Narrative:

I was captain on air carrier flight XXX (radio call flight XXX) lax-sfo on may/xa/00. Just prior to leveloff at FL310, over rzs VOR, noticed 'a' hydraulic quantity declining. Quantity declined to 5%, followed by low pressure warning lights. Ran irregular checklist for 'a' hydraulic system pressure loss. Elected to continue to sfo to allow time for contact with dispatch and maintenance, and to allow flight attendants to 'prepare' the cabin. WX was better at sfo. We coordinated with dispatch and maintenance, declared an emergency with ATC, and asked for a holding pattern on the arrival to manually lower the landing gear. After successfully lowering the landing gear, I flew an ILS/visual approach to runway 28R at sfo. I instructed the flight attendants to have the passenger assume the 'brace' position for landing. I wanted to ensure that the passenger would be in a safe position if a gear collapsed upon landing. We cleared the runway, stopped the aircraft, and were towed to the gate. Upon postflt inspection maintenance found a broken valve in the landing gear hydraulic lines. The 'a' system quantity was zero. Callback conversation with reporter revealed the following information: the reporter stated the loss of 'a' hydraulic pressure and quantity was caused by a shutoff valve that allows the power xfer unit to extend the leading edge devices with loss of 'B' system.

Google
 

Original NASA ASRS Text

Title: A B737-500 IN CRUISE AT FL310 DECLARED AN EMER DUE TO LOSS OF 'A' SYS HYD PRESSURE AND QUANTITY CAUSED BY A FAILED PWR XFER VALVE.

Narrative: I WAS CAPT ON ACR FLT XXX (RADIO CALL FLT XXX) LAX-SFO ON MAY/XA/00. JUST PRIOR TO LEVELOFF AT FL310, OVER RZS VOR, NOTICED 'A' HYD QUANTITY DECLINING. QUANTITY DECLINED TO 5%, FOLLOWED BY LOW PRESSURE WARNING LIGHTS. RAN IRREGULAR CHKLIST FOR 'A' HYD SYS PRESSURE LOSS. ELECTED TO CONTINUE TO SFO TO ALLOW TIME FOR CONTACT WITH DISPATCH AND MAINT, AND TO ALLOW FLT ATTENDANTS TO 'PREPARE' THE CABIN. WX WAS BETTER AT SFO. WE COORDINATED WITH DISPATCH AND MAINT, DECLARED AN EMER WITH ATC, AND ASKED FOR A HOLDING PATTERN ON THE ARR TO MANUALLY LOWER THE LNDG GEAR. AFTER SUCCESSFULLY LOWERING THE LNDG GEAR, I FLEW AN ILS/VISUAL APCH TO RWY 28R AT SFO. I INSTRUCTED THE FLT ATTENDANTS TO HAVE THE PAX ASSUME THE 'BRACE' POS FOR LNDG. I WANTED TO ENSURE THAT THE PAX WOULD BE IN A SAFE POS IF A GEAR COLLAPSED UPON LNDG. WE CLRED THE RWY, STOPPED THE ACFT, AND WERE TOWED TO THE GATE. UPON POSTFLT INSPECTION MAINT FOUND A BROKEN VALVE IN THE LNDG GEAR HYD LINES. THE 'A' SYS QUANTITY WAS ZERO. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE LOSS OF 'A' HYD PRESSURE AND QUANTITY WAS CAUSED BY A SHUTOFF VALVE THAT ALLOWS THE PWR XFER UNIT TO EXTEND THE LEADING EDGE DEVICES WITH LOSS OF 'B' SYS.

Data retrieved from NASA's ASRS site as of July 2007 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.