What happened
Following a missed approach and a subsequent holding pattern, the crew of a BAE 146 began a second approach toward Rockhampton. The captain, acting as pilot flying, initiated a descent upon crossing the SARUS waypoint. However, the descent began from a minimum holding altitude of 3,500 ft, rather than the 5,000 ft altitude typically used for this straight-in approach. This premature descent caused the aircraft to follow a profile that was one approach segment ahead of its actual position.
As a result of this early descent, the aircraft twice dropped below the segment minimum safe altitudes (SMSA). During this period of dark night and cloudy weather, the aircraft lost the protection provided by terrain and obstacle separation. The error was only identified when the first officer, acting as pilot monitoring, noticed discrepancies in the control display unit and the ground proximity warning system issued a 'terrain' alert. The first officer then called for a missed approach.
The investigation
The investigation examined several factors that contributed to the descent error. It was found that the first officer's focus was diverted by radio communications during the holding pattern, and the captain's takeover of monitoring tasks temporarily removed the first officer from the approach monitoring role. Furthermore, the next waypoint was not immediately visible on the horizontal situation indicator, which hindered the recognition of the error.
The investigation also looked into the crew's physiological state. Both crew members were operating during their circadian low and were likely experiencing fatigue due to accumulated sleep debt. While the operator's rosters complied with regulations, the irregular nature of the night duties disrupted the crew's ability to obtain adequate sleep. Additionally, the high workload of the second approach—compounded by turbulent conditions and the manual inputs required by the aircraft's analogue instrumentation—contributed to the failure to identify the descent error.
Findings
- The captain initiated a descent using a standard technique but from an incorrect starting altitude.
- The first officer maintained an incorrect mental model of the aircraft's position relative to the approach segments.
- The crew experienced fatigue and sleep debt that likely impaired performance.
- High workload during the second approach, including radio communications and manual flight tasks, hindered effective monitoring.
- The aircraft's analogue systems and autopilot required significant crew input.