The crew of a Saab 340 were arriving at Sydney from the
north-west and had been cleared by air traffic control to make a
visual approach to runway 16R. The controlled airspace steps in
that sector required the crew to maintain an altitude of 2,500ft
until 12 NM, when further descent would be available. As they were
passing through 2,400ft, they observed an airship immediately in
front and below. The aircraft's traffic alert and
collision-avoidance system (TCAS) issued a Resolution Advisory
alert and the crew reduced the rate of descent until they had
passed the airship.
The airship had been tracking outside controlled airspace at a
planned altitude of 1,500ft and was being navigated along the
boundary of that airspace which, at that point, was a 12 NM arc
based on Sydney airport. The crew was required to remain north-west
of that boundary.
Analysis of the recorded data indicated that the airship had
marginally infringed controlled airspace and was 11.9 NM from
Sydney. The Saab 340 had descended marginally through the 2,500ft
limit and was approximately 2,300ft when it passed the airship.
The TCAS actuated based on the reducing altitude differential
and provided the crew with timely information on the airship below
it.