Flight Crew Air Transport Pilot (ATP)
Flight Crew Total 22707; Flight Crew Type 11458
Troubleshooting
Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy
General Maintenance Action
Human Factors; Procedure
While conducting our cockpit preflight of the crew oxygen system we observed a drop from approximately 1700 PSI to 160 PSI. Upon notifying maintenance it was discovered that the crew oxygen valve had been turned nearly completely off. We had flown this same aircraft to ZZZZ three days prior from ZZZ and the O2 had tested properly before departure. ZZZZ Maintenance had not serviced the system upon our arrival. The airplane then returned to ZZZ prior to returning to ZZZZ for today's flight. So I conclude that the servicing error had happened in ZZZ. This is the third instance of this exact scenario that I have discovered while pre-flighting B767 aircraft during approximately the past 5 months; and the prior two instances also took place in ZZZ. Anecdotally my First Officer on today's flight told me that another captain had recently told him the same thing happened on his flight departing from ZZZ.Considering I have flown a relatively small sample of flights from ZZZ during the past 5 months; such a high number of instances of this error is especially alarming and I think needs some high level attention. It is likely that it is happening on other flights if it has happened on 3 of mine. Clearly if not detected during pilot pre-flight it presents a huge safety threat. Two maintenance techs have now told me that their procedure when servicing crew O2 is to open the valve fully; then turn it back a small amount. It appears in my three instances that they did the exact opposite--turning it fully off then barely cracking it open; as the O2 levels did return slowly to normal after depleting during the test.
B767 Captain reported that during preflight the crew oxygen system did not test properly; and the oxygen bottle valve was found to not be fully opened.