A320 GREEN SYSTEM HYDRAULIC FAILURE IS PREDICTED BY LOW QUANTITY INDICATION. NO ECAM WARNING UNTIL GEAR IS EXTENDED FOR LNDG. ADVANCE PREPARATION LIMITED THE IMPACT ON A TIMELY ARRIVAL.

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DURING CRUISE; APPROX 1 HR INTO FLT; DOING ROUTINE CHKS; DISCOVERED GREEN HYD SYS INDICATING ALMOST '0' QUANTITY; NORMAL 3000 PSI PRESSURE; NO ECAM WARNINGS; ALL SYS OPS NORMAL. CONTACTED MAINT AND WAS ADVISED TO 'FOLLOW QRH PROCS.' NO QRH PROCS WERE FOUND FOR THE ABOVE CONDITIONS; CONTINUED TO MONITOR GREEN HYD QUANTITY THROUGHOUT FLT. NO CHANGES OBSERVED. CONCLUDED THAT ECAM INDICATION FOR GREEN HYD QUANTITY WAS MOST LIKELY INACCURATE -- FAILED. AS A PRECAUTION; WE PLANNED ON AND BRIEFED OUR PLAN OF ACTION FOR A COMPLETE GREEN HYD FAILURE; INCLUDING ADVISING THE FLT ATTENDANTS. ON APCH INTO ZZZ; NO CHANGES IN GREEN HYD QUANTITY OR ECAM FAILURES NOTED; EVEN DURING FLAP EXTENSION; AND WE EXPECTED A NORMAL LNDG. WHEN GEAR WAS SELECTED DOWN AT 1500 FT AGL; GREEN HYD QUANTITY LOW LEVEL AND GREEN HYD PRESSURE LOW WARNINGS TRIGGERED ON ECAM; FOLLOWED ECAM PROCS. SINCE WE HAD PREBRIEFED FOR A POSSIBLE HYD FAILURE WE CONTINUED APCH; NOTIFIED TWR OF OURSITUATION; CHANGED RWYS TO RWY XXL (LONGER RWY) AND ADVISED FLT ATTENDANTS. LNDG WAS UNEVENTFUL AND WE WERE ABLE TO CLR THE RWY ON A HIGH SPD TXWY. COMPANY WAS NOTIFIED THAT WE WOULD NEED TO BE TOWED TO THE GATE (NO STEERING) AND KEPT PAX UPDATED ON OUR CONDITION. 30 MINS LATER; WE ARRIVED AT THE GATE.