Incident Overview
An aircraft sustained substantial damage when its cargo door inadvertently opened during flight, resulting in a box exiting the cargo hold and striking the left horizontal stabilizer. The pilot reported that after loading, he and the loaders had to force the cargo door closed due to the large volume of freight. The taxi and takeoff proceeded normally.
In-Flight Events
A few minutes after leveling at cruising altitude, the door ajar warning light illuminated. Shortly after, the pilot heard a loud thud and concluded that a box had departed the aircraft and struck the tail. Control of the aircraft remained normal. The pilot executed the door ajar checklist, diverted to an intermediate airport, and landed without further incident. Upon exiting, the pilot observed damage to the left horizontal stabilizer.
Post-Incident Inspection
An examination of the cargo door installation found the door structure and locking mechanism intact and undamaged. The door closed and latched properly during testing. However, the inspection revealed that the door design allowed enough flexibility for the forward frame to be seated in the door jamb and latched while the aft frame remained outside the jamb and unlatched. The micro-switch for the door warning annunciator light was located on the forward jamb, making it possible for the aft cargo door latch to be disengaged without illuminating the warning light.
Outcome
The aircraft landed safely, and no injuries were reported. The cargo door was found to be functional, but the design allowed a condition where the door could appear closed while not fully secured.