AN ATR72 PIC ATTEMPTS TO DEPRESSURIZE THE CABIN AFTER TKOF WHEN A CARGO DOOR WARNING LIGHT COMES ON. THE ACFT OVERPRESSURIZES AND HAS TO BE DEPRESSURIZED; CREATING A SMOKE-LIKE ATMOSPHERE. AN EMER IS DECLARED FOR THE RETURN LAND.

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ON TKOF AT DFW AT PM18 ON JUL/XA/98; A CARGO DOOR UNLOCK LIGHT CAME ON APPROX 30 SECONDS AFTER TKOF. THE CAPT REACHED OVER AND PRESSED THE MANUAL BUTTON ON THE AUTOPRESSURIZATION SYS. THIS CAUSED THE CABIN TO START OVERPRESSURIZING; CAUSING PRESSURE SPIKES; AS NOTED BY THE FLT ATTENDANT. SOON THEREAFTER; THE FLT ATTENDANT CALLED RPTING HOT AIR BLOWING IN THE CABIN. THIS WAS CAUSED BY THE EXCESSIVE PRESSURE IN THE CABIN. SUBSEQUENTLY; THE CAPT ELECTED TO DUMP THE CABIN; CAUSING CONDENSATION FROM RAPID DEPRESSURIZATION; WHICH WAS CONFUSED WITH SMOKE BY THE FLT ATTENDANTS. THE CREW TREATED THIS AS ACTUAL SMOKE IN THE CABIN AND RETURNED TO DFW. AN EMER WAS FILED; HOWEVER; DFW WAS INFORMED TO NOT TREAT IT AS AN EMER ANYMORE RIGHT BEFORE LNDG; BECAUSE THE FLT ATTENDANTS RPTED THE SMOKE HAD DISAPPEARED WHICH IS WHEN THE CREW REALIZED IT WAS CONDENSATION FROM DUMPING THE CABIN. THE ACFT LANDED SAFELY WITHOUT INCIDENT. IT IS IN MY OPINION THAT THIS WHOLE INCIDENT COULD HAVE BEEN AVOIDED BY A REVIEW OF PROPER PROCS OF THE CARGO DOOR LIGHT ILLUMINATING INFLT. SUPPLEMENTAL INFO FROM ACN 408806: ALL WAS NORMAL UP TO STARTING ENG #1. ALL DOOR LIGHTS WERE OUT. RAMPER OK'S START #1. LOOKED UP JUST BEFORE START; NOTED CARGO DOOR LIGHT ON (THOUGHT GREMLIN). ASKED RAMPER TO RECHK DOOR; LIGHT WENT OUT. TAXIED OUT; GREMLIN CAME BACK 1/2 WAY TO RWY 18L. RESET CIRCUIT BREAKER FOR CARGO DOOR; LIGHT STAYED ON. RESET MFC; LIGHT WENT OUT. BOTH OF US AGREED CARGO DOOR WAS LOCKED; BUT REVIEWED QRH BEFORE DEP JUST IN CASE IT APPEARED INFLT. PLEASE NOTE: AT THIS POINT I MUST HAVE REVIEWED THE WRONG QRH ITEM BUT STILL UNDER PRESSURIZATION. CORRECT SAYS STAY IN AUTO MODE TO DEPRESSURIZE. I QUICKLY SAW ITEM IN WHICH IT WAS DONE IN MANUAL MODE. AUTO MODE IS SET AT A RATE OF 550 PER MIN MAX; WHERE MANUAL MODE IS ADJUSTABLE TO 2500 MAX.) AFTER TKOF DID NORMAL CHKLIST WHEN CARGO DOOR LIGHT CAME BACK ON. FO COMPLETED CLB CHKLIST. CAPT ASKED FOR QRH FOR CARGO DOOR LIGHT. I FLEW; FO FIXED. WHILE NOTICING ACFT OVERPRESSURIZING; CAPT REQUESTED THIS ITEM TO BE COMPLETED FIRST. FLT ATTENDANT CALLED TWICE WHILE DOING THE CHKLIST REGARDING PRESSURE BUMPS. JUST AFTER; FLT ATTENDANTS CALLS AGAIN ABOUT EXCESS TEMP. FO PROCEEDS TO NEXT ITEMS ON QRH. THE ONLY ONE THAT APPLIED WAS DIRECT OVERHEAT. CAPT ASKED FO TO SHUT OFF ALL BLEEDS; WHILE CAPT ELECTED TO DUMP THE CABIN. IMMEDIATELY AFTERWARD; THE FLT ATTENDANT CALLED SMOKE IN THE CABIN. WENT TO APPROPRIATE QRH; COMPLETED IT; PROCEEDED TO ASK FLT ATTENDANT ABOUT SMOKE. REPLY 'GONE.' MADE GENERAL PA TO PAX. ADVISED APCH AND TWR SMOKE GONE BUT KEEP EMER TRUCKS OUT. ASKED FLT ATTENDANT IF AN EVAC NEEDS TO BE DONE WITH SMOKE GONE; ANSWER WAS NO.