What was found
FL 350 OBSERVED GREEN HYD SYS QUANTITY DECREASING. SHORTLY THEREAFTER ECAM TRIGGERED AND QUANTITY DROPPED TO -RED DOT-. FOLLOWED ECAM & QRH. MANUALLY EXTENDED GEAR ON FINAL AND GREEN HYD SYS QUANTITY RETURNED TO TOP HAT. UNEVENTFUL LANDING AND TOW-IN TO GATE. SERVICED GREEN SYSTEM AND FOUND LEAK IN MAIN GEAR WHEEL WELL AT THE LANDING GEAR SELECTOR VALVE. FOUND ONE OF THE LANDING GEAR SELECTOR VALVES HYDRAULIC UNION'S JAM NUT BACKED OFF AND THE UNION LOOSE. REPACKED UNION AND REINSTALLED. PRESSURIZED GREEN SYSTEM, NO LEAKS NOTED.