A319 CAPT REPORTS NAV ERRORS DURING RNAV ARRIVAL WITH NO FMGC CAUTIONS. NAV IS FOUND TO BE SELECTED ON THE RMP PANEL.

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ON THE ARR. FO FLYING; AUTOPLT #2 SELECTED. 1.0 NAV ACCURACY WAS INSERTED INTO THE 'REQUIRED' SIDE OF THE PROG PAGE. NO MESSAGES. ATC STARTS MAKING STRANGE QUERIES. FINALLY THEY TURN US OFF THE ARR TO AVOID MIL AIRSPACE GIVEN DIRECT AND HANDED OFF TO APCH. THEY ASK IF WE ARE ON THE ARR AND THINGS OUT THE WINDOW DON'T LOOK 'QUITE' RIGHT. I CHK THE PROG PAGE AND THE 'ESTIMATED' SIDE SAYS 8.7 ON BOTH MCDU'S. THE IR EPE'S ARE ALL IN THE LOW 2 RANGE. NAV DISPLAY SHOWS RIGHT ON THE GREEN LINE. FINALLY DISCOVERED THE FO SIDE RMP HAD THE NAV BUTTON PUSHED. RESET THE BUTTON. APCH GIVES US A VECTOR. AND WITHIN A FEW MI THINGS ALL GET SORTED OUT. THE FMGC'S FIGURE OUT THEY ARE 4 MI OFF COURSE AND THE 'ESTIMATED' DISPLAYS ON THE PROG PAGES GO BELOW 1.0. AND WE GET A NAV UPGRADED MESSAGE. MADE AN ENTRY IN THE ACFT LOG. NAV BUTTON ON RMP PUSHED. I NOR THE FO HAVE ANY IDEA HOW THIS HAPPENED. ACFT WAS RECEIVED FROM MAINT. THE CAPT'S RMP NAV BUTTON WAS FOUND TO BE PUSHED; WHICH IS A NORMAL CONDITION COMING FROM MAINT. IT WAS RESET AND THE VOR WAS HARD TUNED ON THE MCDU FOR THE DEP; WHICH FURTHER PROVES THAT THE MANUAL TUNING MODE WAS NOT ACTIVE. THE FO BUTTON WAS NOT FOUND TO BE PUSHED DURING COCKPIT SETUP. THE SWITCH IS GUARDED; SO IT CANNOT BE ACCIDENTALLY PUSHED. THE FO ALSO USED THE MCDU FOR TUNING VOR'S ENRTE. FAILURE OF THE FMGC'S TO ACCURATELY MAINTAIN ACCURATE POS. EVEN WITH NO RADIO POS UPDATING THE FMGC POS SHOULDN'T DRIFT THAT FAR. NO NAV ACCURACY DOWNGRADE MESSAGE. ANOMALY. NO IRS NAV ONLY MESSAGE EITHER. THIS SHOULD COME UP AFTER 10 MINS WITH NO RADIO NAV.