Narrative:

In cruise; at FL350 we started getting fluctuations in our crew oxygen pressure indications between 800-1000 psi. We contacted dispatch/maintenance and informed them of the problem and maintenance informed us that there was history regarding several svcings of the system. However; nothing had appeared in the log history informing us of this. As a crew and with dispatch/maintenance concurrence we decided to return to ZZZ. After some difficulty and time via HF we finally obtained a clearance to return and initiated a right 180 degree turn back to ZZZ. Approximately 5 mins later at XA12Z; the pressure reading dropped below 500 psi; which generated an EICAS message which we followed with the fom irregular procedure to descend to 10000 ft. After several unsuccessful attempts to obtain this clearance; declared an emergency and initiated a descent to 10000 ft. At this point the oxygen pressure read 0 psi. I had the relief pilot obtain 2 cabin oxygen bottles; in the event a pressure problem might occur on the descent. After leveloff we returned the bottles to the cabin as they were not secured adequately; and there was no further need for them. The remainder of the flight was uneventful and made a normal landing at ZZZ. The entire flight/cabin crew and dispatch/maintenance is to be commended for their utmost professionalism and invaluable support. Unfortunately; upon arrival at ZZZ; maintenance checked the oxygen level and it was normal; and attempted to hurry us out by just deferring the indication; which we would have found unacceptable had we been legal to continue. With no adequate method of knowing how much pressure was available; and given the history of this system; further investigation was certainly warranted.

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Original NASA ASRS Text

Title: A B777 OVER THE OCEAN ON AN INTERNATIONAL FLIGHT EXPERIENCED FLUCTUATING CREW OXYGEN SYSTEM PRESSURE. THEY CONSULTED WITH DISPATCH AND MAINTENANCE; DECLARED AN EMERGENCY; AND DIVERTED TO A NEARBY AIRPORT; WHERE MAINTENANCE FOUND THE INDICATING SYSTEM AT FAULT.

Narrative: IN CRUISE; AT FL350 WE STARTED GETTING FLUCTUATIONS IN OUR CREW OXYGEN PRESSURE INDICATIONS BTWN 800-1000 PSI. WE CONTACTED DISPATCH/MAINT AND INFORMED THEM OF THE PROB AND MAINT INFORMED US THAT THERE WAS HISTORY REGARDING SEVERAL SVCINGS OF THE SYS. HOWEVER; NOTHING HAD APPEARED IN THE LOG HISTORY INFORMING US OF THIS. AS A CREW AND WITH DISPATCH/MAINT CONCURRENCE WE DECIDED TO RETURN TO ZZZ. AFTER SOME DIFFICULTY AND TIME VIA HF WE FINALLY OBTAINED A CLRNC TO RETURN AND INITIATED A R 180 DEG TURN BACK TO ZZZ. APPROX 5 MINS LATER AT XA12Z; THE PRESSURE READING DROPPED BELOW 500 PSI; WHICH GENERATED AN EICAS MESSAGE WHICH WE FOLLOWED WITH THE FOM IRREGULAR PROC TO DSND TO 10000 FT. AFTER SEVERAL UNSUCCESSFUL ATTEMPTS TO OBTAIN THIS CLRNC; DECLARED AN EMER AND INITIATED A DSCNT TO 10000 FT. AT THIS POINT THE OXYGEN PRESSURE READ 0 PSI. I HAD THE RELIEF PLT OBTAIN 2 CABIN OXYGEN BOTTLES; IN THE EVENT A PRESSURE PROB MIGHT OCCUR ON THE DSCNT. AFTER LEVELOFF WE RETURNED THE BOTTLES TO THE CABIN AS THEY WERE NOT SECURED ADEQUATELY; AND THERE WAS NO FURTHER NEED FOR THEM. THE REMAINDER OF THE FLT WAS UNEVENTFUL AND MADE A NORMAL LNDG AT ZZZ. THE ENTIRE FLT/CABIN CREW AND DISPATCH/MAINT IS TO BE COMMENDED FOR THEIR UTMOST PROFESSIONALISM AND INVALUABLE SUPPORT. UNFORTUNATELY; UPON ARR AT ZZZ; MAINT CHKED THE OXYGEN LEVEL AND IT WAS NORMAL; AND ATTEMPTED TO HURRY US OUT BY JUST DEFERRING THE INDICATION; WHICH WE WOULD HAVE FOUND UNACCEPTABLE HAD WE BEEN LEGAL TO CONTINUE. WITH NO ADEQUATE METHOD OF KNOWING HOW MUCH PRESSURE WAS AVAILABLE; AND GIVEN THE HISTORY OF THIS SYS; FURTHER INVESTIGATION WAS CERTAINLY WARRANTED.

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.