Descent below the minimum permitted altitude, Airbus A320, VH-VNC, 15 km south-south-east of Avalon Airport, Victoria, on 30 June 2011

What happened

At 2302 Eastern Standard Time on 30 June 2011, an Airbus A320 aircraft, registered VH-VNC and operated by Tiger Airways Australia Pty Ltd (Tiger Airways) on a regular public transport flight, conducted a go-around procedure after an unsuccessful approach to land on runway 18 at Avalon Airport, Victoria. The flight crew contacted air traffic control (ATC) and were directed to climb to 3,000 ft and, after further discussion, to visually position as required for return and landing on Avalon runway 36. During that re-positioning, the flight crew descended below the assigned altitude without clearance. After a brief discussion with ATC, the flight crew were cleared for a conditional visual approach. In continuing the approach, the aircraft descended to 1,600 ft at a point where the minimum permitted altitude was 2,000 ft. The aircraft subsequently landed on runway 36.

What the ATSB found

The ATSB found that the flight crew, despite recognising the potential that a go-around may be required, did not plan for a return to runway 36 before commencing their first approach. The workload associated with the execution of the go-around prevented them from planning the return to land until levelling at 3,000 ft. The flight crew’s comprehension of the aircraft’s position during the second approach was probably influenced by the workload associated with the approach. Similarly, this high workload may have resulted in their not recognising the ramifications of descending to 1,600 ft.

The ATSB also found that the controller did not provide the flight crew with the required minimum vector altitude as part of the visual approach clearance, which resulted in a missed opportunity for the flight crew to identify the lowest altitude to which the aircraft could be descended. In addition, despite the controller identifying that the aircraft had descended below the minimum permitted altitude and querying the flight crew, a safety alert was not issued. This meant that the aircraft remained at heightened risk for an extended period of time.

Finally, a number of discrepancies in the operational and air traffic services documentation relating to night visual approaches and Avalon airspace were identified.

What's been done as a result

Tiger Airways advised that they have implemented a process to identify and manage the risks associated with any organisational change. This process will apply to any future changes to operational documentation.

Airservices Australia has completed a review of the Manual of Air Traffic Services and incorporated a number of amendments to improve its readability.

Safety message

This incident highlights the importance of preparation by flight crew in order to avoid the adverse effects of high workload. The potential safety benefit of intervention by air traffic controllers is also highlighted.