A DC9-50 IN CLB AT 10000 FT EXPERIENCED THE CABIN ALT WARNING AND MASTER WARNING CAUSED BY THE MANUAL PRESSURE CTL INADVERTENTLY ENGAGED.

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ATC GAVE US CLB TO 17000 FT BUT WHEN WE GOT TO 10000 FT THE MASTER WARNING LIGHT ILLUMINATED ALONG WITH THE RED CABIN ALT LIGHT. I LOOKED AND IMMEDIATELY SAW THAT THE PRESSURIZATION CONTROLLER ON PEDESTAL WAS IN MANUAL POS AND THE CABIN ALT WAS AT 10000 FT. WE TOLD ATC WE NEED TO STAY AT 10000 FT AND BEGAN THE PROCESS IF BRINGING THE CABIN BACK DOWN. I DO NOT KNOW HOW THE CONTROLLER GOT IN MANUAL BUT THERE ARE 3 POSSIBILITIES. 1) I WAS INTERRUPTED DURING PREFLT AND MISSED IT. I DON'T THINK THIS IS THE CASE BECAUSE I REMEMBER NO SUCH INTERRUPTION AND PREFLT WENT NORMAL WITH LOTS OF TIME. 2) I WAS FLYING WITH NEW COPLT WHO COULD HAVE HIT IT WHILE CLBING INTO SEAT. (ALL YOU HAVE TO DO IS PUSH HANDLE DOWN.) 3) A MECH CAME UP AFTER PREFLT AND WHILE LEANING OVER THE PEDESTAL COULD HAVE INADVERTENTLY KNOCKED IT DOWN TO MANUAL. SCENARIO #2 AND #3 SEEM VERY PLAUSIBLE BUT THE BOTTOM LINE IS; PNF MISSED IT ON THE CLB FLOW OUT OF 3000 FT AGL. I SHOULD HAVE DONE MY OWN FLOW TO BACK HIM UP SINCE HE WAS VERY NEW TO ACFT.