Narrative:

In preparation for a relocation flight the crew had performed a thorough preflight check of the oxygen system and checked both masks; but then turned the oxygen off to return to the FBO. When we returned to the aircraft to depart; I pulled the oxygen back on; and even checked the position of the handle while running checklists; but did not realize the system was not pulled quite all the way on. While enroute at FL280; approximately 10 minutes into cruise; the passenger oxygen masks dropped in the cabin. The cabin altitude indicated 12;000 ft and the cabin differential was around 5.0 psi. I did not notice the cabin rate. I attempted to lower the cabin altitude with the altitude control knob; and adjusted the cabin rate controller to no avail. I then chose to attempt to cycle the bleed air valves. I turned the left bleed air valve to inst and enviro off; then back on. At approximately the same time the cabin attendant altitude hi master caution light illuminated (as would be expected with decreased airflow). There was dialog between the crew about feeling hypoxic; and I then realized that my faculties were notably decreased. I felt light headed and on the verge of tunnel vision. I assumed control of the aircraft; began a rapid descent; and [advised ATC]. During the descent the cabin altitude continued to rise. The highest cabin altitude indication that I noticed was 15;000 ft.we began to use oxygen; and the first officer pointed out that there was no oxygen flowing (I think that I got the last breath or two of residual oxygen pressure remaining in the system). I saw there was still 1700 psi oxygen; had received some oxygen; and visually saw that the oxygen handle was on. I was not able to wrap my head around the idea that the handle was not pulled all the way on; and was not able to figure it out until we were level at 10;000 ft. If I had handled the situation better; I would've directed the first officer to focus on why there was no oxygen; while I focused on the descent. Regarding my communications with ATC controllers and requested lower. I received multiple step-down altitudes; but should've initially indicated to ATC that I was going to descend to 10;000 ft i.e. My first radio call should've been; 'aircraft X is descending to 10;000 ft' instead of 'aircraft X; we need lower'. Regarding CRM issues; we go to safety conferences and train as single pilot operators. I think that we can improve our operation by spending time simulating emergency situations as a crew and more clearly defining expectations of each crew member. The aircraft was safely returned to the departure airport without further incident.

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Original NASA ASRS Text

Title: BE20 Captain reported a loss of pressurization at FL280; but the crew could not get oxygen from their masks.

Narrative: In preparation for a relocation flight the crew had performed a thorough preflight check of the oxygen system and checked both masks; but then turned the oxygen off to return to the FBO. When we returned to the aircraft to depart; I pulled the oxygen back on; and even checked the position of the handle while running checklists; but did not realize the system was not pulled quite all the way on. While enroute at FL280; approximately 10 minutes into cruise; the passenger oxygen masks dropped in the cabin. The cabin altitude indicated 12;000 ft and the cabin differential was around 5.0 PSI. I did not notice the cabin rate. I attempted to lower the cabin altitude with the altitude control knob; and adjusted the cabin rate controller to no avail. I then chose to attempt to cycle the bleed air valves. I turned the left bleed air valve to Inst and Enviro Off; then back on. At approximately the same time the CAB ALT HI master caution light illuminated (as would be expected with decreased airflow). There was dialog between the crew about feeling hypoxic; and I then realized that my faculties were notably decreased. I felt light headed and on the verge of tunnel vision. I assumed control of the aircraft; began a rapid descent; and [advised ATC]. During the descent the cabin altitude continued to rise. The highest cabin altitude indication that I noticed was 15;000 ft.We began to use oxygen; and the FO pointed out that there was no oxygen flowing (I think that I got the last breath or two of residual oxygen pressure remaining in the system). I saw there was still 1700 PSI oxygen; had received some oxygen; and visually saw that the oxygen handle was on. I was not able to wrap my head around the idea that the handle was not pulled all the way on; and was not able to figure it out until we were level at 10;000 ft. If I had handled the situation better; I would've directed the FO to focus on why there was no oxygen; while I focused on the descent. Regarding my communications with ATC controllers and requested lower. I received multiple step-down altitudes; but should've initially indicated to ATC that I was going to descend to 10;000 ft i.e. my first radio call should've been; 'Aircraft X is descending to 10;000 ft' instead of 'Aircraft X; we need lower'. Regarding CRM issues; we go to safety conferences and train as single pilot operators. I think that we can improve our operation by spending time simulating emergency situations as a crew and more clearly defining expectations of each crew member. The aircraft was safely returned to the departure airport without further incident.

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.