LGT DECLARES EMERGENCY DUE TO HYDRAULIC SYSTEM FAILURE.

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AT APPROX XA10 DCT; WE EXPERIENCED A RAPID AND TOTAL LOSS OF 'A' SYS HYD PRESSURE AND QUANTITY. WE WERE JUST ENTERING THE NY ATC SECTOR. WE IMMEDIATELY PERFORMED THE INITIAL ACTION ITEMS FOR HYD LEAK OR LOSS; AND THEN COMPLETED THE FOLLOW UP CHKLIST FOR SYS A HYD LOSS. WE INFORMED COMPANY; ATC AND THE 'A' LINE F/A OF THE PROBLEM. DUE TO THE NECESSITY OF A 15 DEG FLAP LNDG; AND ITS ASSOCIATED INCREASED LNDG SPD; WE ELECTED TO DIVERT FROM LGA TO JFK FOR ADDITIONAL RWY LENGTH. UPON ADVISING ATC OF THE NEED FOR AND EXTENDED FINAL APCH DUE TO THE NECESSITY OF MANUALLY EXTENDING THE GEAR; AND THE PROBABILITY OF REQUIRING TOW ASSISTANCE TO CLEAR THE RWY; DUE TO NO NOSE WHEEL STEERING; ATC INFORMED US THAT USCH HANDLING WOULD ONLY BE AVAILABLE IF WE DECLARED AN EMER; RATHER THAN THE PRIORITY HANDLING WE HAD ORIGINALLY REQUESTED. AT THAT POINT; WE DECLARED AN EMER. WE PERFORMED THE APPROPRIATE DSCNT; APCH AND LNDG CHKLISTS; INCLUDING MANUAL EXTENSION OF THE GEAR AND ALTERNATE EXTENSION OF THE FLAPS (15 DEGS). AFTER GEAR EXTENSION; .8 GALS OF FLUID RETURNED TO THE 'A' SYS; WITH CORRESPONDING PRESSURE. AFTER LNDG WE ACTIVATED THE 'A' SYSTEM PUMPS; AND BOTH PRESSURE AND QUANTITY HELD. WE INFORMED JFK TWR WE WOULD NOT REQUIRE TOW ASSISTANCE; SINCE WE HAD RECOVERED NOSE WHEEL STEERING. WE TERMINATED THE EMER UPON CLRING RWY 31L. S/O INSTRUCTED BOTH THE F/A'S AND GND PERSONNEL NOT TO OPEN THE AFT AIRSTAIRS DOOR; DUE TO THE POSSIBILITY OF TOXIC FUMES. PAX DEPLANING WAS DONE AT THE JETWAY; AND THE ABNORMAL SIT WAS CONCLUDED W/O INCIDENT.