AN AIRBUS 330 IN CRUISE AT FL400 DECLARED AN EMER DUE TO LOSS OF GREEN HYDRAULIC SYSTEM QUANTITY AND PRESSURE. CAUSE UNKNOWN. CREW NOTES DIFFICULTY CALCULATING RATE OF LOSS AS QUANTITY INDICATORS HAVE NO INDICES.

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AT APPROX XA15 ON 6/SUN/03; OUR ANNUNCIATOR PANEL (ECAM) INDICATED A POSSIBLE LEAK IN THE 'GREEN' HYDRAULIC SYSTEM. WE STARTED TO MONITOR THE SYSTEM OVER THE NEXT 10 MINUTES. AT THE END OF THAT TIME; PRIOR TO EXITING ZZZZ AIRSPACE; WE DETERMINED THAT THERE EXISTED A HIGH PROBABILITY OF A LEAK. SHUT DOWN THE GREEN SYSTEM; AND REQUESTED PRIORITY LNDG TO OUR DEST; ZZZ; ABOUT 1.5 HR AWAY. (THE 'GREEN' SYSTEM IS PRIMARY ONLY FOR LNDG GEAR AND STEERING; AND NOT CRITICAL TO THE PHASE OF FLT WE WERE IN.) THE FLT CONTINUED NORMALLY UNTIL APCH INTO ZZZ. THERE; THE CAPT ASKED FOR THE 'GREEN' SYSTEM TO BE SWITCHED BACK ON. I DID SO AND WE LOWERED THE LNDG GEAR. THIS EXHAUSTED THE REMAINDER OF OUR AVAILABLE 'GREEN' SYSTEM FLUID; ANNUNCIATING A LOSS. THE CAPT DECLARED AN EMER; AND WE LANDED WITHOUT FURTHER INCIDENT. DIFFERENTIAL BRAKING ALLOWED US TO CLR THE RWY; ALTHOUGH WE HAD TO BE TOWED TO THE GATE. PROBLEM: THE A330 GAGES COVERING HYDRAULIC FLUID ARE NEARLY IMPOSSIBLE TO READ VIS; DETECTING THE RAPIDITY OF A LEAK. BETTER GAGES SHOWING RATE WOULD THE HELPFUL. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE ECAM WARNING ON THE POSSIBLE LEAK IN THE GREEN HYDRAULIC SYSTEM ALERTED THEM TO TURN OFF THE SYSTEM AND SAVE THE REMAINING FLUID FOR GEAR EXTENSION. THE RPTR STATED THERE IS NO WAY TO DETERMINE THE RATE OF LOSS OF FLUID DUE TO THE HYDRAULIC QUANTITY INDICATORS HAVE NO INDICES. THE MAIN CAUSE OF THE HYDRAULIC LOSS IS UNKNOWN AS MAINT HAS MADE NO RPT TO THE CREW.