An incident at Melbourne Airport in which an aircraft on approach descended 500 ft lower than cleared has demonstrated the importance of a resilient safety management system. It also shows the safety benefits of clear operator procedures in the management and use of safety-critical navigation data.
The event took place on 7 June 2011, as an Airbus A320, operated by Tiger Airways, was on an approach to runway 27 at Melbourne Airport, Victoria. Air traffic control (ATC) had cleared the aircraft to descend to 2,500 ft. Shortly after, ATC identified that the aircraft had descended to 2,000 ft, below the limiting altitude for that segment of the approach. ATC notified the flight crew of the deviation. The crew re-established the aircraft at 2,500 ft, then continued the approach and landed.
The ATSB found that the flight crew had based the descent profile on information displayed on the aircraft’s Multipurpose Control and Display Unit (MCDU). The MCDU drew on information stored in the aircraft’s flight management guidance system (FMGS). The FMGS information included data provided through a third party that had a missing altitude limitation. That limitation was, however, included in the paper charts also used by the crew but the data anomaly was not identified by the crew during their preparation for the approach.
The ATSB also found there was an increased risk of inadvertent non-compliance with published instrument approach procedures because of the inconsistent application of the operator’s safety management system to the identification and management of database anomalies. In addition, different assumptions by the data suppliers and the operator compromised the quality assurance of the navigational data.
The action by ATC to alert the flight crew triggered their recovery to the required flight profile.
In response to this occurrence, Tiger Airways implemented an auditable process for identifying and managing navigational database anomalies in its aircraft fleet.
Read the report: Operational non-compliance - Airbus A320, VH-VNG, 17 km east-north-east of Melbourne Airport, Victoria, on 7 June 2011