Narrative:

During climb my 'cabin altitude' annunciator was activated. My cabin pressure had increased to approximately 10;000 ft. Under normal operations my cabin altitude should have been below 8;000 ft.I was already using the supplemental O2 system.I declared an emergency and requested an immediate descent which was approved along with a turn back toward my departure airport when able. I began an emergency descent then turned to the heading given by ATC. At that time I was unsure as to the cause of the sudden rise in cabin pressure and was focused on making a controlled descent while on supplemental O2. I then requested a return to the cessna citation service center airport where my airplane had just completed routine scheduled maintenance. It had not been flown since the maintenance was completed other than a short flight back to my own airport.after landing I began trouble shooting the probable causes associated with a 'cabin altitude' annunciator and determined that the 'air source select knob' had been set to the 'off' position. I then confirmed with the service center that switching the air source select knob to 'off' would result in a lack of pressurization consistent with what I had experienced. I have since flown the aircraft at flight levels without incident.normal operation of the aircraft dictates the air source select knob always is in the 'both' position; although the cessna approved checklist indicates that the knob be set 'as required.' I believe the air source select knob was turned off during the recent maintenance at the cessna citation service center by the avionics technician - probably while he was completing an rvsm documentation that was due. I feel declaring an emergency was an appropriate action.I have added the word 'both' next to the 'as required' on my checklist.

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Original NASA ASRS Text

Title: A Citation Single pilot experienced a cabin altitude warning during his climb; declared an emergency; descended and returned the aircraft to the service center where it had recently undergone routine maintenance service. After a safe landing the pilot discovered the Air Source Select knob had been placed in the 'OFF' position; possibly during the previous maintenance visit; thus depriving the system of the air required to control cabin altitude. Subsequent flights with the knob in the 'BOTH' [sources] position have proven trouble free.

Narrative: During climb my 'Cabin ALT' annunciator was activated. My cabin pressure had increased to approximately 10;000 FT. Under normal operations my cabin altitude should have been below 8;000 FT.I was already using the supplemental O2 system.I declared an emergency and requested an immediate descent which was approved along with a turn back toward my departure airport when able. I began an emergency descent then turned to the heading given by ATC. At that time I was unsure as to the cause of the sudden rise in cabin pressure and was focused on making a controlled descent while on supplemental O2. I then requested a return to the Cessna Citation Service Center airport where my airplane had just completed routine scheduled maintenance. It had not been flown since the maintenance was completed other than a short flight back to my own airport.After landing I began trouble shooting the probable causes associated with a 'Cabin ALT' annunciator and determined that the 'Air Source Select Knob' had been set to the 'OFF' position. I then confirmed with the Service Center that switching the Air Source Select Knob to 'OFF' would result in a lack of pressurization consistent with what I had experienced. I have since flown the aircraft at flight levels without incident.Normal operation of the aircraft dictates the Air Source Select Knob always is in the 'Both' position; although the Cessna approved checklist indicates that the knob be set 'AS REQUIRED.' I believe the Air Source Select Knob was turned off during the recent maintenance at the Cessna Citation Service Center by the avionics technician - probably while he was completing an RVSM documentation that was due. I feel declaring an emergency was an appropriate action.I have added the word 'BOTH' next to the 'AS REQUIRED' on my checklist.

Data retrieved from NASA's ASRS site as of July 2013 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.