PLT OF BE55 BARON HAS DOOR POP OPEN INFLT WHEN ENCOUNTERING TURB. EMER DECLARED. VECTORED TO NEARBY ARPT.

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ON THE BEECH BARON YOU MUST CAREFULLY MONITOR CLOSING AND SECURING THE DOOR. THIS INCLUDES CHKING THE PIN IN THE TOP OF THE DOOR FRAME. MY PAX CLOSED AND LATCHED THE DOOR AS I WATCHED. I REACHED OVER; TOOK THE HANDLE AND TURNED IT COUNTER CLOCKWISE AGAINST THE PIN STOP. IT NORMALLY WILL ROTATE FREELY THROUGH ABOUT 20 DEGS. I DID NOT HOWEVER TURN IT COUNTER CLOCKWISE TO MAKE SURE THAT THE LOCKING SYS WENT 'OVER SQUARE' INTO THE LOCKED POS. I CHKED THE TOP PIN TWICE; AND THE TOP OF THE DOOR WAS VERY SECURE. AFTER LEVELOFF AT 7000 FT I HEARD A WIND NOISE AND FELT THE WIND LACING AROUND THE DOOR. THE DOOR WAS AJAR BUT NOT OPEN. I CALLED APCH; SAID I HAD A PROB; AND WOULD PROCEED TO LANCASTER; PA (100 MI; 25 MINS AWAY AS IT HAD THE BEST WX; NIGHT MARGINAL VFR). THEY ASKED IF I WANTED TO GO TO WINCHESTER AND I DECLINED (300 FT CEILING). MODERATE TURB WAS FORECAST FOR THE TRIP AND MOMENTS (2-5 MINS LATER) WE HIT IT. AS SOON AS WE HIT THE TURB THE DOOR POPPED OPEN. THE SUCTION AND WIND WERE UNBELIEVABLE. THE HEADSET WAS RIPPED OFF THE PAX'S HEAD. EVERY CHART AND PIECE OF PAPER NOT NAILED DOWN WENT OUT THE DOOR. IMMEDIATELY I DECLARED AN EMER AND APCH GAVE ME SEVERAL OPTIONS. THE BEST WAS THE ILS AT HAGERSTOWN; MD. THE CTLR GAVE ME A VECTOR; READ ME THE APCH; AND WE LANDED UNEVENTFULLY. THIS COULD HAVE BEEN A TRAGEDY. THE WIND AND NOISE WERE AWESOME. THE WX LOUSY. IT WAS NIGHTTIME. THE AIRPLANE FLEW STRANGELY. MY MISTAKE AS A PLT WAS ALLOWING ANYONE OTHER THAN MYSELF TO TAKE ANY PART IN SECURING THE DOOR OF THE AIRPLANE. FROM NOW ON; ONLY MY HANDS TOUCH THE DOOR. ALSO THERE IS AN AIRWORTHINESS DIRECTIVE ON THIS DOOR; THAT WAS COMPLIED WITH. THIS DOOR SHOULD HAVE AN ANNUNCIATOR OR WARNING LIGHT IF NOT CLOSED PROPERLY.