Narrative:

Enroute at FL320; first officer noted sound coming from his oxygen mask. [He] looked at flow indicator on regulator which showed flow. Once first officer pulled mask down from hanger; the flowing sound was audible from captain's seat. [He] attempted to reset check valve via various means with no positive outcome [and] secured oxygen supply at regulator. Bottle pressure was at approximately 450 psi. [We] went into operations manual to determine duration of available oxygen. Had some discussion as to how to interpret chart; contacted dispatch who in turn contacted maintenance control. Maintenance control said; 'you have zero.' went into QRH for loss of all flight crew oxygen; which has us land as soon as practical; taking into account oxygen service etc. Dispatch agreed that the nearby airport was best candidate; not only closest but they had an oxygen bottle in stock. [We] landed approximately 25 minutes later. Of note; the maintenance sign off (done by contract maintenance at the divert station) was that the regulator checks ok on deck. The contract maintenance person told first officer and me that maintenance control's position was that if the regulator works on the ground; it'll work in the air. The issue did recur on our way from to our destination again. This time we were watchful for it. Flight was planned at FL260 for maximum ground speed (2 hours late at this point); and cabin altitude at FL260 worked out to approximately 5;000 ft. First officer pulled his mask off its hanger; and at that point it started to flow (flow indicator confirmed this). The flow rate wasn't as high as had been noted on the prior flight when we were at FL320 (with cabin altitude somewhere above 7;000 ft). In this case you had to hold the mask up to your ear to hear the flowing of oxygen. My thoughts are that as the pressure differential across the valve in the regulator increased the more unseated the valve would become. Having no knowledge of the inner workings of the regulator; my guess is that there's a weak spring in there somewhere. We handled this situation thusly: first officer secured oxygen supply at his regulator and hanged the observer's mask at first officer station for quick donning for the remainder of the flight. I concurred this was a reasonable course of action. We landed at our planned destination uneventfully with an oxygen tank showing 1;800 psi. So; two issues here - firstly; the chart I referred to; and secondly the oxygen regulator itself. I have no idea of what the incidence of failure of this particular component is; but if we are starting to see oxygen bottles that are inexplicably depleted; then the failure of this component is of sufficient gravity that perhaps this situation warrants further investigation. It was unclear to us just what the oxygen duration chart was telling us - maintenance control cleared that up for us. Our thinking had been that perhaps we could extrapolate -- after all; does it not stand to reason that if 600 psi gives you; say; an hour of usable oxygen; then 450 psi should give you (1 hour- x)? Given that this is not the case; then why not have it written in the area below the line on the chart some words to the effect of 'assume zero oxygen available in this area.' with such wording there would be no room for discussion and the diversion decision could then be made that much sooner.

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Original NASA ASRS Text

Title: A MD80 First Officer's oxygen mask allowed a large flow discharge while in cruise at FL320. The aircraft diverted to a nearby airport where the oxygen bottle was replaced because it was depleted. Maintenance detected no flow on the ground but when the flight continued to its destination at cruise the mask leak continued.

Narrative: Enroute at FL320; First Officer noted sound coming from his oxygen mask. [He] looked at flow indicator on regulator which showed flow. Once First Officer pulled mask down from hanger; the flowing sound was audible from Captain's seat. [He] attempted to reset check valve via various means with no positive outcome [and] secured oxygen supply at regulator. Bottle pressure was at approximately 450 PSI. [We] went into operations manual to determine duration of available oxygen. Had some discussion as to how to interpret chart; contacted Dispatch who in turn contacted Maintenance Control. Maintenance Control said; 'You have zero.' Went into QRH for loss of all flight crew oxygen; which has us land as soon as practical; taking into account oxygen service etc. Dispatch agreed that the nearby airport was best candidate; not only closest but they had an oxygen bottle in stock. [We] landed approximately 25 minutes later. Of note; the maintenance sign off (done by Contract Maintenance at the divert station) was that the regulator checks OK on deck. The Contract Maintenance person told First Officer and me that Maintenance Control's position was that if the regulator works on the ground; it'll work in the air. The issue did recur on our way from to our destination again. This time we were watchful for it. Flight was planned at FL260 for maximum ground speed (2 hours late at this point); and cabin altitude at FL260 worked out to approximately 5;000 FT. First Officer pulled his mask off its hanger; and at that point it started to flow (flow indicator confirmed this). The flow rate wasn't as high as had been noted on the prior flight when we were at FL320 (with cabin altitude somewhere above 7;000 FT). In this case you had to hold the mask up to your ear to hear the flowing of oxygen. My thoughts are that as the pressure differential across the valve in the regulator increased the more unseated the valve would become. Having no knowledge of the inner workings of the regulator; my guess is that there's a weak spring in there somewhere. We handled this situation thusly: First Officer secured oxygen supply at his regulator and hanged the observer's mask at First Officer station for quick donning for the remainder of the flight. I concurred this was a reasonable course of action. We landed at our planned destination uneventfully with an oxygen tank showing 1;800 PSI. So; two issues here - firstly; the chart I referred to; and secondly the oxygen regulator itself. I have no idea of what the incidence of failure of this particular component is; but if we are starting to see oxygen bottles that are inexplicably depleted; then the failure of this component is of sufficient gravity that perhaps this situation warrants further investigation. It was unclear to us just what the oxygen duration chart was telling us - Maintenance Control cleared that up for us. Our thinking had been that perhaps we could extrapolate -- after all; does it not stand to reason that if 600 PSI gives you; say; an hour of usable oxygen; then 450 PSI should give you (1 hour- x)? Given that this is not the case; then why not have it written in the area below the line on the chart some words to the effect of 'assume zero oxygen available in this area.' With such wording there would be no room for discussion and the diversion decision could then be made that much sooner.

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.