A RETURN LAND EMER LNDG MADE WHEN SMT PAX DOOR BLOWS OFF IN CLB.

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LOADED PAXS; I CLOSED DOOR AND CHKED THE INSPECTION DOOR TO MAKE SURE MECHANISM WAS LOCKED. IT APPEARED LOCKED AND SECURED. PERFORMED CHKLISTS AND CALLED UNICOM; INFORMED IT THAT I WAS DEPARTING RWY 23. AFTER DEPARTING AND REACHING APPROX 1200 FT AGL; THE DOOR BLEW OFF; COMPLETELY DETACHING ITSELF FROM THE ACFT. I TOLD ALL PAXS TO KEEP THEIR SEAT BELTS ON; AND TURNED L BACK TO THE ARPT AND LINED UP AND LANDED ON RWY 5. THE WIND WAS CALM; WX CLR; AND VISIBILITY UNLIMITED; APPROX 6 DEGS FAHRENHEIT. BASED ON MY REVIEW OF THIS SITUATION; I DID NOT FEEL THAT THERE WAS ANYTHING ADDITIONAL I COULD HAVE DONE TO HAVE PREVENTED THIS OCCURRENCE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO. THE LEVER THAT MOVES THE LOCKING MECHANISM INTO THE LATCH HAD SHEARED BUT THE WINDOW SHOWING LOCK POS DID SHOW TO THE RPTR AT THE DEP TIME THAT IT WAS LOCKED. THE DOOR IS AN OUTWARD DOWN TYPE DOOR. WHEN IT OPENED IN FLT IT WAS YANKED OFF THE BOTTOM HINGES. THE DOOR WAS RECOVERED FOR A POST FLT INSPECTION. RPTR CLAIMS THIS HAS OCCURRED BEFORE BUT NOT TO HIM. FAA SENT HANDLE TO MANUFACTURER FOR APPRAISAL.