Narrative:

Aircraft cabin failed to pressurize immediately after takeoff. Performed the pressurization fail checklist and the aircraft would not pressurize. Held close to the departure airport while discussing possible return to the field (rtf) with dispatch. Decided to rtf. While preparing for the rtf; dispatch called us and suggested divert to an enroute airport. We referenced the enroute charts for adequate MEA for us being at 10;000 ft and we were comfortable with proceeding to the divert airport. As we were diverting the cabin rate indicated a 3;000 FPM descent and pressurization returned to normal operation. Maintenance control seemed to believe it was a frozen aft cargo door seal. 40 miles from the divert airport; dispatch called again and ran numbers for continuing to the filed airport. My crew and I were comfortable with this so we continued to our original destination. With all our low altitude maneuvering; we were showing arrival tight on fuel so I declared minimum fuel about 90 miles from my filed destination. About 15 minutes into our divert; I re-ran the pressurization fail checklist to make sure I didn't miss anything and I discovered that I had completed the checklist at the flight attendant notify point. Had I continued the checklist it would have led into the manual press checklist; then unpressurized flight checklist. If it were indeed a frozen door seal; the manual pressurization wouldn't have worked anyway. After discovering this error; the aircraft was pressurizing normally so I felt it was better to not continue the checklist because that would have led us to de-pressurize an airplane which was pressurizing normally.

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Original NASA ASRS Text

Title: A Q400 failed to pressurize after takeoff and while the crew considered either returning to land or diverting the pressurization began operating normally so they continued to the filed destination. A frozen aft cargo door seal was the suspected cause.

Narrative: Aircraft cabin failed to pressurize immediately after takeoff. Performed the Pressurization Fail Checklist and the aircraft would not pressurize. Held close to the departure airport while discussing possible return to the field (RTF) with Dispatch. Decided to RTF. While preparing for the RTF; Dispatch called us and suggested divert to an enroute airport. We referenced the enroute charts for adequate MEA for us being at 10;000 FT and we were comfortable with proceeding to the divert airport. As we were diverting the cabin rate indicated a 3;000 FPM descent and pressurization returned to normal operation. Maintenance Control seemed to believe it was a frozen aft cargo door seal. 40 miles from the divert airport; Dispatch called again and ran numbers for continuing to the filed airport. My crew and I were comfortable with this so we continued to our original destination. With all our low altitude maneuvering; we were showing arrival tight on fuel so I declared Minimum Fuel about 90 miles from my filed destination. About 15 minutes into our divert; I re-ran the Pressurization Fail Checklist to make sure I didn't miss anything and I discovered that I had completed the checklist at the Flight Attendant notify point. Had I continued the checklist it would have led into the Manual Press Checklist; then Unpressurized Flight Checklist. If it were indeed a frozen door seal; the manual pressurization wouldn't have worked anyway. After discovering this error; the aircraft was pressurizing normally so I felt it was better to not continue the checklist because that would have led us to de-pressurize an airplane which was pressurizing normally.

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.