A B737-500'S 'A' SYS HYD QUANTITY DECREASED PROMPTING THE CREW TO DECLARE AN EMER AND SHUTDOWN THE 'A' HYD SYS.

Narrative

JUST AFTER TOP OF DSCNT WE NOTICED THE MASTER CAUTION BRIEFLY ILLUMINATE. THIS HAPPENED ABOUT 3 TIMES WITHIN 30 SECONDS ACCOMPANIED BY THE HYD ANNUNCIATION. LOOKING AT THE OVERHEAD PANEL THE HYD SYS A ENG LOW PRESSURE LIGHT WAS ON AND NOW REMAINED ON. THE SYS A HYD QUANTITY INDICATED 19% AND REMAINED AT 19%. THE FO WAS THE PF AND I INSTRUCTED THE FO THAT HE HAD THE ACFT AND ATC COMS WHILE I RETRIEVED THE FLT MANUAL FOR THE IRREGULAR PROC. THE FO ADVISED ATC OF OUR SITUATION AND REQUESTED VECTORS WHILE WE WORKED THROUGH THE CHKLIST. WE ACCOMPLISHED THE 'HYD SYS A QUANTITY OR PRESSURE LOSS' CHKLIST. WE CONTACTED DISPATCH AND MAINT BY RADIO AND ADVISED THEM OF OUR SITUATION. WE DECLARED WITH ATC THAT WE WERE AN EMER ACFT AND TO HAVE THE FIRE EQUIP STANDING BY. I CALLED THE FLT ATTENDANTS AND EXPLAINED THE DETAILS OF OUR MECHANICAL ABNORMALITY AND AT THIS POINT WE FULLY EXPECT A NORMAL LNDG. ON THE LOC ATC ISSUED A WINDSHEAR ALERT FOR THE APCH END OF OUR RWY OF 'GAIN OF 20 KTS.' WE DISCUSSED THE WINDSHEAR AND ARPT CONDITIONS AND ELECTED TO ABORT THE APCH; ADVISED ATC AND WERE GIVEN VECTORS OFF THE LOC. WE ASKED AND ATC ADVISED THAT THE WINDSHEAR ALERT HAD CEASED AND WE ADVISED WE WERE READY FOR THE APCH. WHILE INBOUND ON THE APCH ATC AGAIN ADVISED OF A WINDSHEAR ALERT FOR THE APCH END OF THE RWY; A 25 KT GAIN. WE AGAIN BROKE OFF THE APCH TO WAIT FOR THE WINDSHEAR TO CEASE. AFTER FURTHER DISCUSSION WITH ATC IT APPEARED THAT THIS WAS A GUST FRONT THAT WOULD PASS AND THERE WAS NO INDICATION OF ANY OTHER GUST FRONTS OR WINDSHEAR OTHER THAN THE ONE LINE THAT WAS PASSING THE ARPT. WHEN THE SECOND WINDSHEAR CEASED WE ASKED FOR THE APCH. AT THIS POINT WE SWITCHED ROLES OF PF AND PNF AND THE CAPT PERFORMED THE APCH AND LNDG.