TEMPORARY LOSS OF CABIN PRESSURE WHEN A B737-700 FLC CLBS TO 10000 FT BEFORE THE PIC REALIZES THAT THE FO LEFT THE CABIN PRESSURE SWITCH IN MANUAL CTL DEP ONT; CA.

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I WAS FLYING THE POMG OUT OF ONT. AT 10000 FT; THE CAPT DIRECTED ME TO LEVEL OFF. HE THEN ASKED ATC FOR AN INTERMEDIATE LEVEL AND ATC APPROVED. HE THEN POINTED OUT THE CABIN PRESSURE. I LOOKED AT THE PRESSURE CTL KNOB AND SAW IT WAS IN MANUAL. WE CLOSED THE OUTFLOW VALVE AND REGAINED CTL OF THE CABIN PRESSURE. WE ASKED ATC TO RESUME CLB AND CONTINUED NORMAL OPS FOR AN UNEVENTFUL FLT. THIS FLT WAS AN ORIGINATOR AND I REMEMBER TESTING THE OUTFLOW VALVE DURING PREFLT. I DON'T REMEMBER WHAT BROKE MY HABIT PATTERN; BUT I FAILED TO RETURN THE MODE SELECTOR TO AUTO. THIS WAS A NEW PRESSURE CTL WITHOUT THE 'FLT-GND' SWITCH. MY FLOW FOR AFTER START IS 'PEETERS; HEATERS; BLEEDS; FLT.' WITHOUT THE SWITCH; I STOPPED CHKING FOR THE 'AUTO.' BELIEVE ME; THIS LESSON WAS LEARNED. MY FLOW IS NOW 'FLT/AUTO' WITH A TWIST OF THE KNOB TO ENSURE IT IS IN AUTO. I HAVE ALSO ADDED A 5000 FT CHK OF CABIN PRESSURIZATION. SUPPLEMENTAL INFO FROM ACN 537942: FO INADVERTENTLY LEFT THE SWITCH IN MANUAL. I; AS THE CAPT; FAILED TO NOTICE THIS ON TAXI OUT. WE CAUGHT THE ERROR DURING OUR 10000 FT CHK. I INSTRUCTED THE FO TO LEVEL OFF. WE TOOK THE APPROPRIATE STEPS TO BRING THE CABIN PRESSURE UNDER CTL. THE FO IS UNCERTAIN WHY HE LEFT THE MODE SELECTOR IN MANUAL; BUT BELIEVES HE WAS DISTR BY A RADIO XMISSION. WE WERE FORTUNATE IN 2 RESPECTS. FIRST; THAT WE CAUGHT THE PROB WHEN WE DID (I ALWAYS TRY TO PHYSICALLY TOUCH BOTH THE PRESSURE AND CLB GAUGE AT 10000 FT). AND SECOND; THAT WE WERE NOT IN RAPID CLB OUT OF ONE OF OUR HIGH ALTS CITIES.