Piper PA-31-350, VH-FIA

Approaching Adelaide on descent, the pilot of the Piper PA31 observed that the main cabin door had opened in flight. A direct track to Adelaide was requested and the aircraft landed uneventfully a short time later.

An inspection by the pilot found that the door handle was still in the locked position and all locking pins were extended. The pilot reported that the door was properly closed and locked prior to departure.

A maintenance inspection was unable to fault the door locking mechanisms and a series of functional tests of the door concluded that the door could not be opened without first moving the handle to the OPEN position.

The aircraft was returned to service. The pilot then flew a total of six sectors over two days without further incident, however at the completion of the seventh sector, the pilot noticed that the locking handle had moved approximately halfway between the fully closed/locked position and fully open/unlocked.

Maintenance personnel again inspected the aircraft, but no fault could be found. During that inspection, it was noted that when closing the stair door from the outside, the forward support cable could become wedged in between the door and the jam. This would not be immediately apparent to the pilot if closing the door from the outside and could prevent the door locks from correctly securing the door. The aircraft was returned to service without further incident.