FLC OF AIR CARRIER MLG ACFT RETURNED TO ARPT AFTER THE ACFT PRESSURIZATION WAS NOT WORKING DURING DEP.

Narrative

WE WERE #3 FOR TKOF AND ABOUT TO START OUR SECOND ENG WHEN CAPT NOTICED ON MLG ACFT IN FRONT OF US; AN OPEN ACCESS PANEL FOR REAR STAIRWAY. COPLT CONTACTED TWR AND THEN CONTACTED THE AFFECTED COMPANY ACFT ON COMPANY FREQ TO EXPLAIN ACCESS DOOR PROBLEM. WHILE COPLT WAS COMMUNICATING CAPT STARTED THE #2 ENG AND SIGNALED THE CABIN CREW TO PREPARE FOR TKOF. WHEN THE PRECEDING ACFT BECAME #1 FOR TKOF THEY ELECTED TO RETURN TO GATE FOR INSPECTION. THIS IMMEDIATELY MADE US (#1) FOR TKOF. TWR CLRED US FOR TKOF AND CAPT CALLED FOR THE BEFORE TKOF CHKLIST (OMITTING THE DELAYED START CHKLIST AFTER HE STARTED THE #2 ENG. AT 3000 FT AGL WE REALIZED OUR ACFT WAS NOT PRESSURIZING AND THE OUTFLOW VALVE WAS CLOSED. THE COPLT SET CLB PWR AS DIRECTED; THEN SWITCHED FROM #1 PRESSURIZATION SYS TO THE #2 PRESSURIZATION SYS; BUT TO NO AVAIL. AT THIS POINT AUTOTHROTTLES AND THRUST MGMNT COMPUTER FAILED. THE CAPT SUSPECTED THERE WAS AN ELECTRICAL MECHANICAL PROBLEM WITH THESE SYS AND ASKED DEP TO LEVEL AT 7000 FT AND REQUESTED CLRNC TO RETURN TO DEP ARPT. DUE TO WORKLOAD OF COMS; CHKLISTS; AND APCH SETUP. THERE WAS NO FURTHER TROUBLESHOOTING OF THE SYS. WHILE ACCOMPLISHING CHKLISTS THE COPLT FOUND BOTH AIR CONDITIONING PACK SWITCHES IN THE OFF POS WHERE THEY HAD BEEN SINCE START OF SECOND ENG. ACFT RETURNED TO ARPT FOR UNEVENTFUL LNDG. THE CREW WAS RUSHED FOR TKOF AND CAPT DID NOT PERFORM DELAYED START CHKLIST AFTER STARTING #2 ENG. CONSEQUENTLY; PACK SWITCHES LEFT IN OFF POS AND ACFT WAS NOT ABLE TO PRESSURIZE ON CLBOUT.