Narrative:

Just before descent into ZZZ the first officer noticed the a hydraulic fluid was down to 25 (from 90). No warning lights were on at the time. We got the book out for the a low quantity/pressure. Shortly after that; we got the engine a hydraulic low pressure light on. We followed the book procedure which turns off both a pumps if the fluid is below 25. The fluid now was at 21. We shut off both pumps and turned on the standby pump and followed the remainder of the flight manual procedure. I notified ATC that we were declaring an emergency for loss of a system. I called the purser and told her our situation and put them on a cabin advisory expecting a normal landing. I notified the passenger that we had lost 1 of our 3 hydraulic system and that we expected a normal landing in ZZZ. I notified dispatch and sent a status report to maintenance that we had declared an emergency and were coming in with a complete loss of a system. ATC was excellent. I asked for a 20 mi final as we had to lower the gear manually. I asked for the longest runway. We were cleared direct ZZZ for runway xxl. I had the fire trucks standing by. Landing gear deployed properly with the t-handles; we had alternate nosewheel steering and it worked properly. Landing was normal and we were able to taxi normally. We went to the B deice pad and maintenance brought a tug. We started the APU; shut down the engines and were tugged to the gate. Apparently the O ring failed at the electric pump and dumped the fluid overboard. The flight attendants were excellent; ATC did a superb job and my copilot (who is new) was stunning. Couldn't ask for better help all around. All backup system worked properly. Remarkable really.

Related reports:

Google
 

Original NASA ASRS Text

Title: AFTER EXPERIENCING THE LOSS OF THE 'A' HYDRAULIC SYSTEM QUANTITY; A B737-300 CREW DECLARED AN EMERGENCY AND LANDED AT THE DEST NEARBY.

Narrative: JUST BEFORE DSCNT INTO ZZZ THE FO NOTICED THE A HYD FLUID WAS DOWN TO 25 (FROM 90). NO WARNING LIGHTS WERE ON AT THE TIME. WE GOT THE BOOK OUT FOR THE A LOW QUANTITY/PRESSURE. SHORTLY AFTER THAT; WE GOT THE ENG A HYD LOW PRESSURE LIGHT ON. WE FOLLOWED THE BOOK PROC WHICH TURNS OFF BOTH A PUMPS IF THE FLUID IS BELOW 25. THE FLUID NOW WAS AT 21. WE SHUT OFF BOTH PUMPS AND TURNED ON THE STANDBY PUMP AND FOLLOWED THE REMAINDER OF THE FLT MANUAL PROC. I NOTIFIED ATC THAT WE WERE DECLARING AN EMER FOR LOSS OF A SYS. I CALLED THE PURSER AND TOLD HER OUR SITUATION AND PUT THEM ON A CABIN ADVISORY EXPECTING A NORMAL LNDG. I NOTIFIED THE PAX THAT WE HAD LOST 1 OF OUR 3 HYD SYS AND THAT WE EXPECTED A NORMAL LNDG IN ZZZ. I NOTIFIED DISPATCH AND SENT A STATUS RPT TO MAINT THAT WE HAD DECLARED AN EMER AND WERE COMING IN WITH A COMPLETE LOSS OF A SYS. ATC WAS EXCELLENT. I ASKED FOR A 20 MI FINAL AS WE HAD TO LOWER THE GEAR MANUALLY. I ASKED FOR THE LONGEST RWY. WE WERE CLRED DIRECT ZZZ FOR RWY XXL. I HAD THE FIRE TRUCKS STANDING BY. LNDG GEAR DEPLOYED PROPERLY WITH THE T-HANDLES; WE HAD ALTERNATE NOSEWHEEL STEERING AND IT WORKED PROPERLY. LNDG WAS NORMAL AND WE WERE ABLE TO TAXI NORMALLY. WE WENT TO THE B DEICE PAD AND MAINT BROUGHT A TUG. WE STARTED THE APU; SHUT DOWN THE ENGS AND WERE TUGGED TO THE GATE. APPARENTLY THE O RING FAILED AT THE ELECTRIC PUMP AND DUMPED THE FLUID OVERBOARD. THE FLT ATTENDANTS WERE EXCELLENT; ATC DID A SUPERB JOB AND MY COPLT (WHO IS NEW) WAS STUNNING. COULDN'T ASK FOR BETTER HELP ALL AROUND. ALL BACKUP SYS WORKED PROPERLY. REMARKABLE REALLY.

Data retrieved from NASA's ASRS site as of May 2009 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.