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.