SCHEDULED CARGO PLT OF A PIPER PA32-300 LOST THE TOP PORTION OF THE CABIN DOOR DURING DSCNT 20 MI SHORT OF DEST. PLT MADE A SUCCESSFUL LNDG AND THE TOP OF THE DOOR HAS NOT BEEN RECOVERED.

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I HAD BEGUN MY DSCNT FOR CONCORD REGIONAL ARPT (JQF); WHEN I HEARD AIR RUSHING IN/OVER THE AIRPLANE. A SECOND OR TWO LATER THERE WAS A POPPING NOISE WHICH INCREASED THE SOUND OF AIR SUBSTANTIALLY (SIMILAR TO THE SEQUENCE OF SOUNDS HEARD WHEN OPENING A COKE CAN). I TURNED AROUND TO FIND THE TOP PART OF THE PAX DOOR ON THE PA32-300 HAD SHEARED OFF. THE AIRPLANE DID NOT FLY MUCH DIFFERENTLY BUT I STILL SLOWED THE ACFT DOWN AS A PRECAUTION AND TO DECREASE AMBIENT NOISE IN THE COCKPIT. AT THE SAME TIME; I WAS HAVING RADIO COM PROBS DUE TO A PUSH TO TALK NOT OPERATING EFFECTIVELY. SIMULTANEOUS; I WAS DSNDING TO 3000 FT; TURNING TO AN ASSIGNED HEADING AND TRYING TO PLUG IN THE AUX MIKE. NO EMER WAS DECLARED AND I WAS ABOUT 20 NM OUT FROM MY DEST. I LANDED UNEVENTFULLY AT THE ARPT AND SPOKE WITH THE TWR REGARDING THE DOOR. THE TWR SPOKE WITH CHARLOTTE APCH TO TRY AND DETERMINE WHERE THE DOOR FELL OFF. THE DOOR BROKE OFF IN A SPARSELY POPULATED AREA AND THERE WERE NO RPTED INJURIES ON THE GND. THE FAA WAS ALSO NOTIFIED (I THINK BY THE TWR). I SPOKE WITH AN FAA EXAMINER A FEW HRS AFTER LNDG REGARDING THE INCIDENT. IN EXAMINING WHAT HAPPENED I HAVE COME UP WITH A FEW FACTORS THAT MAY HAVE COMBINED TO CAUSE THIS INCIDENT. FIRST OF ALL; THE DESIGN OF THE DOOR LEAVES VERY LITTLE SUPPORT FOR THE TOP OF THE DOOR. THE TOP OF THE DOOR HOLDS IN A PIECE OF LEXAN; WHICH SERVES AS THE PAX WINDOW. THERE ARE 2 PERMANENT HINGES CONNECTING THE DOOR TO THE ACFT AS WELL AS 2 LATCHES THAT SECURE THE DOOR WHEN CLOSED. THE BOTTOM HALF OF THE DOOR HAS BOTH PERMANENT HINGES AND ONE OF THE LATCHES; WHICH MAKES IT VERY SECURE. THE TOP HALF OF THE DOOR IS ONLY CONNECTED BY THE LATCH ON THE TOP CTR. WITH BOTH LATCHES SECURED PROPERLY; THERE IS STILL THE POTENTIAL FOR A SMALL GAP FROM WHICH AIR MIGHT ENTER THE CABIN SECTION AND PRODUCE A FORCE THAT MAY WEAKEN THE WEAKEST POINT OF THE DOOR. THE TOP HINGE IS ROUGHLY PARALLEL TO THE LATCH ON THE BOTTOM AND IT IS JUST ABOVE THIS POINT THAT THE WINDOW IS INSTALLED AND MAKES FOR A WEAK SPOT ON THE DOOR. THE AIRFRAME HAS OVER 12200 HRS ON IT AND IS USED IN A PART 135 FREIGHT OP. THE HIGH AIRFRAME TIME; BEING USED MON-FRI WITH THE DOOR BEING OPERATED AT LEAST 6 TIMES A DAY; MUST HAVE FATIGUED THE DOOR IN QUESTION CONSIDERABLY. THE LAST FACTOR THAT MAY HAVE CONTRIBUTED TO THE FAILURE OF THE DOOR IS THE POS OF THE TOP LATCH. THERE ARE 3 POSSIBILITIES. THE LATCH MAY BE LATCHED SHUT; BE PUT IN THE SHUT POS BUT NOT PROPERLY LATCHED TO THE AIRFRAME; OR NOT LATCHED AT ALL. I NORMALLY DOUBLE-CHK THE LATCH TO MAKE SURE IT IS SHUT AND AM CONFIDENT THAT I HAD THE LATCH SHUT.