AN A300 FLIGHT CREW LOST PRESSURIZATION CONTROL IN CLIMB BECAUSE THEY HAD THE APU BLEED SWITCH IN THE ON POSITION.

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ACFT WAS DISPATCHED WITH: GENERATOR #1 INOP. HP BLEED VALVE #1 INOP. FO PF. PASSING FL240 CLBING TO FL290; ECAM WARNING OF BOTH PACK VALVES FAULTED. NOTICED THAT CABIN PRESSURIZATION WAS DECREASING SLOWLY. COMPLIED WITH ECAM IN ATTEMPTING TO RESET BOTH PACK VALVES; HOWEVER BOTH STILL SHOWED FAULT. IN THIS CASE; ECAM DIRECTS DSCNT TO 10000/MEA. WE DONNED OXYGEN MASKS AND I ADVISED THE CABIN JUMPSEATER ON THE INTERCOM TO DON HIS MASK. WE ESTABLISHED COMS AND I ADVISED ATC THAT WE WERE LOSING PRESSURIZATION AND REQUESTED DSCNT TO 10000 FT. THEY ASSIGNED FL180 AS AN INTERMEDIATE LEVELOFF. AT THIS POINT; WITH AN UNCTLED CABIN PRESSURE SITUATION; I DECLARED AN EMER TO CONTINUE THE DSCNT AND THEY COMPLIED IN SHORT ORDER. AT NO POINT DID THE CABIN ALT EVER EXCEED 10000 FT AND AT NO POINT WAS THE PRESSURE LOSS SO SEVERE AS TO REQUIRE AN EMER DSCNT. PASSING 14000 FT I NOTICED THAT THE ECAM AND THE PROB HAD COMPLETELY CLRED AND THE CABIN WAS PRESSURIZING NORMALLY. THE LAST STEP IN THIS PROC WOULD HAVE BEEN TO OPEN THE RAM AIR VALVE. OBVIOUSLY; THAT WAS NO LONGER NECESSARY. ONCE LEVEL AT 10000 FT; I TERMINATED EMER STATUS WITH THE CTR. I CALCULATED FUEL BURN REQUIREMENTS TO CONTINUE TO DEST AND WITH SUFFICIENT FUEL LOAD AND GOOD WX; THIS WAS NOT A PROB. ADVISED OPS OF THE SITUATION VIA ACARS AND THEY PROVIDED REVISED FBO. I THEN REVIEWED THE QRH FOR PACK VALVE FAULT AND SAW NOTHING PERTINENT TO THE PRESENT STATUS OF THE ACFT. MADE AN AML ENTRY OF THE DISCREPANCY AND AT SOME POINT HAD SOME TIME TO REEVALUATE THE SITUATION WHICH HAD OCCURRED. WE HAD A DISCUSSION ABOUT WHAT MIGHT HAVE CAUSED THE FAULTS AND IT WAS AT THIS POINT THAT I NOTICED THAT THE APU BLEED VALVE SWITCH WAS IN THE ON POS. DURING THE ACTUAL EVENT; THE COCKPIT WAS DARK AND I HAD NOT NOTICED THAT THE SWITCH WAS ON. MY CONCERN AT THAT POINT WAS THE SAFETY OF THE ACFT AND CREW AND THE PROPER ACCOMPLISHMENT OF THE ECAM ACTIONS. DURING MY RESET OF THE PACK VALVES; I DO NOT BELIEVE THAT I MISTAKENLY MOVED THE SWITCH TO THE ON POS. ONCE ON THE GND; AFTER DISCUSSIONS WITH THE FO; AFTER REVIEWING THE MEL PROCS FOR DISPATCH WITH A GENERATOR INOP AND AFTER REVIEWING THE AFM CONCERNING THE APU BLEED SYS; WE MAY HAVE INADVERTENTLY LEFT THE APU BLEED SWITCH IN THE ON POS IN THE AFTER START FLOW. JUST AFTER START; THE FO ASKED ME TO CONFIRM THAT THE APU WAS TO BE LEFT RUNNING DUE TO THE MEL PROC. I ANSWERED IN THE AFFIRMATIVE AND IT MAY HAVE BEEN AT THIS POINT THAT HIS FLOW WAS BROKEN AND THE SWITCH WAS LEFT ON. THIS MAY EXPLAIN THE LOSS OF PRESSURE ABOVE 20000 FT AND THEN THE RESTORATION OF NORMAL OP BELOW 15000 FT. ULTIMATELY; THIS MAY HAVE LED TO THE SITUATION WHICH REQUIRED US TO DSND AND FLY AT 10000 FT. MY MISTAKE WAS NOT APPLYING MY SYS KNOWLEDGE TO REMEMBER THAT POSSIBLY A DUAL PACK FAILURE COULD RESULT FROM THIS SWITCH POS AND NOT CHKING IT PRIOR TO DSNDING; ALSO AND ULTIMATELY; NOT MONITORING THE FO IN THE PROPER ACCOMPLISHMENT OF THE AFTER START CHKLIST EVEN THOUGH IT IS NOT CHALLENGE AND RESPONSE IN NATURE. THE MITIGATING FACTORS ARE THAT NORMALLY THE APU BLEED SWITCH IS POSITIONED OFF BY THE FO UNSUPERVISED ON THE AFTER START FLOW AND ALL OF OUR PROCS ASSUME THAT THIS HAS BEEN ACCOMPLISHED BY HIM. THE COCKPIT WAS DARK AND THE SWITCH POS WAS NOT READILY APPARENT. ALSO; THE FACT THAT WE HAD AN INOP HP VALVE WAS ALSO ON MY MIND AS A POSSIBLE CAUSE. AS CAPT; ONCE THE ECAM ALERTS OF A POTENTIALLY SERIOUS SITUATION; MY TRAINING HAS BEEN TO RELY ON THE ECAM TO STABILIZE IT BY ACCOMPLISHING THE ITEMS DIRECTED AND THEN FOLLOWING THAT UP BY REFING THE QRH AND OTHER PUBS AS NECESSARY IN ORDER TO CONTINUE THE FLT SAFELY OR LNDG AT A SUITABLE ARPT. IN OTHER WORDS; ONCE THE ECAM ALERTS; IT BECOMES THE PRIMARY FOCUS TO STABILIZE THE ACFT AND TROUBLESHOOTING IS NOT NORMALLY A PART OF THE THOUGHT SEQUENCE AT THAT POINT. NOTWITHSTANDING THE FACT THAT OUR MAINT PERSONNEL DID FIND A FAULTY PNEUMATIC CTLR ON THIS ACFT AFTER THE INCIDENT; THIS SITUATION MAY HAVE BEEN SELF INDUCED. IN ANY CASE; ALLPARTIES REACTED IN ACCORDANCE WITH APPROVED PROCS AND IN A COMPLETELY PROFESSIONAL MANNER. IN THE FUTURE; IF FACED WITH THIS SITUATION AGAIN AND EVEN THOUGH IT IS NOT A DOCUMENTED PROC; I WOULD CHK THE POS OF THE APU BLEED SWITCH PRIOR TO DSNDING JUST TO MAKE SURE.