What happened
On 6 August 2017, an Airbus A320, registered VH-VGY, was performing a scheduled flight from Wellington to Christchurch. During the arrival phase, the aircraft descended below the established minimum safe altitude for a specific segment of the standard arrival route.
The flight crew had elected to use an 'open descent' mode rather than the fully automated 'managed descent' mode. While the aircraft was cleared to descend to 2,000 feet, it passed through a segment requiring the aircraft to remain above 3,000 feet before reaching the GOMPI waypoint. The aircraft also breached a 2,500-foot minimum altitude limit between the GUKAM and GOMPI waypoints. The crew did not realize the altitude excursion had occurred until after the aircraft had safely landed at Christchurch.
The investigation
The investigation examined the flight crew's use of the aircraft's flight management guidance system and the air traffic controller's response. Investigators reviewed radar data from Airways New Zealand and interviewed the pilots and the air traffic controllers involved.
Analysis focused on why the crew failed to maintain altitude restrictions and why the air traffic controller, who observed the descent on radar, did not alert the pilots during the approach. The investigation also looked at the operator's standard operating procedures regarding the use of automated navigation systems.