Narrative:

After approximately 2 hours 45 mins after takeoff, #3 engine oil pressure started fluctuating between 40-50 psi continuously. It occasionally dropped to 34 psi (red radial) causing oil pressure low light to come on (oil temperature was steady at 72 degrees C and matched other 3 engines. Contacted maintenance and dispatch who directed return to mia. At XA30Z, precautionary shutdown engine as pressure dropped below red radial every 5-10 seconds. We had no idea of oil quantity as quantity gauge was MEL'ed (deferred). Oil quantity was physically checked by maintenance before flight per MEL. As crew, we determined that return to mia was as safe as diverting to any other airport in area. We had no WX or NOTAMS for other airports. We had WX and NOTAMS for mia, and it was a familiar field. The engine could be restarted if necessary. Idle oil pressure was maintained due to windmilling fan blades. ATC advised of precautionary engine shutdown. Special ATC services were declined. No emergency was declared. All appropriate checklists completed or reviewed. Engine problem would have been detected a lot earlier if the oil quantity gauge had been working. CRM worked. Each member of crew contributed to successful resolution of problem by working as a team and by making individual contributions to the decision making process. Callback conversation with reporter revealed the following information: the reporter stated the loss of oil was caused by a failed o-ring seal in the oil system somewhere. The reporter said maintenance advised an o-ring seal had failed but did not report the component or line. The reporter said no warning of the loss of oil was possible as the #3 engine oil quantity indicator was deferred as inoperative.

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Original NASA ASRS Text

Title: A B747-200 IN CRUISE AT FL330 DIVERTED AND SHUT DOWN #3 ENG DUE TO LOSS OF OIL PRESSURE CAUSED BY A FAILED O-RING SEAL IN THE OIL DISTRIBUTION SYS.

Narrative: AFTER APPROX 2 HRS 45 MINS AFTER TKOF, #3 ENG OIL PRESSURE STARTED FLUCTUATING BTWN 40-50 PSI CONTINUOUSLY. IT OCCASIONALLY DROPPED TO 34 PSI (RED RADIAL) CAUSING OIL PRESSURE LOW LIGHT TO COME ON (OIL TEMP WAS STEADY AT 72 DEGS C AND MATCHED OTHER 3 ENGS. CONTACTED MAINT AND DISPATCH WHO DIRECTED RETURN TO MIA. AT XA30Z, PRECAUTIONARY SHUTDOWN ENG AS PRESSURE DROPPED BELOW RED RADIAL EVERY 5-10 SECONDS. WE HAD NO IDEA OF OIL QUANTITY AS QUANTITY GAUGE WAS MEL'ED (DEFERRED). OIL QUANTITY WAS PHYSICALLY CHKED BY MAINT BEFORE FLT PER MEL. AS CREW, WE DETERMINED THAT RETURN TO MIA WAS AS SAFE AS DIVERTING TO ANY OTHER ARPT IN AREA. WE HAD NO WX OR NOTAMS FOR OTHER ARPTS. WE HAD WX AND NOTAMS FOR MIA, AND IT WAS A FAMILIAR FIELD. THE ENG COULD BE RESTARTED IF NECESSARY. IDLE OIL PRESSURE WAS MAINTAINED DUE TO WINDMILLING FAN BLADES. ATC ADVISED OF PRECAUTIONARY ENG SHUTDOWN. SPECIAL ATC SVCS WERE DECLINED. NO EMER WAS DECLARED. ALL APPROPRIATE CHKLISTS COMPLETED OR REVIEWED. ENG PROB WOULD HAVE BEEN DETECTED A LOT EARLIER IF THE OIL QUANTITY GAUGE HAD BEEN WORKING. CRM WORKED. EACH MEMBER OF CREW CONTRIBUTED TO SUCCESSFUL RESOLUTION OF PROB BY WORKING AS A TEAM AND BY MAKING INDIVIDUAL CONTRIBUTIONS TO THE DECISION MAKING PROCESS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE LOSS OF OIL WAS CAUSED BY A FAILED O-RING SEAL IN THE OIL SYS SOMEWHERE. THE RPTR SAID MAINT ADVISED AN O-RING SEAL HAD FAILED BUT DID NOT RPT THE COMPONENT OR LINE. THE RPTR SAID NO WARNING OF THE LOSS OF OIL WAS POSSIBLE AS THE #3 ENG OIL QUANTITY INDICATOR WAS DEFERRED AS INOP.

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.