What happened
The FAA’s narrative, in sentence case. The FAA writes it in capitals and removes private details.
The aircraft cargo door unlatched and opened in cruise flight. A passenger attempted to close the door but was unable to. The flight continued at cruise speed and the door separated from the aircraft after the center rail bent up to allow the door to slide past aft stop. The aircraft landed safely with no injuries and no additional damage to the aircraft. The aircraft damage consisted of deformed upper and lower rear door rails, and bent center rail. The cargo door has not been recovered. Operation at sustained cruise speed with an unlatched door contributed to the door fully opening and departing the aircraft. The cause of the door unlatching has not been determined.
GA-8 flight manual supplement – inflight rear door open operations limitations: when opening or closing the door: 1.reduce speed to 80 KIAS or below. 2. Fly straight and level. 3. Open or close door fully (never fly for extended periods with door partially open).
As the FAA filed it
THE AIRCRAFT CARGO DOOR UNLATCHED AND OPENED IN CRUISE FLIGHT. A PASSENGER ATTEMPTED TO CLOSE THE DOOR BUT WAS UNABLE TO. THE FLIGHT CONTINUED AT CRUISE SPEED AND THE DOOR SEPARATED FROM THE AIRCRAFT AFTER THE CENTER RAIL BENT UP TO ALLOW THE DOOR TO SLIDE PAST AFT STOP. THE AIRCRAFT LANDED SAFELY WITH NO INJURIES AND NO ADDITIONAL DAMAGE TO THE AIRCRAFT. THE AIRCRAFT DAMAGE CONSISTED OF DEFORMED UPPER AND LOWER REAR DOOR RAILS, AND BENT CENTER RAIL. THE CARGO DOOR HAS NOT BEEN RECOVERED. OPERATION AT SUSTAINED CRUISE SPEED WITH AN UNLATCHED DOOR CONTRIBUTED TO THE DOOR FULLY OPENING AND DEPARTING THE AIRCRAFT. THE CAUSE OF THE DOOR UNLATCHING HAS NOT BEEN DETERMINED.
GA-8 FLIGHT MANUAL SUPPLEMENT – INFLIGHT REAR DOOR OPEN OPERATIONS LIMITATIONS: WHEN OPENING OR CLOSING THE DOOR: 1.REDUCE SPEED TO 80 KIAS OR BELOW. 2. FLY STRAIGHT AND LEVEL. 3. OPEN OR CLOSE DOOR FULLY (NEVER FLY FOR EXTENDED PERIODS WITH DOOR PARTIALLY OPEN).