A B747-400 flight crew on an international route reported noticing an EICAS message 'crew oxygen low...

Attributes
ACN	894727
Time
Date	201006
Local Time Of Day	1201-1800
Place
Locale Reference	ZZZZ.ARTCC
State Reference	FO
Aircraft 1
Make Model Name	B747-400
Operating Under FAR Part	Part 121
Flight Phase	Cruise
Component
Aircraft Component	Oxygen System/Crew
Person 1
Function	Captain Pilot Flying
Qualification	Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Last 90 Days 155 Flight Crew Total 21000 Flight Crew Type 2900
Person 2
Function	Pilot Not Flying First Officer
Qualification	Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Last 90 Days 200
Events
Anomaly	Aircraft Equipment Problem Critical

Narrative:

90 minutes after takeoff at FL300 cruise; EICAS caution of crew oxygen low. We looked at the status message and observed 0 psi O2 pressure. During preflight it was approximately 1750 psi. Status message also observed. Passenger O2 was observed at 1800 psi. We asked for a lower altitude and started a descent initially to FL200. During descent crew O2 pressure came back to normal at 1700 psi. After initial level off crew O2 pressure then went to zero again. At this point we had no real confidence in our O2 quantity. I felt we had crew oxygen just not a reliable indication. I contacted dispatch and reported the situation. I made it clear I would not go polar or over high terrain. I suggested we return to [departure airport]. It was agreed to return so the airplane could be repaired and we could accommodate the crew and passengers. We dumped 150;000 pounds of fuel and an uneventful landing was made. There was a history of this malfunction last week.

Original NASA ASRS Text

Title: A B747-400 flight crew on an international route reported noticing an EICAS message 'Crew Oxygen Low;' and the pressure fluctuated from zero to normal and back again. They returned to their departure airport after consulting with Dispatch.

Narrative: 90 minutes after takeoff at FL300 cruise; EICAS caution of Crew Oxygen low. We looked at the status message and observed 0 psi O2 pressure. During preflight it was approximately 1750 PSI. Status message also observed. Passenger O2 was observed at 1800 PSI. We asked for a lower altitude and started a descent initially to FL200. During descent Crew O2 pressure came back to normal at 1700 PSI. After initial level off crew O2 pressure then went to zero again. At this point we had no real confidence in our O2 quantity. I felt we had crew oxygen just not a reliable indication. I contacted Dispatch and reported the situation. I made it clear I would not go polar or over high terrain. I suggested we return to [departure airport]. It was agreed to return so the airplane could be repaired and we could accommodate the crew and passengers. We dumped 150;000 pounds of fuel and an uneventful landing was made. There was a history of this malfunction last week.

Data retrieved from NASA's ASRS site as of April 2012 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.

A B747-400 flight crew on an international route reported noticing an EICAS message 'crew oxygen low;' and the pressure fluctuated from zero to normal and back again. They returned to their departure airport after consulting with dispatch.