NEW SO DID NOT TURN ON #1 AND #3 AIR-CONDITIONING SYS IN ACCORDANCE WITH AFTER TKOF SILENT CHKLIST. AT ABOUT FL300 1 AIR-CONDITIONING PRESSURIZATION SYS UNABLE TO MAINTAIN 10500 FT ALT IN THE CABIN. CABIN ALT WARNING PROMPTS CREW TO DON OXYGEN MASKS AND COMMENCE EMER DSCNT. #1 AND #3 AIR-CONDITIONING SYS TURNED ON AND NORMAL PRESSURIZATION RESTORED.

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SO DIDN'T TURN ON AIR-CONDITIONING PACKS #1 AND #3 AFTER TKOF. WE LEVELED OFF AT FL280 (I THINK) AND STARTED A CLB TO FL310. AT FL300 TO FL310 A HORN WENT OFF. I SAID 'ISN'T THAT THE CABIN HORN?' THE SO SAID THE CABIN WAS AT 10500 FT AND WE DONNED OXYGEN MASKS AND STOPPED CLB WHILE GETTING CLRNC TO DSND TO FL240. SOMEWHERE DURING THIS TIME I SAID TO 'MAKE SURE YOUR PACKS ARE ON.' THE SO SAID SOMETHING ABOUT THE OUTFLOW VALVES WERE CLOSED AND HE COULDN'T CTL THE CABIN. WE DECLARED AN EMER AND CTR HAD US TURN TO APPROX E 90 DEGS OR R 110 DEGS. AS WE CONTINUED TO DSND; I NOTICED 2 ACFT ON TCASII. I BELIEVE THE CLOSEST WAS 2300 FT BELOW AND 5-7 MI AWAY (YELLOW OR TA) ON TCASII. AT THIS POINT; I THOUGHT THE SO HAD ACKNOWLEDGED MY 'PACK CHK;' BUT HE HADN'T. THE CAPT TOLD HIM TO TURN ON PACKS #1 AND #3 AND THE CABIN RECOVERED IMMEDIATELY. WE TOLD CTR WE WERE OK AND GOT AS LOW AS 19800 FT AND LEVELED AT FL200 AND PROCEEDED ON COURSE. DURING DSCNT WE TOLD THE PAX WE WERE HAVING DIFFICULTY CTLING THE CABIN PRESSURE AND AFTER LEVEL OFF AT FL200 WE TOLD THEM EVERYTHING WAS OK. PROBS: SO DIDN'T TURN ON PACKS AFTER TKOF (AFTER TKOF CHKLIST). SO DIDN'T HEAR ME TELL HIM TO CHK THE PACKS WERE ON. HEARING. SO MISSED THE PACKS ON THE EMER CHKLIST. PACK SWITCHES ARE DIFFICULT TO PICK OUT WHEN VIEWED FROM CAPT SEAT AND IMPOSSIBLE TO SEE FROM FO SEAT WITHOUT GYMNASTIC TRAINING. SO WAS (MAYBE) NERVOUS AS IT WAS HIS FIRST SO FLT AFTER IOE (DOWNBID TO SO). (ALSO; NERVOUS ABOUT FAA INSPECTOR ON BOARD MAYBE.) SOLUTION: DIFFERENTIATE PACK SWITCHES. PAINT A GREEN BORDER AROUND THEM. BE AWARE OF NEW GUY/NERVOUS SYNDROME. BE AWARE OLDER SO'S MAY NOT HEAR WELL.