FLT CREWOF A320 EXPERIENCED HYD SYS FAILURE ON DSCNT TO ORD. DECLARED EMER; LANDED SAFELY AND WAS TOWED TO GATE.

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ON INITIAL DSCNT INTO ORD; AN ECAM INDICATING GREEN HYD LOW PRESSURE MESSAGE APPEARED. COMPLETED ECAM PROC WHILE REFING THE FLT MANUAL. IN ATTEMPTING TO RESTORE PRESSURE TO THE GREEN SYS; THE HYD QUANTITY FURTHER DECREASED. COMPLYING WITH ECAM AND FLT MANUAL PROCS; THE PTU AND GREEN ENG HYD PUMP WERE TURNED OFF; AND THE REMAINDER OF THE ECAM WAS COMPLETED. DISPATCH AND MAINT CTL NOTIFIED OF OUR INTENTIONS TO DECLARE AN EMER. REQUESTED COORD TO INSURE THE EMER EQUIP WOULD BE STANDING BY. NOTIFIED DISPATCH OF THE NEED TO LAND ON RWY 27L FOR ITS LENGTH AND ADVANTAGEOUS WINDS. ADVISED OUR INTENTIONS TO COME TO A COMPLETE STOP ON THE RWY AND THE NEED FOR A TOW OFF AS THE NOSEWHEEL STEERING HAD BEEN DISABLED BY THE IRREGULARITY. DECLARED EMER WITH ZAU; REQUESTED ONE TURN IN HOLDING TO COMPLETE CHKLISTS AND REVIEW ECAM STATUS PAGE. FLT ATTENDANTS AND PAX BRIEFED. A CABIN ADVISORY WAS ISSUED. THE SUBSEQUENT APCH AND LNDG WERE NORMAL AND UNEVENTFUL. ACFT STOPPED ON RWY 27L; AND WAS TOWED TO THE GATE AFTER ENG SHUTDOWN.