FLC OF ACR LTT TURBOPROP ACFT MADE AN EMER LNDG AFTER THE MAIN CABIN DOOR CAME OPEN DURING CLB OUT. FLC DID NOT REALIZE THAT THE DOOR ACTUALLY FELL OFF THE ACFT.

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WE HAD NO PAX FOR THE FLT BUF TO ROC. AFTER CLOSING OF THE MAIN CABIN DOOR BY THE BUF RAMP PERSONNEL (WHICH IS STANDARD PROC) WE HAD A 'DOOR' LIGHT ON THE CAP PANEL INDICATING THE MAIN CABIN DOOR WAS OPEN; OR 1 OF THE 7 MICRO SWITCHES ON THE DOOR WAS NOT IN THE PROPER POS. HAVING FLOWN THE SUBJECT ACFT THE DAY BEFORE; AND HAVING EXPERIENCED A SIMILAR OCCURRENCE ON 3 OUT OF 4 LEGS; IT WAS MY BELIEF THAT THE PROB WAS A MICRO SWITCH. WITH NO PAX ON BOARD; AND A SHORT 20 MIN FLT TO ROC AT A FILED ALT OF 5000 FT; I MADE THE ASSUMPTION; BASED UPON THE PRIOR DAY'S EXPERIENCE; THAT THE PROB WAS A FAULTY MICRO SWITCH. IT WAS MY BELIEF THAT BY PULLING AND RESETTING THE APPROPRIATE CIRCUIT BREAKER; IF THE PROB WAS IN FACT A FAULTY MICRO SWITCH; FOLLOWING THE ABOVE PROC WOULD ALLOW THE MICRO SWITCH TO RESET ITSELF AND IF THE 'DOOR' CAP LIGHT EXTINGUISHED; THE DOOR WAS CLOSED; AND SAFE FOR FLT. THE LTT ACFT APPROVED PROC IS TO PULL THE CIRCUIT BREAKER; REOPEN THE DOOR; LEAVE IT OPEN FOR 15 SECONDS AND RECLOSE THE DOOR. AFTER DEP FROM BUF; CLBING THROUGH APPROX 4000 FT MSL; WE EXPERIENCED A RAPID DECOMPRESSION. LOOKING TO THE REAR OF THE CABIN I COULD TELL THE MAIN CABIN DOOR WAS AJAR. BECAUSE OF THE INHERENT DANGER; I ELECTED NOT TO GET OUT OF MY SEAT TO INSPECT THE DOOR; BUT INSTRUCTED THE FO TO SLOW THE ACFT TO 130 KIAS AND PROCEED TO ROC. BECAUSE THE FO WAS LOW TIME IN THE ACFT; I TOOK THE CTLS ON FINAL APCH TO MAKE A SLOW; FLAT LNDG TO MINIMIZE DAMAGE TO THE DOOR. AFTER EXITING THE ACTIVE RWY AT ROC; THE FO WENT TO THE REAR OF THE CABIN TO INSPECT THE SIT AND CLOSE THE DOOR; ONLY TO FIND THE DOOR WAS COMPLETELY MISSING. AFTER DISCUSSING THE INCIDENT WITH OUR CHIEF PLT AND MAINT PERSONNEL; I WAS INFORMED THAT MY PULLING AND RESETTING OF THE ENTRY DOOR WARNING CIRCUIT BREAKER ONLY SERVED TO DISABLE THE SYS; AND THE SYS WAS IN FACT NOT ARMED. THERE IS NOTHING IN THE ACFT LITERATURE NOR OUR INSTRUCTION THAT MAKES THIS CLR. FURTHER DISCUSSION WITH OTHER COMPANY CREW MEMBERS REVEALED THAT OTHER CREWS HAVE HAD SEVERAL 'DOOR' CAP LIGHTS COME ON IN FLT (ON CLB OUT) ON THIS SAME ACFT WITHIN THE PRECEDING WK; BUT THE PROB WAS NEVER WRITTEN UP BECAUSE SUBSEQUENT FLTS WERE NORMAL.