On 7 September 2013, an Airbus A320, registered VH-VFJ, was on descent into Auckland, New Zealand via a Required Navigation Performance (RNP) approach to runway 23L. During the later stages of their descent, the crew managed the aircraft speed to meet an Air Traffic Control request and according to applicable company speed restrictions.

The auto-flight system sequenced to final approach mode passing about 4,200 ft, but exited final approach mode when the crew subsequently levelled the aircraft approaching 3,000 ft. The crew levelled the aircraft to reduce speed to comply with a company speed restriction of 210 kt maximum below 3,000 ft. Having slowed sufficiently, subsequent manipulation of the auto-flight system resulted in the inadvertent engagement of open climb mode, which resulted in an increase in engine thrust and aircraft acceleration.

Attempting to avoid exceeding the limiting speed applicable to the existing aircraft configuration, the captain retarded the thrust levers to the idle stop, inadvertently disconnecting the auto-thrust system. The crew resumed the approach, unaware that the auto-thrust system was disconnected, and therefore no longer controlling aircraft speed. As the aircraft continued to decelerate, soon after the final stage of flap was selected for landing, the Flight Management Guidance System generated a low energy warning. As the crew was responding to the low-energy warning, alpha-floor auto-thrust mode engaged. The crew accelerated the aircraft to approach speed using manual thrust control, and was able to continue the approach for an uneventful landing.

The operator’s investigation into the incident found that, among other things, there may be some commonly held misunderstandings with respect to some aspects of instrument approach procedures, particularly their application to RNP approaches. The operator planned to communicate relevant procedural information to flight crew, with appropriate explanatory information, and communicate with flight crew regarding procedural requirements associated with auto-flight system mode awareness and speed monitoring. The operator also planned to include more guidance in appropriate documentation dealing with transfer of aircraft control between flight crew.

This incident highlights the need for robust and clear instrument approach and auto-flight system management procedures. It also highlights the need for consistent attention to aircraft auto-flight modes and energy state.

Aviation Short Investigation Bulletin - Issue 35