A320 FLT CREW AT FL350 NOTICE YELLOW HYDRAULIC SYSTEM QUANTITY DECLINING. APPROACHING DESTINATION; SYSTEM FAILS AND THE FLT CREW DECLARES AN EMERGENCY. A NORMAL LANDING ENSUES AT DESTINATION.

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DURING CRUISE FLT HRLY CHK OF SYS PARAMETERS; BOTH FLT CREW MEMBERS NOTICED THE YELLOW SYS QUANTITY DECLINING. AT PUSHBACK YELLOW SYS QUANTITY WAS 'IN THE GREEN' RANGE. MONITORED AFFECTED SYS OVER THE COURSE OF THE NEXT 1/2 HR AND IT SHOWED A STEADY DECLINE. CAPT OPTED TO REVIEW QRH AS WELL AS FLT MANUAL AND CONTACTED MAINT CTL VIA ACARS. WE ELECTED TO CONTINUE TOWARDS OUR DEST AS WE DID NOT HAVE ANY ECAM WARNINGS OR CAUTIONS AT THAT TIME. FACTORS FOR THIS DECISION INCLUDED TIME INFLT LEFT (ABOUT 1:30) AND THE AVAILABILITY OF MANY SUITABLE ARPTS IN THE IMMEDIATE AREA. WE ALSO CONSIDERED MAINT AVAILABILITY AND THE FACT THAT WE STILL HAD 2 OTHER HYD SYS OPERATING NORMALLY AND DISPLAYING NORMAL QUANTITY INDICATIONS. ABOUT 350 MI FROM THE DEST ARPT WE RECEIVED AN AMBER ECAM FOR LOW YELLOW SYS PRESSURE; LOW Y SYS QUANTITY AND A PTU FAULT. CAPT AGAIN DETERMINED NO ACTION REQUIRED BY QRH AND PERFORMED ECAM ACTIONS WHILE I CONTINUED TO FLY TOWARDS DEST. CAPT TRIED TO CONTACT MAINT CTL AND DISPATCH WITHOUT ANY SUCCESS. ACARS WAS USED SLOWING DOWN THE COM PROCESS. CAPT AND I DISCUSSED DECLARING AN EMER AT THAT POINT AND AGREED THAT IT WAS IMPORTANT TO HAVE EQUIP STANDING BY IN CASE OF ANOTHER HYD SYS FAILURE. I NOTIFIED CTR WHO IN TURN NOTIFIED APCH CTL AND THE TWR. THE CAPT COMMUNICATED OUR PLAN WITH THE FLT ATTENDANTS AND MADE A VERY WELL THOUGHT OUT PA TO THE PAX. RELATED TO HUMAN FACTORS; I MUST SAY THAT THE CAPT PROVIDED AN EXCELLENT EXAMPLE OF HOW TO MANAGE AN ABNORMAL SITUATION. HE WAS VERY PROACTIVE IN REVIEWING THE ECAM PROCS EVEN BEFORE THE ECAM OCCURRED. HE TOOK EVERYONE'S INFO AND MADE A CLR AND CONCISE DECISION WHICH RESULTED IN A NORMAL LNDG AND TAXI IN TO THE GATE. GREAT JOB! ON THE OTHER SIDE; DURING MY COM WITH ATC; I WAS ASKED FOR FOB (IN HRS) AND SOULS ON BOARD. I COMMUNICATED THIS INFO TO THE HIGH SECTOR AND WHEN SWITCHED TO THE LOW SECTOR CTLR WAS ASKED FOR THE NATURE OF THE EMER; FOB AND SOULS ONBOARD AGAIN. THIS OCCURRED AGAIN WITH APCH CTL. ALSO APCH CTL HELD US UP TOO HIGH TO MAKE A NORMAL DSCNT BECAUSE OF TFC. WHEN AN ACFT DECLARES AN EMER THEY SHOULD BE GIVEN PRIORITY OVER ALL TFC. IN OUR SLIGHTLY DEGRADED MODE THIS WAS NOT AN ISSUE; HOWEVER; IT IS NOT THE RESPONSIBILITY OF THE APCH CTLR TO ASSESS OUR SITUATION. ATC NEEDS TO DO BETTER COMMUNICATING INFO ABOUT FLTS WHO DECLARE EMERS BECAUSE AFTER GIVING FOB IN MIN/HRS; SOULS ON BOARD AND THE NATURE OF THE EMER 3 TIMES TO 3 SEPARATE CTLRS (HIGH SECTOR; LOW SECTOR AND APCH CTL) PROVIDING THIS INFO BECAME A DISTR TO THE FLT CREW AT A TIME WHEN OUR ATTN NEEDED TO BE FOCUSED ON THE SITUATION.