Narrative:

I went through my preflight like normal on aircraft X. I did the interior preflight up front as my first officer was in the back putting the stock back on board. I ran through the checklist and did the oxygen check like normal on both masks. They checked fine and normal. Both inflated and had pressure when checked oxygen showed in the green arc at 1;600 psi. Going through FL180 we did our checklist and checked the oxygen. My first officer did his first and was normal sound that came out. I did mine and a little bit of hiss was heard but not what I was used to hearing. I then went to test it again and no more hiss was heard. Pulled the mask out and could not get any flow. Tried my first officer's mask again and no flow would come out of his either. At this point in time since I realized they had changed the bottle on the ground at the airport we departed that it may not have been turned on. We asked ATC for lower and at this point I called the company and told them the situation. After conferencing with acp; maintenance; dispatch and customs; we were only 85 miles from ZZZ and coming back down through about FL190; that it was decided to divert to ZZZ. We landed at ZZZ without incident. Upon shutting down and getting the plane into the hangar; we could hear hissing coming from the pilot's oxygen mask. Tried to test the mask again and it would inflate (harness part that goes around the head) but you could only do it once and then there was no more left to do it a second time. Let it sit a few more times (usually 5 minutes or longer) and it would do the same thing but would not inflate a second time or right away afterwards. Once you inflated once nothing else would come out unless you would let it sit. In talking with the maintenance guy at ZZZ he was saying it should not be able to do that if the oxygen was turned off at the bottle. He hadn't gotten to the bottle by the time I left to know for sure what the problem was at the time. Callback conversation with reporter revealed the following information: reporter stated he did not know if the crew oxygen bottle was a steel or composite type bottle. After landing; he could not tell if the hissing sound he heard coming from his mask was from the mask hose or the mask oxygen regulator.

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Original NASA ASRS Text

Title: Captain reports diverting his Cessna 560 Citation Ultra due to lack of crew oxygen from his and the First Officer's oxygen masks. Captain also notices a hissing sound coming from his mask both in-flight and after landing.

Narrative: I went through my preflight like normal on Aircraft X. I did the interior preflight up front as my First Officer was in the back putting the stock back on board. I ran through the checklist and did the oxygen check like normal on both masks. They checked fine and normal. Both inflated and had pressure when checked oxygen showed in the green arc at 1;600 PSI. Going through FL180 we did our checklist and checked the oxygen. My First Officer did his first and was normal sound that came out. I did mine and a little bit of hiss was heard but not what I was used to hearing. I then went to test it again and no more hiss was heard. Pulled the mask out and could not get any flow. Tried my First Officer's mask again and no flow would come out of his either. At this point in time since I realized they had changed the bottle on the ground at the airport we departed that it may not have been turned on. We asked ATC for lower and at this point I called the company and told them the situation. After conferencing with ACP; Maintenance; Dispatch and Customs; we were only 85 miles from ZZZ and coming back down through about FL190; that it was decided to divert to ZZZ. We landed at ZZZ without incident. Upon shutting down and getting the plane into the hangar; we could hear hissing coming from the Pilot's oxygen mask. Tried to test the mask again and it would inflate (harness part that goes around the head) but you could only do it once and then there was no more left to do it a second time. Let it sit a few more times (usually 5 minutes or longer) and it would do the same thing but would not inflate a second time or right away afterwards. Once you inflated once nothing else would come out unless you would let it sit. In talking with the maintenance guy at ZZZ he was saying it should not be able to do that if the oxygen was turned off at the bottle. He hadn't gotten to the bottle by the time I left to know for sure what the problem was at the time. Callback conversation with Reporter revealed the following information: Reporter stated he did not know if the crew oxygen bottle was a steel or composite type bottle. After landing; he could not tell if the hissing sound he heard coming from his mask was from the mask hose or the mask oxygen regulator.

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.