A BEECH KINGAIR 300 IN CLB AT 6000 FT RETURN LAND BECAUSE THE L AFT COWLING DOOR DEPARTED THE ACFT.

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ON JUN/SAT/02 AT XA30; KINGAIR 350 DEPARTED ZZZ INTL ARPT ON AN OWNER FLT. AT 6 MINS INTO THE FLT; AND AT 6000 FT MSL; THE PLT NOTICED THE L AFT COWLING DOOR WAS PARTLY OPEN; HE DECIDED TO RETURN TO ZZZ AND DURING THE TURN TO RETURN TO THE ARPT; THE COWLING DOOR OPENED COMPLETELY AND DEPARTED THE ACFT SOON AFTER COMPLETELY OPENING. ON JUN/WED/02; THE COWLING HAD BEEN OPENED BY MAINT TO PERFORM A LIFE LIMITED COMPONENT CHANGE. BELOW IS A COPY OF THAT LOGBOOK ENTRY OF WORK PERFORMED. AFTER MAINT WAS PERFORMED; THE COWLINGS WERE CLOSED AND ACFT WAS RETURNED TO SVC. THE ACFT WAS NOT FLOWN BEFORE THE DATE OF THE INCIDENT. IT IS HARD FOR ME TO BELIEVE THAT CLOSING OF THE LATCHES WAS OMITTED IN SECURING THE COWLING FOR FLT. IF IT WAS DELETED FROM MY ACTIONS; IT WAS DUE TO SOME SORT OF DISTR. I FEEL THAT IF THE COWLING HAD NOT BEEN LATCHED IT WOULD NOT HAVE REMAINED ON THE ACFT FOR AS LONG AS IT DID. AFTER THIS INCIDENT; I QUICKLY RESEARCHED TO SEE IF THIS HAD HAPPENED BEFORE. I FOUND THAT THE LATCHES HAVE BEEN REDESIGNED BY RAYTHEON SEVERAL TIMES INDICATING THAT THERE MIGHT BE A PROB WITH THE LATCHING SYS. ALSO THEY HAVE ADDED EXTRA SCREWS AND QUARTER TURN FASTENERS TO HELP SECURE COWLING. DURING THE REPAIR OF THIS ACFT; I WILL INCORPORATE ANY AND ALL MANUFACTURER'S RECOMMENDED SVC LETTERS OR SVC BULLETINS TO IMPROVE THE LATCHES OF THE COWLINGS AND ACCESS DOORS INSTALLED ON THIS ACFT. ALSO; I WILL PAINT THE LATCHES RED ON THE PORTIONS OF THE LATCHES THAT ARE NOT VISIBLE WHEN THE LATCHES ARE PROPERLY LATCHED. THIS WILL BE VISIBLE AT A GLANCE IF LATCHES ARE NOT LATCHED PROPERLY.