Narrative:

When we selected slats extend for the approach, the left hydraulic pressure dropped to zero and the left hydraulic pressure low message came on in the annunciator panel. Left hydraulic quantity also increased sharply from 9 quarts to 18 quarts. We accomplished the abnormal procedures wihtout recovering the system and declared an emergency for landing due to the system being inoperative. However, due to the remaining quantity of fluid, I contacted our company maintenance at the same time we reported our impending emergency landing status. Maintenance agreed that this appeared to be a cavitated system, and they suggested cycling the hydraulic pumps on again. After a min, pressure recovered, and the system operated normally for the rest of the flight. Upon reaching the chocks, maintenance found a significant amount of air in the system and went through a bleed procedure. Following that a check of hydraulic component operation was completed by maintenance, and the aircraft was returned to service. I have suggested to our company's technical staff that the abnormal procedures might benefit from alteration to permit multiple cycling attempts if quantity remains in the system. It appears recovery required a period of several mins with the pumps off prior to reactivation.

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Original NASA ASRS Text

Title: AN MD88 ON APCH AT 7000 FT DECLARED AN EMER DUE TO LOSS OF L HYD PRESSURE, QUANTITY OK. FOUND EXCESSIVE AIR IN HYD SYS.

Narrative: WHEN WE SELECTED SLATS EXTEND FOR THE APCH, THE L HYD PRESSURE DROPPED TO ZERO AND THE L HYD PRESSURE LOW MESSAGE CAME ON IN THE ANNUNCIATOR PANEL. L HYD QUANTITY ALSO INCREASED SHARPLY FROM 9 QUARTS TO 18 QUARTS. WE ACCOMPLISHED THE ABNORMAL PROCS WIHTOUT RECOVERING THE SYS AND DECLARED AN EMER FOR LNDG DUE TO THE SYS BEING INOP. HOWEVER, DUE TO THE REMAINING QUANTITY OF FLUID, I CONTACTED OUR COMPANY MAINT AT THE SAME TIME WE RPTED OUR IMPENDING EMER LNDG STATUS. MAINT AGREED THAT THIS APPEARED TO BE A CAVITATED SYS, AND THEY SUGGESTED CYCLING THE HYD PUMPS ON AGAIN. AFTER A MIN, PRESSURE RECOVERED, AND THE SYS OPERATED NORMALLY FOR THE REST OF THE FLT. UPON REACHING THE CHOCKS, MAINT FOUND A SIGNIFICANT AMOUNT OF AIR IN THE SYS AND WENT THROUGH A BLEED PROC. FOLLOWING THAT A CHK OF HYD COMPONENT OP WAS COMPLETED BY MAINT, AND THE ACFT WAS RETURNED TO SVC. I HAVE SUGGESTED TO OUR COMPANY'S TECHNICAL STAFF THAT THE ABNORMAL PROCS MIGHT BENEFIT FROM ALTERATION TO PERMIT MULTIPLE CYCLING ATTEMPTS IF QUANTITY REMAINS IN THE SYS. IT APPEARS RECOVERY REQUIRED A PERIOD OF SEVERAL MINS WITH THE PUMPS OFF PRIOR TO REACTIVATION.

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.