Narrative:

We took the aircraft in the afternoon; to complete our sequence back to the domicile. During our preflight preparation; the first officer noted an unsatisfactory crew oxygen system test. We verified that the oxygen knob (in the cockpit) was fully open; and contacted maintenance. I had already tested my oxygen mask before the first officer noted the needle movement on the pressure gauge (on the overhead panel). I did not see a pressure drop during my test. I looked at the left side panel; moved the reset/ test lever; then depressed the emergency/test selector; verifying oxygen flow on the indicator. Then; with the oxygen still flowing; I looked at the gauge on the overhead panel. It was not moving; indicating constant pressure during the test. What appears significant to me is that the first officer looked overhead at the gauge while sliding the reset/test lever and depressing the emergency/test selector; this revealed a pressure drop of about 200 psi; which then stabilized (the pressure indication did not continue to decrease with oxygen flow). Had he checked the oxygen flow indication first (on the mask panel); then looked overhead; as I had; he would not have seen a pressure change. We would have departed having completed what appeared to be a satisfactory test (it was in accordance with our aom procedures). Our mechanic double-checked that the crew oxygen valve in the cockpit was open; and then checked the valve at the crew oxygen cylinder (accessed through the baggage compartment). He found that valve to be only partially open. He completed the necessary maintenance action; documented it in the logbook; and we departed. During this process our mechanic checked the date of the last oxygen service. It appeared several crews had flown the airplane in this condition. I do not find this surprising. Conducting the oxygen test in accordance with our procedure did not reveal an abnormality; as displayed on the overhead [O2] gauge. I had already completed the test as I always do; and had seen a proper indication. Only by looking at the gauge while initiating the test did we note anything amiss. Our mechanic was very concerned that this had occurred; and that a number of flights had been completed in this condition. I share that concern; and hope we are able to correct this without applying harsh personnel actions. I would proudly fly an aircraft serviced by the mechanics who last worked on this one. I am sure their sense of professionalism will be evident in all their work. Perhaps we could emphasize a check of the oxygen pressure while initiating the test (checking the flow indication afterward; thus modifying our procedure.

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Original NASA ASRS Text

Title: A B737 Captain reports how a minor difference in a crew oxygen mask check procedure revealed a 200 PSI drop in oxygen pressure on the overhead oxygen gauge. Pilot recommends an emphasis on observing overhead O2 gauge when initiating the mask check procedure.

Narrative: We took the aircraft in the afternoon; to complete our sequence back to the domicile. During our preflight preparation; the First Officer noted an unsatisfactory Crew Oxygen System test. We verified that the oxygen knob (in the cockpit) was fully open; and contacted Maintenance. I had already tested my oxygen mask before the First Officer noted the needle movement on the pressure gauge (on the overhead panel). I did not see a pressure drop during my test. I looked at the left side panel; moved the reset/ test lever; then depressed the emergency/test selector; verifying oxygen flow on the indicator. Then; with the oxygen still flowing; I looked at the gauge on the overhead panel. It was not moving; indicating constant pressure during the test. What appears significant to me is that the First Officer looked overhead at the gauge while sliding the reset/test lever and depressing the emergency/test selector; this revealed a pressure drop of about 200 PSI; which then stabilized (the pressure indication did not continue to decrease with oxygen flow). Had he checked the oxygen flow indication first (on the mask panel); then looked overhead; as I had; he would not have seen a pressure change. We would have departed having completed what appeared to be a satisfactory test (it was in accordance with our AOM procedures). Our Mechanic double-checked that the Crew oxygen valve in the cockpit was open; and then checked the valve at the Crew oxygen cylinder (accessed through the baggage compartment). He found that valve to be only partially open. He completed the necessary Maintenance action; documented it in the logbook; and we departed. During this process our Mechanic checked the date of the last oxygen service. It appeared several Crews had flown the airplane in this condition. I do not find this surprising. Conducting the oxygen test in accordance with our procedure did not reveal an abnormality; as displayed on the overhead [O2] gauge. I had already completed the test as I always do; and had seen a proper indication. Only by looking at the gauge while initiating the test did we note anything amiss. Our Mechanic was very concerned that this had occurred; and that a number of flights had been completed in this condition. I share that concern; and hope we are able to correct this without applying harsh personnel actions. I would proudly fly an aircraft serviced by the Mechanics who last worked on this one. I am sure their sense of professionalism will be evident in all their work. Perhaps we could emphasize a check of the oxygen pressure while initiating the test (checking the flow indication afterward; thus modifying our procedure.

Data retrieved from NASA's ASRS site 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.