VA1388 - Report: Alliance F100 at Adelaide on Aug 30th 2023

An Alliance Airlines Fokker 100 on behalf of Virgin Australia, registration VH-FGB performing flight VA-1388 from Brisbane,QL to Adelaide,SA (Australia) with 91 passengers and 5 crew, was on a standard instrument approach to Adelaide with the automation in lateral and vertical navigation modes and the correct sector altitude selected, when the aircraft descended below the selected 3800 feet. The captain noticed the descent below the selected altitude and selected to hold altitude, the first officer noticed however the aircraft continued to descend, disengaged the autopilot and performed a manual climb. The aircraft subsequently continued for a safe landing.

On Apr 12th 2024 the ATSB released their final report remarking this occurrence highlights the important to monitor automation at all times and concluding the probable causes of this incident were:

For reasons that could not be determined, the flight management system did not capture the selected altitude. This resulted in the aircraft descending about 480 ft below the segment minimum safe altitude.

The ATSB analysed:

During the standard instrument arrival to Adelaide Airport, the aircraft was operating in an automated flight mode, in which the flight management system controlled the flight path.

Approaching waypoint KERRS, although the automatic flight system entered altitude hold mode, the aircraft continued to descend, but met the 5,000 ft minimum altitude restriction at KERRS. The automatic flight system then returned to the descent mode and did not capture the selected 3,800 ft altitude, which was the segment minimum safe altitude between KERRS and waypoint GULLY.

Despite the ATSB consulting with the aircraft manufacturer, Fokker, and the flight management computer manufacturer, Honeywell, the reason the aircraft did not level at the selected altitude could not be determined.

Fortunately, in this incident the flight crew were monitoring the instruments and disconnected the automatic flight system when they detected the descent below the selected altitude and initiated a climb. By the time the flight crew had completed resolution actions and reinstated the automatic flight system, the aircraft had passed GULLY and intercepted the lateral localiser track. During that period, the aircraft was in and out of cloud and the flight crew were able to visually assess terrain clearance. When the flight crew reconnected the autopilot, the aircraft intercepted the glideslope and completed the instrument landing system approach.
Related Flight: VA1388, Virgin Australia News