DHC8 FLT CREW TAKE OFF WITH BOTH ACFT PACKS IN THE OFF POS.

Narrative

BOTH ACFT PACKS WERE INADVERTENTLY LEFT IN 'OFF' POS. NORMAL POS IS 'AUTO' OR 'MANUAL.' BLEEDS SWITCHES WERE ON. AT 15000 FT MSL (CRUISE ALT) CABIN ALT WARNING LIGHT CAME ON. CABIN ALT SHOWED 9300 FT AND 2.5 PSID. ACFT PACK SWITCHES IMMEDIATELY SELECTED ON (MANUAL POS). ACFT RE-PRESSURIZED NORMALLY. COMMENTS: THE POS OF PACK AND BLEED SWITCHES IS NORMALLY CHKED ON THE CAPT'S ORIGINATING FLOW (NOT A CHKLIST ITEM). BLEED SWITCH POS IS A CHKLIST; BUT PACK SWITCH POS; ON THE 'AFTER START; 'BEFORE TKOF;' AND 'AFTER TKOF' CHKLISTS. IT IS ALWAYS ASSUMED TO CHK THE PACK SWITCH POS ALONG WITH BLEED SWITCH POS. NORMALLY; THE PACK SWITCHES ARE ALWAYS ON IN EITHER AUTO OR MANUAL POS AND NEVER SELECTED OFF EXCEPT FOR ABNORMAL OP. ONLY BLEEDS SELECT ON/OFF FOR TKOF/LNDG PERFORMANCE REQUIREMENTS. HOWEVER; A NEW COMMON PRACTICE IS TO CYCLE THE PACK SWITCHES OFF FOR APPROX 10 SECONDS; THEN BACK ON; TO CLR FAULTS IN THE DIGITAL PACK CTLS THAT SOMETIMES CAUSE TEMP CTL PROBS. THIS MAY BE WHY THE PACK SWITCHES WERE LEFT IN 'OFF' POS. IT IS ALSO INTERESTING TO NOTE THAT WE WERE MAINTAINING APPROX 2.5 PSI AT 15000 FT MSL WITH BLEEDS ON; BUT ACFT PACKS OFF.