AN ACR CAPT; PNF; DECLARED AN EMER AFTER THE COMPLETE LOSS OF HYD SYS A DURING APCH TO BOS.

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ABOUT 30 SECONDS TO 1 MIN AFTER GEAR WAS SELECTED DOWN; WE HAD A COMPLETE LOSS OF SYS 'A' HYD. THIS OCCURRED BTWN 1600-1800 FT MSL; ON ABOUT A 5 MI FINAL TO RWY 27 IN BOS. THE FO WAS PF; AND I WAS THE PNF. I DECLARED AN EMER; TOLD THE TWR THE NATURE OF THE EMER; AND WE THEN EXECUTED A MISSED APCH. WE WERE HANDED BACK TO BOS APCH CTL AND I INFORMED THEM THAT I WOULD NEED ABOUT 5 MINS TO COMPLETE CHKLISTS; BEFORE RETURNING TO BOS FOR LNDG. I THEN INFORMED THE PAX; FLT ATTENDANTS ON THE SIT AND REASON FOR THE GAR. I SAID THAT AFTER I RUN SOME CHKLISTS; WHICH WOULD TAKE ABOUT 5 MINS OR SO; I WOULD GET BACK TO THEM AND LET THEM KNOW WHAT WOULD HAPPEN NEXT. RAN THE LOSS OF SYS A HYD CHKLIST IN THE QRH. GEAR WAS ALREADY DOWN; AND FLAPS WERE AT 5 DEGS. I INFORMED THE COMPANY VIA RADIO THE NATURE OF THE EMER; AND TO INFORM OPS. REQUESTED RADAR VECTORS TO RWY 22L. I MADE A PA TO THE PAX AGAIN; LETTING THEM KNOW WE WERE ALL SET UP TO RETURN TO THE FIELD FOR LNDG. I SAID EVERYTHING ABOUT THIS APCH AND LNDG SHOULD FEEL NORMAL TO THEM. THE ONLY DIFFERENCE IS THAT AFTER LNDG; WE WILL BE STOPPING ON THE RWY TO AWAIT A TOW TRUCK (TUG) TO TAKE US TO THE GATE. I ALSO MENTIONED THAT EMER VEHICLES WOULD BE ON THE GND ON MY REQUEST; JUST FOR ADDITIONAL SAFETY. I INFORMED THE FLT ATTENDANTS THAT EVERYTHING WAS GOOD; AND THE LNDG SHOULD BE A NON EVENT; LNDG IN ABOUT 5-7 MINS. WAS TURNED ON ABOUT 15-18 MI FINAL TO INTERCEPT RWY 22L. VMC CONDITIONS. RADAR VECTORS AT 3000 FT MSL THE ENTIRE EVENT. THE RWY WAS 10000 FT IN LENGTH; AND AFTER LNDG WE ALLOWED THE ACFT TO ROLL TOWARD THE END OF THE RWY FOR THE FULL STOP. SET BRAKES; AND SHUT DOWN THE ENGS (APU ON LINE). EMER CREWS INSPECTED THE ACFT; SAID THERE WAS A LITTLE SMOKE COMING FROM THE R MAIN GEAR; BUT AFTER A CLOSER LOOK; SAID IT WAS FROM HYD FLUID DRIPPING ON THE BRAKE AREA; AND THERE WAS NO PROB. MAINT SHOWED UP AND PINNED THE GEAR AND TOWED US TO GATE X IN BOS. WERE MET BY STATION PERSONNEL; OFFERED ANY ASSISTANCE; I THEN WENT TO CALL OPS WITH A RPT.