Incident Overview
On 6 August 2017, at approximately 1845 local time, an Airbus A320-232 (registered VH-VGY) was on approach to Christchurch, New Zealand, with 128 passengers and 6 crew on board. During the arrival procedure, the aircraft inadvertently descended below the published minimum safe altitude of 2,500 feet for a segment of the standard instrument arrival route.
Sequence of Events
The flight had originated from Wellington at about 1825, with the first officer as pilot flying and the captain as pilot monitoring. The crew loaded the MESIX Five Alfa arrival and were cleared by air traffic control to descend via the standard arrival route. The first officer initiated an open descent mode on the flight control unit, which required manual control of altitude while the autopilot managed lateral navigation.
After crossing the GUKAM waypoint at 4,625 feet, the aircraft descended below 3,000 feet before reaching the GOMPI waypoint at 1848:32, violating the minimum safe altitude of 3,000 feet for that segment. The controller noticed the descent below the safe altitude at about 1850, when the aircraft was passing through 2,300 feet, but did not immediately alert the crew. The aircraft leveled at 2,000 feet and passed GOMPI at that altitude. After landing, the tower controller informed the captain of the altitude exceedance.
Findings
The Transport Accident Investigation Commission (TAIC) determined that the flight crew did not maintain adequate situational awareness of the aircraft's position relative to the standard arrival route. Additionally, the operator's procedures did not encourage appropriate use of automated navigation systems. The air traffic controller observed the deviation but did not follow required procedures to alert the crew in a timely manner.
Safety Actions
The operator took safety actions in response to the incident, and TAIC did not issue new recommendations. A key lesson identified was that proper use of automated flight navigation systems reduces crew workload and enhances safety; when not used, heightened vigilance is necessary.