On 26 September 2011 at 1151 Eastern Standard Time, the crew of an Airbus A321 (Airbus) aircraft, registered VH-VWW, received on the aircraft's traffic alert and collision avoidance system (TCAS) a traffic advisory (TA) alert, for an aircraft that was approaching from their front-right position at 2 NM (3.7 km) at 2,700 ft. The Airbus was maintaining 3,000 ft on an ARBEY standard terminal arrival route to runway 34 Left at Melbourne Airport, Victoria and was in Class C controlled airspace. The upper limit of non-controlled Class G airspace in the area was 2,500 ft.

It was later found that the pilot of the other aircraft a Piper PA-28-181 Archer, registered VH-TXD, had incorrectly set the aircraft's altimeter barometric pressure subscale setting, resulting in the aircraft entering controlled airspace without a clearance and a violation of controlled airspace occurring.

The occurrence was quickly resolved due to the traffic alert from the Airbus A321 aircraft's TCAS and the subsequent response from the flight crew.

This incident acts as a reminder to pilots of the importance to cross-check cockpit settings before flight.