DOOR BLEW OPEN AND OFF ACFT AFTER TKOF.

Narrative

IN PREPARATION FOR FLT I CLOSED THE DOOR AND GAVE A BRIEFING TO PAX. WHEN I REACHED THE COCKPIT THE CAPT HAD STARTED THE CHKLIST. WE USUALLY DO IT ALL WITH BOTH PLTS IN THEIR SEATS. THE CAPT HAD PASSED THE DOOR SAFE CHK AND GAVE ME THE CHKLIST AT ENG STARTING. AFTER THE PROBLEM WITH THE DOOR HE SAID HE DID NOT REMEMBER CHKING THE DOOR SAFE LIGHT; THUS WE MISSED A CRITICAL CHK TO SEE IF THE DOOR CLOSED PROPERLY. LATER DURING TAXI THERE IS AN ANNUNCIATOR RECALL CHK; WHICH WAS INTERRUPTED BY A RADIO CALL TO LNDG TFC (UNCTLED ARPT). I DO NOT REMEMBER SEEING THE DOOR UNSAFE LIGHT. WE DO NOT KNOW IF IT WAS ON AND I OVERLOOKED IT OR IF THE DOOR WAS OK AT THIS POINT. WE MADE A NORMAL TKOF AND AT 4000 FT WE BEGAN TO HEAR AN AIR LEAK. IT RAPIDLY GOT LOUDER. WE TURNED BACK TO FIELD AND WERE GOING TO DEPRESS BUT THE DOOR BLEW OPEN BEFORE WE COULD DO IT. LNDG WAS NORMAL AND THERE WERE NO INJURIES. COULD WE HAVE CAUGHT THE DOOR FAILURE BY BETTER CHKLIST DISCIPLINE? WAS IT OK -- AND WE DID DO THE CHK -- I CAN'T REMEMBER! LESSONS: STICK TO STANDARD PROCS. LOOK AND SEE WHEN YOU DO THE CHKLIST. EXPECT TO SEE A PROBLEM SO YOU DON'T LOOK AND NOT SEE A PROBLEM.