A B737-700 DOOR SEAL FAILED PREVENTING CABIN PRESSURIZATION AFTER TKOF. THE CABIN ALT WARNING HORN SOUNDED. RETURNED TO DEP ARPT.

Narrative

THIS WAS A REPOSITION FLT. ON THE CLBOUT WE NOTICED THAT THE ACFT SOUNDS SEEMED A LITTLE LOUDER THAN NORMAL. MY EARS POPPED; WHICH IS UNUSUAL. WE DOUBLE-CHKED THE DOOR LIGHTS; ALL DOORS WERE INDICATING CLOSED. WE LOOKED AT THE CABIN ALT AND PRESSURE DIFFERENTIAL GAUGES AND THE INDICATIONS DIDN'T LOOK TOO FAR ASTRAY AT THE TIME. AT FIRST WE ATTRIBUTED THE EAR POPPING TO AN UNUSUALLY FAST RATE OF ASCENT (SINCE WE WERE SO LIGHT). AS WE GOT AROUND 10000 FT WE CHKED THE GAUGES AGAIN AND NOTICED THAT THE CABIN ALT WAS HIGHER THAN IT SHOULD BE. WE DECIDED TO GET CLRNC TO RETURN TO ZZZ. AS THE CAPT WAS CONTACTING ATC ON THE RADIO; THE CABIN ALT WARNING HORN SOUNDED. I PULLED THE PWR BACK AND IMMEDIATELY STARTED A DSCNT TO BELOW 10000 FT. THE CAPT GOT A LOWER ALT (5000 FT) AND A HDG FROM CTR TO RETURN TO THE ARPT. WE LANDED UNEVENTFULLY. WHEN WE ARRIVED THE MECHS INSPECTED THE FORWARD ENTRY DOOR. THEY DETERMINED THAT A PART OF THE DOOR RIGGING WAS OFF ENOUGH TO CREATE A SMALL GAP AT THE AFT PORTION OF THE DOOR; BUT STILL SEAT THE FORWARD PART OF THE DOOR SO IT EXTINGUISHES THE FORWARD ENTRY DOOR LIGHT IN THE COCKPIT. THE GAP WAS SMALL ENOUGH THAT COMBINED WITH OUR RAPID CLB; THE CABIN ALT LAGGED SIMILAR TO HOW IT WOULD NORMALLY LAG BEHIND THE ACFT ALT. HOWEVER; IT IS LARGE ENOUGH THAT THE ACFT CANNOT MAINTAIN PRESSURIZATION AND ONCE YOU LEVEL OR SLOW YOUR CLB; THE CABIN ALT WILL CONTINUE TO INCREASE. IT MAY HAVE BEEN PREVENTED IF THERE WAS A PROC FOR THE FLT ATTENDANTS TO CHK THE SEALS ON THE DOORS AFTER SECURING THEM. JUST COMING OUT OF FLYING THE B-200 FOR YRS; IT WAS EASY FOR ME TO DISMISS THE INITIAL LOUD CABIN NOISE SINCE THERE IS A WIDE RANGE OF NORMAL CABIN NOISE IN THE B-200 FLEET. IN SPITE OF THIS; WE WERE SUSPICIOUS AND DID NOT COMPLETELY DISMISS WHAT WAS GOING ON. IN AN ATTEMPT TO STAY ON SCHEDULE; WE MADE THE DECISION TO KEEP FLYING SINCE WE WEREN'T 100% POSITIVE THERE WAS A PROB. SINCE WE WERE AWARE THAT SOMETHING MIGHT NOT BE RIGHT; WE KEPT AN ONGOING AND OPEN DISCUSSION ON THE ISSUE TO TRY TO BE SURE ABOUT WHAT WAS REALLY HAPPENING AND CONTINUALLY RECHKING THE GAUGES. IT WAS BECAUSE OF CRM; THE OPEN EXCHANGE OF THOUGHTS AND IDEAS AND NOT BEING SATISFIED UNTIL BOTH PEOPLE WERE SATISFIED; THAT WE WERE ABLE TO CATCH IT WELL BEFORE THE MASKS DROPPED. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE REPORTER DID NOT KNOW THIS ACFT'S HISTORY AS IT RELATED TO THIS DOOR ANOMALY. HE DID REPORT THAT AFTER MAINTENANCE EXAMINED THE DOOR; THE ACFT WAS REMOVED FROM SVC AND THE CREW ASSIGNED ANOTHER ACFT.