RETURN LAND MANDATED IN A DC9 CARGO FLT WHEN FLT CREW IS UNABLE TO PRESSURIZE THE ACFT; FINDING OUT WHEN SHUTTING DOWN THAT BOTH PACK SWITCHES WERE IN THE 'OFF' POS AT ILN; OH.

Narrative

AFTER TKOF FROM ILN TO MSY; FO NOTICED THE CABIN WAS NOT PRESSURIZING WHEN CLBING THROUGH ABOUT 5000 FT AFTER DEP. I NOTICED THE DIFFERENTIAL WAS 'ZERO' PSI AND THE CABIN WAS CLBING WITH THE VSI. WE REQUESTED FROM ATC A LOWER LEVELOFF; SO WE STOPPED AT 10000 FT. I THEN REQUESTED THE CABIN PRESSURE (ABNORMAL) UNCONTROLLABLE; AND THE AUTO PRESSURIZATION INOP CHKLIST. WE CALLED FLT CTL TO SEE IF THEY WANTED US TO CONTINUE UNPRESSURIZED AT 10000 FT. (CHK ON FUEL; WX; ETC) AT WHICH TIME WE WERE TOLD TO RETURN. AFTER SHUTDOWN AT THE GATE; I NOTICED BOTH OF THE PACKS WERE OFF. I HAD TURNED THEM OFF ON TAXI OUT OF ILN WHEN PASSING BEHIND AN ACFT THAT HAD BEEN DEICED AND DID NOT TURN THEM BACK ON. AT THE TIME OF DEP; ILN HAD LOW VISIBILITY; STRONG WINDS AND HVY SNOW. THE RAMPS AND THE RWY WERE ALL WHITE; CONTRIBUTING TO BEING PREOCCUPIED WITH BRAKING CONDITIONS; ABORT; DEICE HOLDOVER TIMES AND GENERAL FATIGUE FOR THAT TIME IN THE MORNING. 1) I SHOULD HAVE SET SOME SORT OF REMINDER THAT THE SWITCHES WERE OFF. 2) I SHOULD HAVE DONE THE CHKLISTS ALSO TO MAKE SURE NOTHING WAS MISSED. 3) AN ADDITIONAL ABNORMAL SIMPLE-LIKE ACFT NOT PRESSURIZING. CHK OUTFLOW VALVE; CHK AIR CONDITIONING PACKS BOTH ON.