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An ATSB investigation has been unable to determine why an Alliance Fokker 100 flew lower than the minimum safe altitude on a service from Brisbane to Adelaide.
The descent was controlled by an auto-flight system that had to be disconnected when the aircraft dipped to 480 ft below the minimum safe level and wouldn’t respond to crew instruction.
The aircraft then rose to a safe height before conducting an uneventful landing.
The ATSB’s director of transport safety, Stuart Macleod, said, “This incident reinforces to pilots the importance of continuously monitoring auto-flight systems and reacting quickly when the aircraft is not on the expected flight path to ensure that limits are not exceeded.”
The full report reveals how, on the morning of 30 August 2023, the Alliance Airlines-operated Fokker 100 flight crew was conducting a BLACK 3A standard instrument arrival into Adelaide’s runway 23 at the conclusion of a passenger service from Brisbane.
“The FMS failed to capture the 3,800 ft minimum safe altitude between the waypoints KERRS and GULLY, and the aircraft descended through this altitude,” Macleod said.
The flight crew were monitoring the altitude and observed the breach.
“In response, the captain pressed the altitude control knob to command the aircraft to hold the current altitude, but this did not arrest the descent,” Macleod said. “The first officer then increased the selected altitude to 4,700 ft, but the aircraft continued to descend.
“During this time, the aircraft was in and out of cloud, but the flight crew reported they had sufficient visibility with the ground and terrain to assess that adequate separation existed.”
Assessing that the aircraft was continuing to descend in auto-flight mode, the first officer disconnected the autopilot and initiated a climb, with the aircraft at about 3,487 ft—approximately 480 ft below the minimum safe altitude for the segment.
The aircraft then entered a gradual climb, but at about the same time, it passed through waypoint GULLY, entering a new segment with a minimum safe altitude of 3,200 ft, so no further climb was necessary.
The flight crew then proceeded with the instrument approach and conducted an uneventful landing.
“Fortunately, 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,” Macleod said.
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.
