Narrative:

We were cleared to climb to 12000 ft and were around 11000 ft when the cabin altitude warning was illuminated on the cwp. I instructed the first officer to stop the climb and started the QRH. Cabin differential was at .5 and cabin altitude was at 9500 ft. I requested 10000 ft from center. I assessed the situation and determined that the manual outflow valve was open and the automatic pressurization took over. The flight proceeded and landed without incident. The manual outflow valve was left open by someone previous to us. On preflight; the first officer mistook full open for full closed. This can happen as the manual knobs are sometimes rigged so that full closed is actually behind the white line on the pressurization panel. As the knob is turned to full open; it is about a 330 degree twist that will also be somewhere behind the white line. On climb out; neither of us detected that pressurization was nil or negligible. Flight crew suggestions/narrative: both the first officer and I were unnerved that such an error could happen. We both plan to exercise extra diligence. I think it could also be helpful if the last item on the after takeoff checklist was a check of the pressurization panel to be sure that cabin differential is going up and the cabin altitude isn't rising too rapidly.

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Original NASA ASRS Text

Title: A SAAB SF340 CAPTAIN REPORTED HIS AIRCRAFT FAILED TO PRESSURIZE IN THE CLIMB. HE FOUND THE MANUAL PRESSURIZATION OUTFLOW VALVE IN THE OPEN POSITION.

Narrative: WE WERE CLRED TO CLB TO 12000 FT AND WERE AROUND 11000 FT WHEN THE CABIN ALT WARNING WAS ILLUMINATED ON THE CWP. I INSTRUCTED THE FO TO STOP THE CLB AND STARTED THE QRH. CABIN DIFFERENTIAL WAS AT .5 AND CABIN ALT WAS AT 9500 FT. I REQUESTED 10000 FT FROM CTR. I ASSESSED THE SITUATION AND DETERMINED THAT THE MANUAL OUTFLOW VALVE WAS OPEN AND THE AUTO PRESSURIZATION TOOK OVER. THE FLT PROCEEDED AND LANDED WITHOUT INCIDENT. THE MANUAL OUTFLOW VALVE WAS LEFT OPEN BY SOMEONE PREVIOUS TO US. ON PREFLT; THE FO MISTOOK FULL OPEN FOR FULL CLOSED. THIS CAN HAPPEN AS THE MANUAL KNOBS ARE SOMETIMES RIGGED SO THAT FULL CLOSED IS ACTUALLY BEHIND THE WHITE LINE ON THE PRESSURIZATION PANEL. AS THE KNOB IS TURNED TO FULL OPEN; IT IS ABOUT A 330 DEG TWIST THAT WILL ALSO BE SOMEWHERE BEHIND THE WHITE LINE. ON CLBOUT; NEITHER OF US DETECTED THAT PRESSURIZATION WAS NIL OR NEGLIGIBLE. FLT CREW SUGGESTIONS/NARRATIVE: BOTH THE FO AND I WERE UNNERVED THAT SUCH AN ERROR COULD HAPPEN. WE BOTH PLAN TO EXERCISE EXTRA DILIGENCE. I THINK IT COULD ALSO BE HELPFUL IF THE LAST ITEM ON THE AFTER TKOF CHKLIST WAS A CHK OF THE PRESSURIZATION PANEL TO BE SURE THAT CABIN DIFFERENTIAL IS GOING UP AND THE CABIN ALT ISN'T RISING TOO RAPIDLY.

Data retrieved from NASA's ASRS site as of May 2009 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.