILL DISPLAY IF THE DATA IS OUT-OF-DATE; I ALSO BELIEVE THAT WHEN THE FO INITIALLY ENTERED THE FLT PLAN INTO THE GFMS IT MUST HAVE HAD THE CORRECT CYCLE IN AT THAT TIME. I DO NOT KNOW IF CHANGING OUT THE CAPT'S NAV COMPUTER COULD HAVE HAD AN EFFECT; BUT THE ACTIVE FLT PLAN HAD NOT CHANGED AFTER THE COMPUTER WAS REPLACED AND WE CHKED THE RTE AFTER THE MAINT WAS COMPLETED. ZZZ1 FBO MAINT PERSONNEL WERE ALSO UNABLE TO EXPLAIN HOW THE NAV DATA COULD BE SO FAR OUT-OF-DATE WITH NO ERROR MESSAGES DISPLAYED; OR HOW; WHY; OR WHEN IT COULD HAVE CHANGED FROM THE CURRENT CYCLE. THIS PARTICULAR GFMS SYS HAS A LONG HISTORY OF PROBS AND WAS PLACARDED IN ZZZ1.