Narrative:

Shortly after reaching cruise altitude one hydraulic B system low pressure light illuminated large transport accompanied by a slight drop in B system pressure. This was followed about 20 seconds later by a pressure low light on the second hydraulic B pump and a complete loss of system B hydraulic pressure. B system hydraulic quantity remained full throughout the incident. We descended initially to a lower altitude for 1 yaw damper inoperative limitations. The captain and F/east worked through the checklists with no result. A system quantity was slightly low at 2 gals but quantity and pressure were steady. This problem was perplexing because B quantity remained full. We suspected that there was actually a loss of B system fluid that was not reflected on the gauge due to gauge failure. This seemed much more likely than failure of 2 independent electric pumps within a short period of time. After consulting with company we continued on to the destination due to WX conditions. Passenger and F/a's were briefed on the situation. ATC was advised and arranged for crash trucks. Landing was normal except for flap 15 degrees per the checklist and some uncertainty about normal brake operation since brakes are normally pwred by B system. The hydraulic brake interconnect which allows a system hydraulic pressure to operate the brakes was opened on short final per the checklist and the brakes worked normally on roll out. We had a brief discussion with the mechanics. They indicated that there was a loss of fluid in the system. The hydraulic system had been serviced the night before (our's was the first departure of the day) and there had been a previous write up for some type of hydraulic low pressure.

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Original NASA ASRS Text

Title: ACFT EXPERIENCED LOSS OF HYDRAULIC PRESSURE IN SYSTEM B.

Narrative: SHORTLY AFTER REACHING CRUISE ALT ONE HYD B SYS LOW PRESSURE LIGHT ILLUMINATED LGT ACCOMPANIED BY A SLIGHT DROP IN B SYS PRESSURE. THIS WAS FOLLOWED ABOUT 20 SECS LATER BY A PRESSURE LOW LIGHT ON THE SECOND HYD B PUMP AND A COMPLETE LOSS OF SYS B HYD PRESSURE. B SYS HYD QUANTITY REMAINED FULL THROUGHOUT THE INCIDENT. WE DSNDED INITIALLY TO A LOWER ALT FOR 1 YAW DAMPER INOP LIMITATIONS. THE CAPT AND F/E WORKED THROUGH THE CHKLISTS WITH NO RESULT. A SYS QUANTITY WAS SLIGHTLY LOW AT 2 GALS BUT QUANTITY AND PRESSURE WERE STEADY. THIS PROB WAS PERPLEXING BECAUSE B QUANTITY REMAINED FULL. WE SUSPECTED THAT THERE WAS ACTUALLY A LOSS OF B SYS FLUID THAT WAS NOT REFLECTED ON THE GAUGE DUE TO GAUGE FAILURE. THIS SEEMED MUCH MORE LIKELY THAN FAILURE OF 2 INDEPENDENT ELECTRIC PUMPS WITHIN A SHORT PERIOD OF TIME. AFTER CONSULTING WITH COMPANY WE CONTINUED ON TO THE DEST DUE TO WX CONDITIONS. PAX AND F/A'S WERE BRIEFED ON THE SITUATION. ATC WAS ADVISED AND ARRANGED FOR CRASH TRUCKS. LNDG WAS NORMAL EXCEPT FOR FLAP 15 DEGS PER THE CHKLIST AND SOME UNCERTAINTY ABOUT NORMAL BRAKE OPERATION SINCE BRAKES ARE NORMALLY PWRED BY B SYS. THE HYD BRAKE INTERCONNECT WHICH ALLOWS A SYS HYD PRESSURE TO OPERATE THE BRAKES WAS OPENED ON SHORT FINAL PER THE CHKLIST AND THE BRAKES WORKED NORMALLY ON ROLL OUT. WE HAD A BRIEF DISCUSSION WITH THE MECHS. THEY INDICATED THAT THERE WAS A LOSS OF FLUID IN THE SYS. THE HYD SYS HAD BEEN SERVICED THE NIGHT BEFORE (OUR'S WAS THE FIRST DEP OF THE DAY) AND THERE HAD BEEN A PREVIOUS WRITE UP FOR SOME TYPE OF HYD LOW PRESSURE.

Data retrieved from NASA's ASRS site as of August 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.