On 1 January 2014, the crew of an Airbus A320, registered VH-VGU, was making a standard arrival route (STAR) descent into Melbourne, Victoria. The STAR was cancelled by air traffic control (ATC) before the procedure was complete, and descent continued under radar vectors to the south-west of Melbourne. The STAR was cancelled to allow ATC to sequence VH-VGU with a number of other aircraft arriving into Melbourne at about the same time.
During radar vectoring, the crew was cleared to descend to 3,000 ft. As they neared that altitude, ATC cleared the crew to track direct to waypoint MEXUN, then cleared the crew to conduct the Area Navigation –P (Required Navigation Performance) (RNAV-P (RNP)) approach to runway 34. Waypoint MEXUN was the latest point at which an aircraft could join the RNAV-P (RNP) procedure.
The crew entered MEXUN as the next waypoint in the Flight Management Guidance System (FMGS) flight plan and armed the auto-flight system final approach mode. Final approach mode engaged almost immediately and the aircraft continued descent towards 2,000 ft, which was the next FMGS altitude constraint. As the aircraft continued descent, it temporarily left controlled airspace.
As they descended through about 2,400 ft, ATC cautioned the crew that they were nearing the relevant lowest safe altitude. In response, the crew stopped the descent at about 2,100 ft, and continued toward MEXUN for an otherwise uneventful approach. The aircraft was outside of controlled airspace for about 45 seconds and there was no conflict with any other known air traffic.
A number of factors were identified as having contributed to the incident, including an elevated crew workload associated with numerous speed restrictions during the arrival sequence, and windy, turbulent conditions. The conditions also prompted a relatively late decision to change the flap selection for landing. Airservices Australia later commented that despite being cleared to conduct the approach, ATC expected the crew to maintain 3,000 ft until either further descent clearance was provided, or the aircraft was established on the approach procedure (which would have been when the aircraft reached MEXUN).
The incident highlights the need for clear procedural guidance regarding auto-flight system management under circumstances where transition from a STAR to an instrument approach is interrupted. The incident also highlights the importance of clarifying an ATC clearance if the clearance appears incomplete.