Narrative:

During climb through 10;000 ft the red cabin altitude annunciator came on; followed by the emergency pressurization being activated. Since we had just entered a cloud we continued climbing to 15;000 ft to get out of the tops of that cloud and the moderate turbulence taking place at the same time all of this started. We leveled off and were in the clear and asked for a descent to 9;000 ft. During the descent everything fixed itself... Emergency pressurization-off; red cabin altitude annunciator-off and the plane started pressurizing. Everything was back to normal. During this time we ran the cabin altitude 10;000 checklist and called the company. We also found that the cabin pressurization switch was in 'manual' instead of 'auto' which inadvertently caused the issue. This switch isn't on the emergency checklist; just the pressurization source selector is noted but luckily we happened to notice it as well. Neither of us knew how this switch got moved out of the auto position so we aren't sure when this occurred. Our guess is that the switch got moved accidentally sometime while on the ground between flights. Since the pressurization switches are only checked on the first flight of the day checklist the switch position was missed on this leg which was #2 for the day and our first flight at FL400 went without any problems. We did divert to a nearby airport and we advised the company that we found the switch in manual upon landing. Adding an annunciator to alert the crew before takeoff that the pressurization system is selected to manual would prevent this from occurring. Also adding pressurization switch position to the before start checklist could help as well.

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Original NASA ASRS Text

Title: A Cessna 560XL diverted to a nearby airport when the aircraft's Red Cabin Altitude annunciator advised of excessive cabin altitude. The Flight Crew discovered the Cabin Pressurization switch was inexplicably in the Manual vice Automatic Mode.

Narrative: During climb through 10;000 ft the Red Cabin Altitude annunciator came on; followed by the Emergency Pressurization being activated. Since we had just entered a cloud we continued climbing to 15;000 ft to get out of the tops of that cloud and the moderate turbulence taking place at the same time all of this started. We leveled off and were in the clear and asked for a descent to 9;000 ft. During the descent everything fixed itself... Emergency Pressurization-OFF; Red Cabin Altitude Annunciator-OFF and the plane started pressurizing. Everything was back to normal. During this time we ran the Cabin Altitude 10;000 Checklist and called the company. We also found that the Cabin Pressurization switch was in 'manual' instead of 'Auto' which inadvertently caused the issue. This switch isn't on the emergency checklist; just the Pressurization Source Selector is noted but luckily we happened to notice it as well. Neither of us knew how this switch got moved out of the auto position so we aren't sure when this occurred. Our guess is that the switch got moved accidentally sometime while on the ground between flights. Since the pressurization switches are only checked on the first flight of the day checklist the switch position was missed on this leg which was #2 for the day and our first flight at FL400 went without any problems. We did divert to a nearby airport and we advised the company that we found the switch in manual upon landing. Adding an annunciator to alert the crew before takeoff that the pressurization system is selected to manual would prevent this from occurring. Also adding pressurization switch position to the BEFORE START checklist could help as well.

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.