A PASSENGER ON A TBM-850 FAILS TO PROPERLY SECURE THE DOOR BEFORE TAKEOFF AND THE DOOR CAME OPEN IN FLIGHT.

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1) TOOK OFF FROM GRUVER; TX; FILED IFR TO FL260. 2) MY PAX SECURED THE DOOR; I DID NOT VERIFY PERSONALLY. HE HAS FLOWN MANY TRIPS AND I BELIEVED THAT HE UNDERSTOOD THE PROCESS. 3) AS WE WERE CLBING THROUGH FL240 AN ABNORMAL AIR NOISE WAS NOTICED IN THE CABIN; ALL GAUGES WERE NORMAL. I ALONG WITH MY PAX NOTICED THAT THE DOOR HANDLE WAS NOT DOWN AS USUAL. 4) MY PAX ATTEMPTED ON SEVERAL OCCASIONS TO PUSH THE HANDLE DOWN FARTHER; NO SUCCESS. 5) I IMMEDIATELY DSNDED TO FL200 AND THE AIR NOISE COMPLETELY SUBSIDED; ALL GAUGES INDICATED NORMAL. I LEFT THE PRESSURIZATION SET AT FL270 IN ORDER TO OBTAIN A LESS THAN MAX PSI DIFFERENTIAL. 6) I BEGAN THE DSCNT INTO STERLING; CO; AFTER ABOUT 45 OR 50 MINS INTO THE FLT. AS THE PLANE DSNDED THROUGH ABOUT 8000 FT MSL THE DOOR CAME OPEN. THE DAMAGE AS IS NOTED: A) RAIN GUTTER; LOCATED ON THE TOP OF THE DOOR HINGE AND ON THE LOWER PORTION OF THE DOOR HINGE; WAS BENT AND BROKEN. B) INSIDE PLASTIC PANEL OF DOOR CAME OFF. C) BOTH R AND L GAS STRUTS THAT HOLD THE DOOR UP CAME OFF. D) CABLE THAT PULLS THE DOOR CLOSED BROKEN. E) ONE OF THE PARTS THAT CAME OFF THE DOOR HIT MIDWAY UP THE VERT STABILIZER CAUSING A 3/4 INCH BY 3 INCH CUT. 7) I REDUCED THE PWR SUBSTANTIALLY; SLOWING TO ABOUT 110 KIAS; NORMAL LNDG AT STERLING WITH NO NOTICEABLE CTL PROBS.