B737-800
Flight Crew Air Transport Pilot (ATP)
Aircraft Equipment Problem Less Severe
General Maintenance Action
Aircraft; Procedure
I thought we had ironed out the cockpit oxygen bottle valve issues but they must still be around. Today during our preflight we both just happened to initiate the Cockpit Oxygen Prefight at exactly the same instant. Looking at the gage during the test the pressure dropped from 1300 PSI to 150 PSI. Realizing we were doing the test simultaneously we re-attempted it individually. This time the pressure dropped slightly more than 100 PSI. Referencing the test procedure in the FOM (Flight Operations Manual) I called Maintenance out to check the valve. The limit if you want to interpret it as such is a decrease of more than 100 PSI which we barely had. Maintenance reported that the valve was not fully opened. He opened it all the way and we tested each of our systems successfully. We then retested simultaneously and as in the individual tests there was no pressure drop at all. So to recap; we might have missed a partially open valve as our 'individual' tests displayed only slightly greater than a 100 PSI drop. However; I believe that had we had explosive depressurization at FL380 and were both trying to breath at the same time (simultaneous test at sea level) we would not have been able to breathe let alone survive with a T.U.C. of less than 20 seconds.I believe we should expand our test procedures to include the following: 'In the event of a pressure drop of approximately 100 PSI during the oxygen test; perform a simultaneous test of both pilots oxygen. If this results in a rapid change in pressure greater than 100 PSI make a logbook entry and inform Maintenance.' Maintenance should also increase our mechanics awareness of the hazard to the flight by not confirming that the valve is fully open. Perhaps this should be a maintenance item that requires a post installation sign off by a different individual than the mechanic who installed the new bottle.
B737 Captain reports noting a large pressure drop during a simultaneous test of the oxygen system; although the drop is normal when one system is tested at a time. Maintenance is called and the supply valve is found to be not fully open.