AIR INTERRUPT; LOSS OF CABIN PRESSURE; RETURN LAND CAUSED BY INCOMPLETE CLOSING OF AFT CARGO DOOR ON A S80 DEP FROM DFW; TX.

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AC XXX IS AN S80. AT 5000 FT ON DEP FROM DFW; THE PRESSURIZATION FLOW LIGHT ILLUMINATED AND THE CABIN ALT WAS CLBING AT ABOUT 1200 FPM; MUCH HIGHER THAN NORMAL. WE MAINTAINED 5000 FT AND ACCOMPLISHED ABNORMAL PROCS. THE CABIN STOPPED ASCENDING AND FLOW LIGHT EXTINGUISHED; BUT THE CABIN ALT AND ACFT ALT WERE NOT IN A NORMAL RATIO. WE RECOMMENCED OUR CLB; BUT THE FLOW LIGHT REILLUMINATED AND NOW THE XFER LOCK OUT ANNUNCIATOR CAME ON. THE PLANE WAS OBVIOUSLY NOT PRESSURIZING SO I ELECTED A RETURN TO DFW. FORTUNATELY; WE WERE LIGHT ENOUGH SO THAT WE COULD IMMEDIATELY LAND WITHOUT MAKING AN OVERWEIGHT LNDG. UPON GATE ARR; A GENERAL MECH IMMEDIATELY APCHED AND INSPECTED THE CARGO DOORS. HE DISCOVERED THAT THE AFT CARGO DOOR WAS CLOSED IN SUCH A WAY THAT IT WAS 'COCKED' OR AJAR SO THAT THE DOOR OPEN SWITCH PUT OUT THE LIGHT IN THE COCKPIT; BUT LEFT A SUBSTANTIAL GAP WHICH DID NOT ALLOW THE PLANE TO PRESSURIZE. IT IS POSSIBLE TO LOCK THE CARGO DOOR AND EXTINGUISH THE DOOR WARNING ANNUNCIATOR IN THE COCKPIT AND STILL HAVE THE DOOR JAMMED OR COCKED OPEN. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: PIC STATED THAT THE COMPANY CONSIDERED THIS INCIDENT A 'ONE TIME ANOMALY'; PROBABLY CAUSED BY A PIECE OF METAL OR A COKE CAN IN THE DOOR SO HAS DONE NOTHING TO GET THE INFO OUT. THE RPTR HAD TALKED WITH THE MECH UPON HIS RETURN TO DFW AND THE MECH CERTIFIED THAT THERE WAS NO OBSTACLE IN THE DOOR FRAME. THE DOOR APPEARS TO BE OF SUCH A DESIGN THAT IT CAN BE PARTIALLY CLOSED AND YET STILL EXTINGUISH THE DOOR WARNING SYS. RPTR FEELS THAT THIS ISSUE NEEDS TO BE BROADCAST OUT THERE FOR THE AVIATION COMMUNITY TO KNOW ABOUT. THE AFT CARGO DOOR IS THE SAME ON ALL OF THE MD80/88 MODELS.