Date: 201512
Local time of day: 1801-2400
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Night
Aircraft: B737-800
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Oxygen System/Crew
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Confusion
Anomaly: Aircraft Equipment Problem Critical; Flight Deck / Cabin / Aircraft Event Other / Unknown; Inflight Event / Encounter Other / Unknown
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Overcame Equipment Problem; General Maintenance Action
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: Prior to flight; Crew Oxygen system gauge read 800+ psi. Both CA and FO successfully accomplished Oxygen System check IAW FM. Flight was normal until 1:45 into flight at FL380 when we planned to take a restroom break. The FO discovered his oxygen mask had no flow. I checked mine; discovering the same thing. Further inspection revealed the Crew Oxygen gauge read Zero. We checked the QRH; discovering no guidance on this malfunction. Though no current issue existed; we were concerned about a potential cabin altitude problem or rapid depressurization; so we coordinated a descent to lower altitude while we discussed continuing to destination or diversion. A quick review of the FOM and FM revealed no guidance on this particular malfunction. We ultimately decided we needed to get to 10;000 feet since we had no emergency back-up source of crew oxygen. We needed 5.9 fuel at destination and FMC said we should land with 6.4 after uploading current winds. The TAF for ZZZ1 said we should have 6nm 8 SCT 15 BKN; which was also the current METAR. We coordinated with Dispatch; he confirmed we should land with 6.4 and could find no guidance against continuing to original destination. We briefed the FAs; but not the passengers (our descent was gradual; it was night with under cast and we thought it would be on social media immediately upon landing. No passenger commented on being at lower altitude during the remainder of flight or during deplaning). We also sent an ACARS message to Maintenance Control about the problem and made a logbook entry (we did not get a response from Maintenance Control). We had to maneuver to avoid some low altitude storms around ZZZ2; but the flight was otherwise uneventful through landing. We actually landed with 6.5. Maintenance did not meet us so I called Maintenance Control to ensure they knew of the problem.

Callback: Reporter stated that he and the First Officer performed a 15 second Crew O2 pressure drop check during their preflight; even though their company procedures only require a 10 second test. They both noted the crew oxygen pressure held steady during that test with no pressure drop indicated and holding at 800 psi. Reporter stated they were quite surprised when they realized the crew O2 was not only reading 0 PSI on the indicator; but they definitely could not get any oxygen from their O2 masks while flying at 38;000 feet. They do not have a portable oxygen bottle in the cockpit. He realizes that common sense should prevail and flight crews should begin descending rather quickly when faced with no crew oxygen. But; not having any QRH; FOM; or FM guidance procedure for their type of situation seemed unusual. Especially since some type of procedure would help flight crews coordinate with ATC; Dispatch; Maintenance; and issues involving fuel burn at lower altitudes or if smoke or fire should occur in the cockpit when no oxygen is available. Reporter stated the crew O2 bottles are removed for servicing. He didn't know if the crew O2 bottle was a steel or composite material. Maintenance performed some troubleshooting on the crew O2 System and later informed him they did not find any malfunctions; loose fittings; or the O2 bottle handle not fully opened. Aircraft was released for service.



Synopsis: Captain reports that he and the First Officer noticed the crew oxygen pressure gauge in their B737-800 aircraft read zero at FL380. They had performed a 15 second crew O2 mask check during preflight with the O2 pressure holding steady at 800 psi. A quick review of their QRH; FOM; and FM revealed no guidance existed for that type of malfunction. Maintenance troubleshooting could not determine the cause of oxygen loss. The aircraft was released for service.