BE90; A PRESSURIZED TWIN ENG TURBOPROP; HAD AN EXPLOSIVE DECOMPRESSION WHEN THE PAX DOOR FAILED AND CAME OFF THE ACFT.

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IN THE MIDDLE OF JULY; I HAD FLOWN THIS PARTICULAR GROUP OF PAX SEVERAL TIMES TO A GAMBLING CASINO. ON TWO OF THE PREVIOUS FLTS THE PAX HAD COMPLAINED ABOUT THE HEAT IN THE AIRPLANE. TO ACCOMMODATE THE PAX ON THIS FLT; I DEMONSTRATED TO ONE PAX HOW TO CLOSE THE CABIN DOOR. THIS ALLOWED ME TO BE STRAPPED IN AND READY TO START THE ENGS SO I COULD GET THE ACFT RUNNING WHILE THE PAX CLOSED THE DOOR. I STARTED THE R ENG AS SOON AS POSSIBLE. BEFORE I STARTED THE L ENG I TURNED AROUND AND NOTICED THE PAX STILL FUMBLING WITH THE DOOR HANDLE. I MOTIONED WITH MY HAND HOW TO LOCK THE DOOR WHILE I STARTED THE L ENG. THE CABIN DOOR LIGHT WAS ILLUMINATED AND I WAS ABOUT TO SHUT DOWN WHEN IT APPEARED THAT THE PAX HAD FINALLY CLOSED THE DOOR. I ASKED THE PAX IF ALL THE PINS WERE IN PLACE AND HE ACKNOWLEDGED. I ALSO NOTICED THAT THE CABIN DOOR LIGHT EXTINGUISHED AND CONCLUDED THAT THE DOOR WAS SECURED. I DEPARTED RWY 17 AND PROCEEDED ON A VECTOR FROM TWR (290 DEGS). AT 1500 FT MSL I WAS HANDED OFF TO APCH WHO CLRED ME TO 11000 FT ON A HDG OF 350 DEGS. UPON REACHING 4000 FT; PRESSURIZATION WAS LOST; ALONG WITH THE CABIN DOOR. I EXPLAINED TO ATC AND REQUESTED A TURN BACK TO THE ARPT. I LANDED AT THE ARPT AND FOUND LITTLE DAMAGE; BUT FOR THE FACT THAT THE AIRSTAIR DOOR HAD FALLEN OFF THE AIRPLANE. THIS OCCURRED JUST N OF THE ARPT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THERE IS A MICRO SWITCH IN EACH DOOR PIN. ALSO THE DOOR HANDLE HAS ONE. ALL MUST BE CLOSED TO EXTINGUISH THE DOOR WARNING LIGHT. MAINT FOUND THAT SOME SWITCHES WERE FAILED IN THE CLOSED POS. THIS WOULD CAUSE A DOOR CLOSED INDICATION; WHEN IN FACT THE DOOR WAS NOT SAFELY SECURED.